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Jim Bettison
Posted on Tuesday, 07 August, 2001 - 00:35:   Edit PostDelete PostView Post/Check IP

Can anyone please tell me the ignition advance/engine rpm relationship for a MkVI (chassis B20JO)?
The background is that we couldn't get the engine to idle smoothly. Eventually we found that one of the springs which causes the centrifugal weights in the distributor to do their thing in the designed manner, had been broken and then badly repaired so that it just wasn't at all effective abd the weight wasn't returning at low rpm ... the motor was hunting because the ignition at idle was grossly advanced. We've done a temporary fix but would like to know what the design relationship for rpm/advance is ...
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Jim Bettison
Posted on Thursday, 16 August, 2001 - 11:05:   Edit PostDelete PostView Post/Check IP

For anyone interested: I found from the RREC (UK) that the MkVI should have ignition full advance at 3000 engine rpm. At full advance, distributor advance should be 18 degrees (36 degrees crankshaft.
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Norman Geeson
Posted on Sunday, 19 August, 2001 - 06:17:   Edit PostDelete PostView Post/Check IP

Jim... the following may assist, but don't try to time this engine with the engine running, the acceleration on the cam lobes as the valves close and open wind up the front spring drive! I hope the figures are not mashed by format alteration on the servers, if so contact me direct by e-mail

Adv degs......Dist min rpm....Dist max rpm


Maximum advance should not exceed 17.5 distributor degrees

Norman Geeson, Peterborough, England
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Jim Bettison
Posted on Sunday, 19 August, 2001 - 23:37:   Edit PostDelete PostView Post/Check IP

Thanks for the timing info. It arrived in what I think must be the format you sent it - quite tabular and clear. It is precisely what we had hoped to elicit.
Your remark about the "front spring drive" does mystify me, however. Do you mean the harmonic damper?
We have been timing the engine while running, for the basic timing, using a timing light (Xenon strobe lamp) triggered from a transducer clip-on pickup clipped around a plug lead and illuminating the flywheel through the inspection cover.
(This had, we found, an additional virtue; by moving the pickup from No1 to No6 lead we could check both the 'A' and the 'B' contacts independently, using the flywheel TDC mark. As mentioned in my first posting, we've been able to fudge a temporary spring to replace the dud one on one of the weights. This has smoothed things out almost completely; although there is still some rough running, we need to do a needle/jet/seals and gaskets job on the carburettors before getting back to ignition.)
But to return; surely by using the strobe lamp we are isolating our perception from all extraneous factors, because the lights fires when the apark occurs, and instantaneously illuminates the flywheel position? Doesn't this mean that we need have no concern about what goes on between crankshaft and spark plug - provided that the spark arrives at the cirrect time? (I am of course only talking about initial timing at this stage.)
Your comments would be most welcome. Thanks again for the info.

Jim Bettison Adelaide South Australia.
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Bill Vatter
Posted on Monday, 20 August, 2001 - 13:28:   Edit PostDelete PostView Post/Check IP


The timing information provided by Norman is quite valuable data and will assist me as well as you in checking the distributor performance on my own early post-war six cars.

The harmonic damper on these cars is quite complex when compared to that of modern cars. It incorporates weight, springs, and friction damping to reduce the intensity of torsional vibration. Torsional vibration, in addition to being unpleasant for the car occupants can lead to crankshaft breakage if the engine is operated for an extended period at its natural frequency, where the vibration amplitude will be quite significant. In fact, crankshaft vibration and crankshaft failure was a major problem in the early days of the automobile, and Henry Royce experienced some of these failures himself along with the others with his Royce cars that preceeded the Silver Ghost. Engines with very long crankshafts were the most problematic, because it was very difficult to design the crankshaft so that the natural frequency was above the normal operating speeds.

This vibration damper is most often referred to as a "slipper flywheel" on pre-war cars. This is because it is a small flywheel that is driven through springs, and damped with friction discs. The camshaft, and hence the distributor also, is driven from the slipping, flywheel, or driven part of the damper, which will move ahead and behind the driving part of the damper when the crankshaft is vibrating torsionally. One might think this would adversely affect the performance of the engine, but I don't think it is significant. More importantly, it prevents the torsional vibration from being transmitted to the camshaft, causing unwanted acceleration and deceleration of all associated components.

The starting handle, called by some the hand crank, also connects to the driven part of the damper. Therefore, when you use the starting handle to turn the engine, you are moving the crankshaft through the damper, and this can significantly affect the position of the camshaft with respect to the crankshaft, and any timing measurements made subsequently are likely erroneous. This is why the factory service manual instructs turning the engine by means of the rear wheel when moving it to the correct position to set the distributor points. Once "cranked" by the starting handle, the engine must be run to shake the damper back to its neutral position for accurate timing.

The vibration damper is located inside the front cover of the engine, also called the gearcase (because for the early cars it was filled with quite a few gears) or the timing cover. The damper is immediately behind the fan belt pully (sheave).

The very best way to measure distributor performance using Norman's data would be with a distributor timing maching such as was made by the Sun Manufacturing Company many years ago. With this you remove the distributor from the car and run it with the machine, which displays advance as a function of RPM. With Norman's data it would be then quite easy to verify the advance is correct as the distributor is sped up through its operating range. Unfortunately, there are not many Sun machines left, being obsolete and hard to find parts. Not unlike our cars.

Short of a Sun Machine, you could use a modern electronic engine analyzer, the kind that has an oscilloscope display of ignition functions. This would be susceptible to the vibration induced inaccuracies, However, the analyzer would either damp out these variations showing the average, (which is what you want anyway) or you would be able to "eyeball" the average from a blurred trace on the scope. The engine analyzer maching is handy because it checks things while the engine is running.

Regarding distributor springs, I am curious: are the springs no longer available from the Rolls-Royce dealer? I would expect Albers RR in the US to have these parts. They have got a lot of bits for the early post-war cars. If it were me, I would just put in new, correct springs from the factory, and assume the centrifugal advance would be correct.
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Jim Bettison
Posted on Monday, 20 August, 2001 - 22:07:   Edit PostDelete PostView Post/Check IP


Your comments and observations are appreciated. Just to comment on some of the points you raise:
I've had first hand experience of a broken crankshaft, and the cause was, I now suspect, a faulty vibration damper. I was much younger and greener when it happened (about 1954) and the vehicle was a Chevrolet "Blitz Buggy".
(A remarkable vehicle in itself. Built by GM Canada using the ubiquitous Chev 6 cylinder power unit mounted into a channel section chassis whose members were about 8" x 3" x 3/8", supporting an all-steel body, it had a tare of 60cwt and all-up (with load) of 75cwt - 3 tons to carry 15 cwt. 4WD through a standard 3-speed + reverse gearbox and a transfer/reduction case to front and rear axles with differentials, road wheels being 9.00 x 16 8-ply ... with luck, a following wind and a good gradient you would get 50 - 55mph, with a fuel consumption of around 12mpg. Mine was the short wheelbase version ...)
Anyway, the crankshaft broke in two about 1pm on a Saturday afternoon about 10 miles from Blayney en route to a student congress with 12 aboard . On reflection I think that the damper was corroded solid ...
We also have a 20/25, and it's interesting to follow the evolution of the vibration damper for those cars, and also to compare it with the MkVI. In my (disorganised) literature I've got a transcript of a talk by Grylls which includes an account of development of the damper. Fascinating.
In our case, we rebuilt the damper some time ago, as part of a complete engine rebuild; the engine has since done about 300 miles. The damper was set up to the spec. for slip; and yes, we knew about not turning the engine over by the starting handle when setting up the distributor!
I tend to agree with you about the motion of the damper not significantly affecting the engine's performance via camshaft and distributor. From my limited experience and in the greater wisdom of our mechanic, more significant sources of irregularity can be the distributor bearings, the coupling of the distributor drive shaft to the offtake gearset, the state of contact rocker bushings; all have been seen to be potent sources of timing "jitter" (on other engines, not this one).
Yes, a Sun machine would be very good to have or to have accessible. We are fortunate to have an electronic engine analyser - bought cheap, used, because it has black and white CRO displays, not modern multicolour solidstate displays, but none the less workable/reliable for all that! Most importantly, we have an operator who knows the machine and how to use it.
You asked about springs from the dealer - didn't try there, because of past experience of "Sorry, beyond our range ..." But with Norman's info we should be able to check whether a home-made spring gives the correct advance curve. Luckily, we have one good spring from which we should be able to get a measurement of rate, if necessary.
There is, however, one point that has emerged since I sent my last note to Norman. It's no big deal, but I would have expected the plot of advance vs rpm to be a straight line. I will probably try to get a "best-fit" straight line graph using Microsoft Excel, just to see what it looks like. Norman, when you see this, do you have any comments? Or Bill? I look forward to them.



PS The engine is gradually becoming smoother as we tie down these little worries. It's pretty close to being able to balance a 20-cent piece (10P to you) on edge - well, with only a _little_ piece of chewing gum under it ...
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Norman Geeson
Posted on Thursday, 23 August, 2001 - 07:18:   Edit PostDelete PostView Post/Check IP


First of all I believe Bill is quite correct if you can obtain a correct spring you can almost presume the advance curve is correct.As you will be aware there is no advance at idle and providing your springs eliminate any idle speed advance then any problems with idling will not be advance related.

The spring drive I mentioned is the portion of the crankshaft front gear that is spring loaded radially. The purpose of which is to keep the tooth contact even between the alloy camshaft drive wheel and the crankshaft gear. The other part of the front damper or slipper drive is another story, note that it is not called a harmonic damper. Although it carries out the same job a true harmonic is rubber based usually within the front pulley. R-R actual refered to a harmonic on the Phantom IV eight cylinder engine which unlike the six cylinder is fitted with a true harmonic damper.

What is not realised on this six cylinder engine is that the effect of the inlet valve springs is around 2.5 times the return poundage of the exhaust springs. When the engine is idling the camshaft torque reversal and hence wind up in the front spring drive is something to behold. In 1952 R-R drew up a scheme to replace the inlet springs with a single constant poundage spring for the Continental R type engine to eliminate some of the problems. The scheme was never introduced due to a few reasons.The exhaust valve springs were subsequently changed along with the bottom caps to constant pressure springs on the 4.5 engine.These exhaust springs and caps will become the standard once the current stock of 4.25 exhaust springs run out, you can bet!
Adding to this problem is the very great possibility of the actual cam riders sticking and "shuttle cocking" up and down their bores instead of lifting and closing smoothly. These cam riders are fitted to very tight tolerances and it is not unusually to find No5 inlet in particular sticking.The area around no 5 bore being subject to uneven expansion and most definately so if the cylinder block is silted up.In addition again when the engines have been stood the area at the top of the exposed cam riders, that is those particularly on "lift", is subject to corrosion and as the engine turns this corrosion is drawn into the cam rider bore.Top end water leaks are attrocious for causing sticking of the riders.Cam riders sticking and the tendency for bowing of the crankshaft on the centre main bearing takes its toll in turn on the spring drive and teeth of the alloy cam gear.
What does this have to do with ignition timing? Well if any of these problems exist or HAVE EXISTED it is possible to obtain erronous readings if the ignition is checked under running conditions. This is important if you are carrying out timing in order to eliminate a problem. Many are the occasions I have statically timed engines to find the ignition is out considerably because the dealer or owner has timed the engine under running conditions. In fact I have only just returned from the U.S.A looking at a Cloud 1 and it had exactly the same problem (along with some more).I can understand exactly why you have tried to time the engine running, but you have a problem and it can cause another misleading circumstance by timing it in running conditions.
On or two tips. Check that the carburetors are NOT fitted with SC needles which are only fitted to the H4 carburettor on the earlier MKVI nad have very often been fitted wrongly to the H6 carb with the larger intake size. DON'T attempt to set the carburettors with the vacuum windscreen wiper tube connected to the manifold.Disconnect the tube and fit a small piece of hose blanked off with a bolt.Examine the inlet manifold gaskets with a dental mirror some of these have been fitted out of position when the fitter has attempted to struggle fitting the centre water connection.Next remove the dashpots and set each jet down between 0.035 and 0.040 inch below the jet bridge, about 0.037 inch is best. Make sure the throttles are exactly equalised, the shaft has only one return spring and twists radially as the throttle closes.With the dash pots removed, slacken the throttle stop screw off then place a 0.002 inch feeler under each throttle butterfly. Screw the throttle stop up slowly and carefully making sure the feelers both come clear exactly together.If I remember correct your car should have a torque adjuster screw in the throttle road link just forward of the rear carb to adjust the windup in the throttle shaft to ease this chore.The reason for initially setting the jets down to 0.037 inch with a depth gauge is because this engine has a particularly large intake pendlum chamber and is very tolerant of having one carb weak and the other rich, in other words it tries and does to some extent compensate the inbalanced mixture. You will find on a good engine the jets will not need no more than 1/8th of a turn to be correct from this start point.If the engine is good when it is set up you can stand two 1/4 nuts up on top of each other on each carb and they will stay in position whilst the engine idles.Make sure by observing the rocker gear that the majority of the return oil is fed to the pushrod cups and not the end tip of the rocker at the valve end. Many of these engines have had rockers and bushes swapped and many of the bushes have been fitted back to front.If the majority of the oil return is not to the push rods the cam riders will stick.
The hot engine compression pressure should be about 95 lbs.Next beware of the exhaust silencer collapsing internally this will give poor idling to start with and slight overheating on the road, the next sign will be exhaust gaskets blowing.I have tried to attack your original problem with one or two thoughts.If the part regarding cam accelerations is still not clear advise. The distributors are indeed set up off the car and although I have not plotted the figures I think you will find they are not quite straight line, very few are exactly straight and after fitting a spring I think your problems will be found elsewhere.
I quite understand your crank breakage problem on the car you mentioned. However I can assure you that these cranks tend to bend when the damper is seized, normally the case.They can be straightened easilly and it is normal to have to press them no less than 1/2 inch in the opposite direction to get them straight. In other words they are so flexible that breakage is not really an option and I have yet to find one of these cranks that has been broken through a seized damper.
Advise if I have gone too far off your original problem.

Best Regards
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Jim Bettison
Posted on Thursday, 23 August, 2001 - 22:38:   Edit PostDelete PostView Post/Check IP


I am rather overcome by, and most appreciative of, the extent and quality of the assistance that I am getting - all deriving from the original query which was, we intended, a "belt-and-braces" check that what we had done to correct a fault was in line with the knowledge and experience of others. On the one hand, you and Bill are opening new and undreamed of vistas; on the other, I feel that we here can take some comfort from your comments, even if only that we can mentally check off many of the points you raise. Knowledge is never redundant.
Your point is taken about slipper drive vs harmonic damper and nomenclature. Thanks. I also take your points about the camshaft.
After your comment on exhaust springs for the 4.25 makes me feel that I should go buy a set of new ones while still available, and put them on the shelf ... but I expect that payment might be confused in magnitude with the UK National Debt ...
When we had the engine down one thing we did check was the condition of the cam riders. This was done in about 1985, and I recollect that we did do some honing and replaced (I think) three. Memory doesn't tell me which they were. Certainly, the size increments in the parts catalogue indicated just how tight the tolerances are.
We also did a substantial job of clearing the cylinder block at that time, and indications now are that we don't have a silting problem, nor any top end water leaks. (Here, in South Australia, we have the hardest mains water in Australia - and as a continent, Australia has the world's hardest mains water in the world. Australian rivers move, on an average daily basis, 2,500 tonnes of salt to the sea; and Adelaide gets most of its mains water from the largest system of all, the Murray-Darling. The innocuous-looking service station water can is - still - usually filled from the mains and is a great contributor to engine loss/reconditioning. I use only fresh rainwater or demineralised water. I have met RROC(A) members who didn't know about this ... please excuse the diversion ... waterway corrosion is a very live issue)
I expect that we will now do a static check again, followed by a strobe at idle rpm; however, the first check with a strobe light was after a static timing, and fortunately (or accidentally?) it confirmed the static setting.
Now for some of the fun (or funny?) bits ... We avoided the windscreen wiper trap - thanks. BUT - The carburettors ARE fitted with SC needles; but then, the carburettors fitted ARE H4s of 1.5" bore - notwithstanding that all the documentation we've got indicates that, in standard build for our chassis (B10JO) they should be H6s of 1.75" bore - as fitted from, I think, B251GT ... The car was also fitted with an oil bath air cleaner, presumably as standard (for Colonial conditions!) since the build cards don't indicate that an oil bath cleaner was called out as an option (which I
think was the case). Your comments on needle setting are welcome; I assume that the H4/SC rig may modify the jet set-down dimension. Your comment about the engine's tolerance towards mixture imbalance from the carburettors has already been observed. It has, however, helped us to use an exhaust gas analyser (there is a local product developed by a local electronics engineer who is also an avid trials/rally driver) which has its transducer/sensor mounted at the very top of the exhaust pipe (not manifold) and has a rapid response time in consequence. (The mounting uses a weld nut, which is closed off by a cap when not required.) There's the additional advantage that the unit cane very easily be taken into the car for road testing.
As I said earlier, we have found that the carburettor gaskets fitted in 1985, although the car wasn't really used until very recently, are decidedly suspect. There will be a complete carburettor rebuild in the next few days.
We are sure of the state of the exhaust silencers i.e., good.
The comment about the advance/rpm plot is interesting. I guess that I was not surprised by your figures, but curious.
Noted also about oil feed to the cam riders.

I am beginning to understand why people write articles about their personal restoration ...

Agai, many, many thanks - with best regards.

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Bill Vatter
Posted on Saturday, 25 August, 2001 - 08:14:   Edit PostDelete PostView Post/Check IP

These are nteresting technical comments from Norman. Thanks.

I set the timing on my Silver Wraith using the static method presceibed in the service instructions (turn engine by rear wheel with the spark plugs removed). I personally believe this is more accurate than using the strobe light on a running engine. Once I set it using the strobe light at idle, and then checked with the static method. I found the static timing was a couple of degrees retarded. Probably the engine was running fast enough to be affected by the centrifugal advance mechanism when I set it with the strobe light, or maybe the spring drive wind-up Norman describes was affecting the timing. I am not sure. Anyway, I like to follow directions when they come from a good source. The Service Bulletin that instructs how to synchronize the points using the #6 cylinder is also very good.

Your discussion about even running is also interesting. Several years ago when I acquired this Silver Wraith, it suffered from an irregular miss at idle. This was quite disconcerting as smoothness is one of the things we like to be proud of with these cars. I eventually found that in several places there were high resistance junctions in the secondary ignition wiring, which I corrected, and I finally replaced the risistance distributor rotor with an aftermarket non-resistor rotor. This resulted in a stronger spark that eliminated the miss. If the points are not properly synchronized, the engine also will not run smoothly.

Incidentally, I called the US RR dealer that stocks spares for the older cars, (Alber's RR) and he does not show a part number for the advance springs, implying they are not available from RR. I have a spare Silver Wraith distributor, but of cource I am not ready to strip it for internal parts, and I expect others might feel the same. However, Tony Handler in California might be persuaided to part with a couple of used springs. He has probably got more distributors than he will ever sell whole.

For equalizing SU carburettors, I use a venturi flow meter to have the carburettors both drawing the same air flow at idle. Regarding setting the needles, some of my sporty-car friends use a device called "color tune." This is a special spark plug with a quartz window that allows you to see the color of the flame (yellow= rich to blue= lean), but I have good results listening to the exhaust note while driving. I just lean it out to smooth the loping sound present when a rich-running engine is very lightly loaded. When the engine gives a smooth steady note, that is good for me. However, I will admit this sound method works better on an MG than on a Rolls-Royce, where hardly anything (in proper condition) can be heard over the wind. (On an MG it is almost impossible to hear the wind over the noise collectively coming from every mechanical part of the car.)
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Norman Geeson
Posted on Friday, 31 August, 2001 - 22:37:   Edit PostDelete PostView Post/Check IP


Your car would definately have been fitted with H6 Carbs with a four bolt fixing inlet manifold to the carbs.This change to H6's took place at chassis B83HP.In the case of either H4 or H6 carbs WITH an oil bath air cleaner a SF needle would have been fitted.Both SC and SJ needles are the "odd" needles of all the different needles fitted to these cars as the idle no 1 position on the taper is exactly the same size as the jet at 0.100 inch. All the other needles start at number 1 position around 0.099 inch.It is possible that someone has either changed the manifold and carbs possibly with the cylinder head or indeed fitted an earlier engine.If they have just changed the manifold and carbs on the original engine it is just possible that the low lift camshaft lobes on your late engine are altering the manifold depression just enough to interfere with good idling on the smaller H4's.On a normal installation one has to take extreme care to finally set the carbs with the air cleaner attached, on an installation with either SJ or SC needles it is vital so that the depression is such as to lift the dashpot and needle sufficient to move the needle higher than with a wire mesh cleaner fitted. In fact fitting a SC needle to H6's with a wire mesh cleaner really proves the point they are impossible to tune for idle and lowering the jet does not and will not compensate as many specialists have found to their cost.In short you may not attain perfect idling unless you fit H6's with the appropriate manifold.The H4 installation with a oil bath air cleaner is very restrictive and the outside cylinders No1 and No6 run weak and at idle with your set up this is one of the things that is difficult to overcome without having the inner cylinders too rich, an analyser picking up only the average condition across the cylinders. Bill mentioned the rotor arm and in addition to this you may note the coils suffer badly from overheating changing both very often makes a considerable difference and particularly to the idle.

If the records of your car indicate "for used in Australia" it will certainly have been fitted with an oil bath air cleaner without this having been deatiled on the chassis card seperately.The fitment of colonial front springs and High speed water pump and special fan diameter of 17 inch would generally be listed seperately.In the latter case the modification involving the smaller fan pulley of 1.11:1 ratio and smaller fan,a Swiss specification, would have been requested by the importer.

Another important point to watch is that the choke lever on the steering column has push the carb jets right up when it is three notches from the end of its off position. If the lever is then closed off the last three notch movement to the fully off position will spring load the carb jets upwards and off completely through the linkage.This is important on MkVI because there is no positive spring return on the front carb jet and more than often these cars are running with the front jet partially down. Even a few thousandths of an inch will disturb your slow running.

Anyway keep us informed how you progress.
Best Regards

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Norman Geeson
Posted on Sunday, 02 September, 2001 - 04:54:   Edit PostDelete PostView Post/Check IP


I did miss out one or two points as the subject has drifted off "ignition timing" slightly.The valve springs for the 4.25ltr are unlikely to run into a short supply situation.I suggested that R-R would only recommend the fitting of the constant rate later 4.5 ltr exhaust springs together with matching bottom caps when and if the current large stock of 4.25ltr runs out.

The mechanic was quite correct in suggesting that if the distributor bushes are worn you do not have a chance of obtaining good idling.

In addition I forget to advise you when using the static timing method it is MOST important to hold the distributor rotor in the full retard position. This is best achieved by fitting an elastic or rubber band around the rotor and fixing the other end to the side panel or other fixed point.As the timing progresses to say No 6 cylinder the band will need removing and refitting again or the rotor may get damaged as the engine is turned.

In regards to slightly rough idling, this will always be present if the compression ratio has been raised on a 4.25ltr by say skimming the cylinder head. This does not apply to the 4.5 ltr.A check can be easilly made of whether the head or block have been skimmed on the 4.25 ltr by checking the hot engine compression pressures. In standard form the pressures will be around 95 lbs at best and even in the maximum cylinder bored out state they should not exceed 100 lbs.If they are above these figures you will know some skimming has taken place and absolute perfect idling will not be attainable.These figures are given for testing an engine with fully charged battery and all plugs removed to obtain maximum cranking speed on the starter.R-R knew at the time of production that the 4.25 ltr engine would not respond in power output in proportion to the raise in compression ratio and produced rougher idling.For this reason the 4.25 ltr compression was never raised,neither does it respond particularly well if a twin exhaust is fitted.The reason subsequently found when the 4.8 ltr was introduced was the restriction in the exhaust porting in the head area.

If you are still having problems locating a advance spring contact me and I will see if I can pick one up.BIll appears to be correct in stating that the springs are not normally available new as they do not appear to be listed seperately.

Sorry we have drifted off the subject somewhat but I hope you may be able to pick out some points to at least know what is causing any rough idling.

Best Regards

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Bill Vatter
Posted on Monday, 03 September, 2001 - 09:17:   Edit PostDelete PostView Post/Check IP

The compression pressures may well be above 100 psi even though the cylinder head is unaltered if there is significant carbon deposit in the combustion chanbers.

This common effect makes the absolute value of the compression pressure a relatively unreliable indication of engine condition. Rather one should consider the variation in pressures between vylinders where a difference of 10 psi from the average is usually condidered an indicator something is wrong in the low cylinder.
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Jim Bettison
Posted on Monday, 03 September, 2001 - 13:04:   Edit PostDelete PostView Post/Check IP

Norman and Bill (and anyone else following this saga ...)

In a short e-mail to you, Norman, I said that we're in onion-peeling mode; but at least with the onion one knows when one can't go any further. Both the MkVI activity and the onion can bring tears to the eyes (well, not really) ... it's my practice to spend at least one day a week with my co-worker and friend on our cars (there's a 20/25 next in the queue for attention) and it sometimes makes one wonder whether the purpose of the other days is to pay for that one ...
This subject has certainly drifted from the original title. The advance spring for the distributor which was the starting point of this journey seems possibly to have been sorted out. Norman, if you can without trouble or inconvenience find one I'd be grateful. It's my present intention to take the good spring and the broken one to our local friendly springmaker and get him to replicate the good one. Has the advantage that it's a non-destructive way to go; if he can't do a replica, then the search for a new spring (which I suspect might be more productive through Delco-Remy than through RR-B) will be stepped up. However, using the sick spring, we have run the engine up to about 960 engine rpm, for an indicated 6 deg of distributor advance, which I feel indicates that for the present, and until we can replace that spring, we might assume that ignition timing is acceptable. A couple of comments: quite coincidentally, I have located a distributor timing machine. Don't yet know the make. In my distributor, the original rotor with its resistor failed, and I replaced it with another Delco-Remy of correct dimensions - but no resistor. The HT wiring was suspect, so we replaced it (haven't yet had a chance to look under the bonnet on a good dark night). Plugs are new.

At an earlier stage of this correspondence, I indicated that we intended to rebuild the carburettors with new gaskets, etc., because we had found some fuel dampness where it shouldn't be ... Well, we did that last week. The scene of action has consequently shifted, as you commented, Norman! You will appreciate that , due to our server being down, we didn't have your last two e-mails, or Bill's last one.

We set up the H4s with SF needles. We had also been worried that the front carb didn't have a positive return action, so we instituted a modification by brazing an extension on the return lever (making it look as neat as we could, I might add) and fitting another return spring ex the SU "waifs and strays" box - so each jet is now spring-returned. It hasn't noticeably increased the load on the control on the steering column, and we felt that it did contribute to ensuring even running.

With the SF needles, we could achieve a good idling mixture. However, as soon as we got a few revs up (especially with the oil bath cleaner in the circuit) the fuel/air mixture went terribly rich. At that stage, I went off and ordered a pair of SG needles, due to arrive tomorrow. In the light of what you've said, Norman, I'm not now expecting any miracles. As you'll see, the SG has the same idle position diameter, but is then a "leaner" needle. It will be at least interesting to see what happens.

Norman, I think that you may have put your finger on the real problem (in an "onion-peeling" way). I think that much comes clear when one accepts that the original inlet manifold and H6 carbs (at least) have been replaced by manifold and H4 carbs! The "at least" proviso is because I wonder whether the head has also been replaced! And, to the point of your last e-mail, has it been skimmed? And how much? I had the local agents do a valve grind about a year after I bought the car (that would be about 1966) and I remember being told that the head had been skimmed by .050.
Obviously, we'll do a compression test as a way of roughly diagnosing what might be the case (your point is taken, Bill, about looking for variations rather than absolute pressures. But I remember hearing of one fellow who had a problem, and his pressures were all about the same at 20 psi ...). Norman, do you feel that a restoration to H6s and manifold would have any real benefit if we do have a skimmed head situation? Or should I just say "Well, we've got a car that won't idle as smoothly as it might, but runs well in all other ways"?

There are lots of unanswered (unanswerable?) questions. Why would anyone have changed over the carbs? And who did it? And what's the history about the head? One thing is sure; the engine wasn't changed. I have the build card which shows the present engine no. (B10J) as original. I know that the car went to Kellow Falkiner in Melbourne for sale to a Mrs Thompson of "Corea", Dunkeld, Victoria. I believe that Kellows serviced the car while she had it - see later. She was widowed in the early 60's, and moved to Adelaide where she traded it in on something else. I bought it in 1965 for 1,100 pounds (Aust) and drove it daily until 1974 when I took it off the road following a minor prang. One thing led to another and we started on what became a complete strip to bare metal and engine/gearbox/diff etc followed by rebuild; it has come back on the road this year (Any trophies for time in restoration?). On the way through we had one divorce and three house moves ... and a modicum of business success. Anyway: I am the second owner (with my partner) and I thought I'd found all the dark corners ...
We took about 28lb of lkead out of the nearside rear where Kellow's had done a repair after there driver "lost it" on a gravel road near Dunkeld when bringing the car back from a servicing in Melbourne (about 250 miles)

This has trun away a bit ... Any comments, Norman or Bill, or anyone else?
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Jim Bettison
Posted on Wednesday, 05 September, 2001 - 10:51:   Edit PostDelete PostView Post/Check IP

Norman and Bill,
Somewhere in the internet there are two documents which are correctional edits to my last posting of Monday 3 September. They disappeared after i hit the "post message" button. I'll try to remember them, because they were intended to make that posting more comprehensible.
First, the subject has drifted from ignition timimg to tuning a MkVI. Finding out what is successively revealed is the onion peeling (but one does know when the onion is all peeled).
Second, we will do a distributor test on the timing machine very shortly; also run a compression test.
Thirdly: another observation. After the carbs were reinstalled (with no leaks detectable) we ran a manifold vacuum check (actually from the w/screen washer pipe) and at idle of 600rpm with carbs balanced the manifold was showing 17" water gauge; experience with other engines would have led me to expect around 20 to 21" water. Any comment?
Fourth: comments about return action referred to jet return when the mixture control is taken to "run", as I expect you adduced.
Fifth: I've had a chance to talk to the man who was workshop foreman of the service agent when they dis a valve grind for me in around 1966/67. Although he doesn't remember the car, he says that their usual practice was to take a skim of .010, We'll see.
Sixth: In light of your comments, Norman, we will try exchanging the coil. I have a Bosch Sports Coil with ballast resistor - a combination that has worked well in a Mk2 3.8 Jaguar. Incidentally, the original Lucas coils are still in place, with 1950 date stamps on them.
Seventh: my ramble on about Kellows included some comments about anomalies that we found with the car; like, all tools (except the tyre valve tool) present and hardly used;bad and untended cracks in the chassis; lubrication system, around the kingpins especially a disaster area; a rough spray (or more accurately overspray) in the boot area - leads into my comment about lead removed. All leads to making one suspect that no-one (particularly service agents!) should be trusted (except us, of course!)
Culminates in trying to understand why anyone would want to change manifold and carbs. And did the do manifold, carbs and head as an assembly. And which camshaft is fitted - I don't feel particularly inclined to do a strip-down to locate the part no!
Comments invited!
Very best regards

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Norman Geeson
Posted on Thursday, 06 September, 2001 - 07:41:   Edit PostDelete PostView Post/Check IP


First the good news,I have a secondhand advance spring being sent to me, let me have your address by normal e-mail and I will post it, no charge!

I notice you checked the vacuum at 600 prm idle speed. The engine should idle perfectly at 350 / 375 rpm and in fact a smooth engine can be taken down to less than 225rpm just for test purposes. You are quite correct that you will be unable to attain a vacuum of 20 / 21 at idle, there are many reasons for this, but you will not get a high vacuum at this high engine idling speed.This particular engine has no valve overlap, the valve timing is best described as "retarded" and tuned for very low torque output and in addition the ignition timing is also retarded somewhat.
The high idle speed concerns me as the engine should idle at much lower speed.I am sure a 1950's coil will have gone by its sell by date years ago under the high temperature bonnet conditions of these cars. I would hesitate to fit a ballast resistor coil.

Probably I have assumed too much, but have you set the ignition advance at 2 B.T.D C. as it should be set or have you used the outdated handbook setting?

A 0.010 skim should not be a problem, but sometimes people skim and skim without realising what has gone before.

Unless you know for absolute certain do not presume the engine is original because the number matches.Many of these engines were changed early in their lives and R-R for example always renumbered the engines back to the original number.They would not however have fitted the smaller carbs.It is hard to say why this occurred many ideas spring to mind.Someone may have had an accident with the car and damaged the carbs and not wanting to wait for new carbs may have elected to fit the older type. The head may have been damaged on removal at some time when having a top overhaul, maybe someone decided to fit a complete top end off another car.

R-R fitted both resistor type and straight types of rotor arms especially towards the end of production. In fact the resistor type was used with a 0.030 plug gap whilst the normal type was used with 0.025 plug gap. Yes, I know that is not in the handbook or service manual, neither is the fact that they should have EP80 oil in the gearbox and chassis lube.

I noticed you had the car on the road some 9 years, how did the idle perform during that time?
I believe you also said you had overhauled the engine, have you thought of rechecking the valve timing?
It is difficult to diagnose from afar but if the car has and does perform to your satisfaction I would probably elect to leave the carbs and manifold alone but keep a look out for such an assembly coming onto the market at the right price.
You may find it worthwhile temporarilly filling the dash pots with very heavy oil say EP80 just for testing the idle.I would suspect the inlet vacuum is relatively high and may be causing carb piston flutter at idling. Heavy oil will damp any flutter a little at idle and you may get an idea if it makes any difference to the idle.
If you have no success I would conduct a full engine check, compressions, air pressure check in the cylinders and a cylinder running drop check.The last is achieved by fitting a RPM counter and shorting out each plug in turn, the engine revolutions should drop evenly and the same amount for each cylinder.Any lower RPM drop on a cylinder would indicate problems on that cylinder.

Two other shots, on a number of occasions I have had perfectly good plugs fail when the throttle is opened and they have partially fired at idle speed.This has in particular happened after otherwise brand new plugs have been stood in an engine for some time.

Next,although the S.U's are very tolerant of fuel levels have you checked that one of the float needle valves is not sticking, I had this once on a 4.5 ltr and it took some finding, the needle would stick at idle and would unstick when the engine was rpm was raised.It is better in my view to fit spring loaded needles in these carb float chambers and it is not unusual to find each chamber with different needle valves, and don't use open ended spanners for changing the seat!
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Jim Bettison
Posted on Thursday, 06 September, 2001 - 23:33:   Edit PostDelete PostView Post/Check IP

Norman and Bill,
Norman, thankfully your last posting arrived before we spent the day (Thursday, Oz time) on the car.

Also, thanks greatly for locating a Delco-Remy spring. I will e-mail address, etc, direct to you.

For the record, this is what we did today - and the apparent outcomes. First, while the engine was hot from the run from my garage/workshop to Danny's workshop (he's my technically proficient restorer/specialist etc - perhaps more about him in another context), about 20 miles away, we did the cylinder compressions. From No1, they were: 110psi, 110psi, 110psi, 110psi, 114psi and 104psi. I must say that I still have a recollection of being told that the head had been shaved/milled down by .050" "because it was so corroded".
Then we replaced the carb needles substituting SGs for SFs. I think that was an effective move. There were some problems with adjusting and synchronising, of which more later. At the end of the day we took the car for a short (6 - 8 mile) run with the exhaust analyser connected. We had uphill, downhill and level terrain, with allowable speeds up to 50mph. The car performed impeccably; and, most interestingly, the air/fuel mixture whether idling, pulling uphill, running downhill or coasting remained in the range 14.0 to 14.7. (
This is a performance that we would expect from a modern fuel-injected motor which, I understand, is aimed to run at 14.3.) I drove home later; the road has about 8 miles of a gradient (average) uphill of 1:7, and the car was pulling ahead on all parts of the uphill sector ... I've finished the day with the feeling that we had a "win" in that area. But there are some outstanding questions, and some penalties ... see later.
Next area of attention was the coil. You were right, Norman; the old coils have now been museumed (if that's a word) and the Bosch replacement runs cooler (with, appropriately, a lower power consumption). I suspect, Norman, that your reservation about a resistively ballasted coil is the likelihood of electrical stress on the distributor EHT components. In our case, the dist cap and rotor are virtually new, and I take some care to keep bakelite components free of dust - the precursor of a "track" in my experience. However, this change also produced an improvement. Engine note became more pleasant (if that's an objective term!) and idling was perceptively better.
Finally, we put the distributor on the timing machine (it still has the scrap-repaired spring that will be replaced with Norman's "find"). The plot of advance vs engine rpm was a good straight line "fit" from zero advance at just under 500rpm to 17.5 deg at 3500rpm, except that the amount of advance was less, by just under 3 deg, than that given by Norman in his posting back on 19 August. Yes, we had taken the ignition advance at 2 deg BTDC. In the light of this discovery, we have advanced ignition by a further 2 deg. The engine seems to like it.
We plan to run the car in this configuration for a while and see how it settles down; the only chsnge planned is to replace the Delco-Remy spring.
At the end of all this we did an exercise by disregarding all the instrumentation, and setting up carbs (mixture, not balance) and timing to where the engine was most comfortable. This came out to be effectively the same as previously. So we now have a car which performs much better than before (and better than I remember it ever being in the 1965-74 period), and has good engine on-road characteristics.
BUT ...
We have found some problems that still need resolution. Idling performance - particularly engine smoothness - is not really acceptable. The engine, when idling at around 450rpm, is very "lumpy". No way could two nuts stay on top of the carbs!
Our problems now are mostly centred on fuel and carburetion. First, we had great difficulty in balancing the carbs (as you predicted, Norman) and evrn now they're not totally right. The engine is much less responsive to mixture adjustment at the front carb than the rear one. We had, last week, replaced the needle valves and set the float levels (to nominal 7/16" - actually 11mm). However, to get some semblance of order with the front carb, we set the float down a further 1mm, which seems to have done the trick (i.e., establish similar response in both carbs). Leaves a question, however; Are there two "wrongs" concealing a "right"?
It seems that we are confirming your earlier advice, Norman - H4s and high compression don't mix. Question: would H6s (and appropriate manifolding) do significantly better on this engine?
In this context, one plan of action might bew to acquire the relevant manifold/carb assembly and instal it to the present head. At least it should be no worse. If we felt that we wanted even better performance, I guess that we could then look for a better head ...
We have replaced the light oil in the dashpots with SAE30 and, judging by performance and instrumentation, that seems to have been a good move.
One other thing; we now achieve an inlet manifold vacuum of about 18 to 18.5 mmHg.
I do feel confident that the engine number is original - even if just because that tampering with engine numbers was virtually a capital crime her until the last few years - engine nos used to appear in registration discs. I do agree with Norman that it seems that a complete top end may have been substituted; possibly because of extreme waterway corrosion of the original.

Thanks again. Your comments awaited with interest.

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Norman Geeson
Posted on Saturday, 08 September, 2001 - 07:23:   Edit PostDelete PostView Post/Check IP


I note from the compressions that the engine is down on No 6 cylinder and having regard for a ten percent tolerance I still think further checks may be appropiate.As you may be aware these engines are notorious for bore distortion on the rear cylinders causing piston damage.I haver known many R-R engines (dozens) pass a compression test and show up problems on the air pressure test outlined below.Whilst rear piston rings are intact, but a split skirt piston has distorted under liner cramping so much as to reduce the piston split to zero it is possible to obtain quite good compression readings. An air pressure test will however show up the problem and any others.

I would carry out a cylinder air pressure test, it will tell you much about the cylinders and valves.Rig up an airline adapter to an old spark plug.Turn the engine to EXACTLY the TDC firing condition on No1 cylinder and lock up the engine flywheel. Screw in the adapter to the plug hole on No1 cylinder and couple up an air line to the adapter,use a spare tyre inflated to about 60 lbs if you do not have an air compressor to provide the air.It is possible to use 120 lbs off the compressor, but keep out of the way of the fan etc as the engine will rotate rapidly if it is not locked up or is off T.D.C.Try to use a tap in the air line and apply the air pressure steadily.Get everything ready and warm up the engine first.

In T.D.C firing condition both valves will be closed. Next remove the radiator cap,the oil filler cap and the air cleaner connection.Then apply the air.

Any leakage down the exhaust indicates a leaking exhaust valve, any leakage into the inlet manifold indicates a leaking inlet valve.Excess leakage into the crankcase heard from the oil filler indicates piston / liner problems.You will need to check all the cylinder in turn to get some idea of piston blow by, expect more blow by on No 6 cylinder.Any bubbles in the radiator indicates a leaking head gasket.

I have presumed you have set all tappet clearances, made absolutely sure that both exhaust silencers are clear through and checked the valve timing.The engines will respond a little to excess ignition advance depending on the fuel,but it is possible to get detination without pinking.I normally run one at no more than 3 degs advance.

H6 carbs should perform much better but I should onion peel to determine if any other problem exists.I also presume that you have done all your testing with the windscreen washer pipe still disconnected!

Remove the dash pots turn on the ignition and view the fuel level by looking straight down the jets. Both jet fuel levels should be equal and only about 1/16th to 1/8 down from the jet bridge.

Less response from the front carb mixture indicates normally a leaking windscreen washer tube if its connected.It does however get confusing if the piston on No 6 cylinder is on its way out.Make sure you have carried out all the checks suggested before including looking around each inlet to cylinder head joint with a small mirror, the gaskets are inclined to be out of position.The excess ignition advance will not help smooth idling!If everything seemed O.K on this engine I would still suspect that the carb throttles (and obviously mixture) are not sychronised.I note you have fitted a SG needle which was never listed for any of these engines is there a reason why you chose this needle? It has particularly weak characteristics and the mixture strength is too weak as you recorded right across the range.

The only other thing I can think of from afar is that the ignition condenser may be acting up on you.

The main point is to ensure nothing is wrong before opting for H6 carbs.I know the performance will improve, surprisingly so will the cooling but check off first.

Keep on peeling onions!



Norman Geeson