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Mark Taxis
Prolific User
Username: mark_taxis

Post Number: 115
Registered: 4-2007
Posted on Friday, 20 January, 2017 - 09:35 am:   Edit PostDelete PostView Post/Check IP

My Mk6 special has been converted to a single point system using a 6 lobe cam,
I have recently bought a multimeter that can measure dwell angle, what would be an acceptable dwell angle range?
I have read that a 6 cylinder engine should have a dwell angle between 35 - 37 degrees - Would this be correct? I have measure the current dwell angle and I am getting 58 degrees at 550 rpm, decreasing to 52 degrees at 1500 rpm .
This would indicate that I need to reset the points,
Any info on this subject would be well received
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Geoff Wootton
Grand Master
Username: dounraey

Post Number: 1560
Registered: 5-2012
Posted on Friday, 20 January, 2017 - 10:21 am:   Edit PostDelete PostView Post/Check IP

Hi Mark

58 is really high. For a full rotation of the distributor arm that's a total of 348 degrees the points are closed. Is your coil running hot? Too long a dwell angle can burn coils out so I would reduce it anyway. The figure I have read is 36 - 40. 40 would be safer than 58 so I would set it at that whilst you get definitive information for the optimum dwell angle for your engine.

Geoff
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Mark Taxis
Prolific User
Username: mark_taxis

Post Number: 116
Registered: 4-2007
Posted on Friday, 20 January, 2017 - 01:10 pm:   Edit PostDelete PostView Post/Check IP

Hi Geoff
Yes the coil was running very hot.
I could not get the points set properly so I invested in a $45 Auto multimeter, now I can read the dwell angle and the RPM.
This proved that the points gap were wrong and also that the rev counter in the car is wildly inaccurate.
I have set the dwell angle to 36 which I think is nearer the right setting.Also by reducing the dwell angle I now have a steady reading accross all revs.
Regards
Mark
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Geoff Wootton
Grand Master
Username: dounraey

Post Number: 1561
Registered: 5-2012
Posted on Friday, 20 January, 2017 - 01:48 pm:   Edit PostDelete PostView Post/Check IP

Hi Mark

Great news. I agree that measuring dwell angle is a much better way of setting the points than measuring the gap with a feeler gauge. The meter is definitely worth the investment.

Geoff
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1098
Registered: 9-2004
Posted on Saturday, 21 January, 2017 - 07:50 pm:   Edit PostDelete PostView Post/Check IP

Hi Mark pleased that you have gone down the dwell checking and setting contact way.
Lets not forget the dwell varriation:
The dwell is GOOD if changes are less than specified varriations.
The indications of BAD dwell are changes beyond specified varriations.
In most cases this will be mechanical ie worn bearings, bent dist shaft, loose contact plate.
If dwell changes abruptly then contact float maybe the cause, weak spring or binding in the spring pivot.
The fault will cause the contacts to remain open and reduce the dwell time and lower the voltage the coil can produce.

IMO the dwell should be set at 38-40.

Setting the timming after the dwell is carried out with the advance-retard set fully retarded and the the timming set at TDC.
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Mark Taxis
Prolific User
Username: mark_taxis

Post Number: 117
Registered: 4-2007
Posted on Saturday, 21 January, 2017 - 09:54 pm:   Edit PostDelete PostView Post/Check IP

The timing has been a bugbear.
Originally I used a timing light, but the engine just did not sound right when it was supposedly correctly timed with the light.
I can only assume that the timing marks are wrong or that the cam timing has been disturbed during a rebuild (this is all a bit like magic to me)
I have since resorted to static timing at TDC using a bulb, car runs well.
I am pleased that I have finally got a grasp on the Dwell angles and the meter certainly makes setting the points an easier task, but more importantly I know that the gap setting is correct
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Norman Geeson
Unregistered guest
Posted From: 81.97.73.87
Posted on Sunday, 22 January, 2017 - 08:51 pm:   Edit PostDelete PostView Post/Check IP

Mark

The dwell angle for six cylinder Delco Remy UK made distributers is 35 to 37 degrees. Quite whether this requires adjustment given that you are now using single points instead of dual contacts I am not sure.

As you have found out these engines often respond better to static timing with a bulb, and with modern fuels about 4 BTDC. That is given that the advance curve is correct and the crankshaft damper is operating correctly.

Reading up on some earlier issues you have experienced it seems as though you need next to set the carburettors up.

I am aware from reading previous posts that the reason for the float chamber to jet well pipes is not understood by anyone. There is no wonder owners experience problems when we have specialists at large trusted to build and tune these engines, yet possessing no clues as to such simple systems.(Rant over)

That reason is not such an issue as realising that these pipes make a 7% variation in fueling at certain RPM/Load, and yet I understand fitted, or removed, they make no difference to your engine.

Either the pipes have to be removed, and the ADAPTER in the jet well blanked (not the CARB BODY)or they must both be clear and free from obstruction and fitted correctly.

(Message approved by david_gore)
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Mark Taxis
Prolific User
Username: mark_taxis

Post Number: 118
Registered: 4-2007
Posted on Monday, 23 January, 2017 - 04:10 pm:   Edit PostDelete PostView Post/Check IP

Hi Norman
Thank you for the information.
I have been trying to find out what the pipe from the bowl to the carb does, with absolutely no luck, nobody I have spoken to has any idea as to why they are there, can you point me towards an article or explain in simple terms exactly what this vent pipe does?
One of the pipes has broken (me being clumsy) so now I need to either repair this or blank them both off.
I would be most interested to hear what their function is, I assumed that they were some sort of breather pipe to change the vacuun pressure in thd carb, but in all honesty I reallly am just guessing.
Best regards
Mark
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1099
Registered: 9-2004
Posted on Monday, 23 January, 2017 - 10:06 pm:   Edit PostDelete PostView Post/Check IP

And I have always thought the main use of the system was for the variations of atmospheric pressures.
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Norman Geeson
Unregistered guest
Posted From: 81.97.73.87
Posted on Tuesday, 24 January, 2017 - 06:11 am:   Edit PostDelete PostView Post/Check IP

Mark

As you have found the particular float chamber to jet well pipe on EPW six cylinder engines cannot be found on any other engine. The arrangement is unique to the S.U .thermo H series four bolt carburettors used on the four port intake manifold.
These carburettors were first used on the early 1949 MKVI 4.5 Ltr (and subsequent 4.25 ltr) engines. A problem arose with flat spotting when accelerating during trials. Investigation found that if a small bore pipe was linked between the float chamber top and the jet well this would emulsify fuel in the jet well before it egressed from the main jet and this fix eliminated the problem.
During investigation into issues with the late R Type engines it was found that temporary removal and blanking of the jet well adapter still prevented the former flat spotting. Still further trials found that the original problem had been caused because the volume of the original jet well was too small. The jet well volume had been increased by the casting in the boss for the pipe adapter. A small number of R Type Continental engines had the pipes removed to richen the mixture and dampen the exhaust burble on overrun. If the pipes are removed some extra care may be needed in selecting the main needles as in certain conditions the mixture is altered by 7%.
The adaptors, which are screwed into the carburettors, are also fine jets and you would be advised not to try to remove them from the alloy carburettor bodies.
The pipes are nothing to do with chamber filling, chamber breathing, and deliberate weakening or vapour venting. Parts book descriptions apart they could be more accurately described as air injectors, or jet well emulsifiers.
I trust this satisfies your curiosity?

(Message approved by david_gore)
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Mark Taxis
Prolific User
Username: mark_taxis

Post Number: 119
Registered: 4-2007
Posted on Friday, 27 January, 2017 - 04:36 pm:   Edit PostDelete PostView Post/Check IP

Many thanks Norman
I really apreciate your time to explain this to me, if you do not mind I will pass this information on to a few friends who are also baffled by this.
Again many thanks.
regards
Mark
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James Senior
New User
Username: jamess

Post Number: 5
Registered: 9-2016
Posted on Sunday, 05 February, 2017 - 11:03 pm:   Edit PostDelete PostView Post/Check IP

Hi everyone,
I also recently bought a meter which can measure dwell angle (although I wanted it for the rpm). If you have the original 3-lobe cam distributor with two sets of contacts, I imagine that the dwell meter will give you the average of the two contact sets, which potentially could be noisy if they're set different?

Any tips on how one might isolate each of the contacts during measurement? If not I guess the approach is to initially set them with a feeler gauge, then try the dwell meter and if you get a noisy reading, proceed with trial and error adjusting one in and out until the reading is stable?
Regards,
James
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1107
Registered: 9-2004
Posted on Tuesday, 07 February, 2017 - 07:55 am:   Edit PostDelete PostView Post/Check IP

Hello James, first set the contacts to the required gap with feeler gauges.
After the setting the gaps block off one set of contacts with a card and adjust dwell, you can then adjust the dwell on the other set swopping the card.

Run engine to check for total dwell and any variation outside the specified limits.
If ok then set the timing.

You could make a meal of it and remove the distributor and carry out the above on a distributor testing machine however if its not broke don't mend it!
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James Senior
New User
Username: jamess

Post Number: 7
Registered: 9-2016
Posted on Wednesday, 08 February, 2017 - 04:23 am:   Edit PostDelete PostView Post/Check IP

Thanks Patrick.
So if a dwell meter measures the period in which the contacts are closed, if I then hold one set open with card and leave the meter on the '6 cylinder setting', should I then be targeting a reading of 19 instead of 38? Is there any chance of unburnt fuel from the three cylinders without a spark from combusting in the exhaust?

Another question regarding setting the timing using a light (when stationary). The handbook says set to TDC or 2 degrees BTDC for premium fuels. Norman suggests 4 degrees which I shall accept as premium fuels today are presumably more premium than in 1949! Now if your flywheel only has TDC marked on it, what's the best way to determine 4 degrees TDC? With the car in 4th gear (1:1 transmission), turning the engine via a jacked up rear wheel, I could determine 4 degrees using the wheel. Is there a more obvious way I haven't thought of!
Thanks,
James
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Norman Geeson
Unregistered guest
Posted From: 81.97.73.87
Posted on Wednesday, 08 February, 2017 - 06:55 am:   Edit PostDelete PostView Post/Check IP

James

".......what's the best way to determine 4 degrees TDC? With the car in 4th gear....."

Use a flywheel tooth as a reference. I cannot remember the number of teeth on a manual MKVI, but using my poorer memory each tooth on a Bentley R Type Automatic is about 3 degrees.

Only move this engine in its correct rotation, and from the flywheel end. Also put a rubber band on the rotor arm to pull it against rotation, just before rotating the engine up to the firing point. This will hold the rotor and auto advance in the full retard position.

(Message approved by david_gore)
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1108
Registered: 9-2004
Posted on Wednesday, 08 February, 2017 - 07:04 pm:   Edit PostDelete PostView Post/Check IP

"Is there a more obvious way I haven't thought of!"

Yes and more simpler and precise.
Use a timing [strobe] lght, the type that can dial in required degrees before TDC and afterTDC
Set to the rquired needed degrees and align the TDC mark on the flywheel. RPM 500.
Job done.
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Geoff Wootton
Grand Master
Username: dounraey

Post Number: 1591
Registered: 5-2012
Posted on Thursday, 09 February, 2017 - 02:47 am:   Edit PostDelete PostView Post/Check IP

I agree with Patrick on this. I suspect a lot of people are not aware of this facility on modern timing guns. One of my best tool investments was a good quality timing gun. It cost $120 and is so much better and more accurate than the $30 ones. The ability to dial in TDC offset would be really useful in this instance. In Mark's case, where he suspects the timing marks are incorrectly placed due to a previous rebuild of the engine, it would be just a case of finding TDC statically, marking the position and then setting the gun to 4 degrees BTDC.

Geoff
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James Senior
New User
Username: jamess

Post Number: 8
Registered: 9-2016
Posted on Thursday, 09 February, 2017 - 03:57 am:   Edit PostDelete PostView Post/Check IP

Hi,
I think Norman suggested 4 BTDC when doing a static adjustment. If using a timing gun at 500rpm, what would we target then?

Back to an earlier question and suggestion, I like the simplicity of holding one set of contacts open whilst measuring the dwell on the other, but I'm a little concerned that doing this in the engine (cutting the spark to half the cylinders whilst running) might cause too erratic running and possibly backfiring. I have this concern in my mind because I think you're normally advised to limit running when doing a power balance test (in which you only cut one cylinder at a time - not three). Does anyone have any thoughts on this? Is it a good idea to do this adjustment in the car with the engine running or not?
What a great forum!
James
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Mark Aldridge
Grand Master
Username: mark_aldridge

Post Number: 396
Registered: 10-2008
Posted on Thursday, 09 February, 2017 - 05:08 am:   Edit PostDelete PostView Post/Check IP

Having got fed up with synchronising twin points on my S1, I fitted an electronic conversion.
http://www.classicheads.com/ .Not much more expensive than 2 sets of points. Have had this conversion for 12 years and the advantage is once set up, no adjustment required.. Have also changed the Dynamo for a Lucas ACR alternator
Mark.
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1109
Registered: 9-2004
Posted on Thursday, 09 February, 2017 - 09:21 am:   Edit PostDelete PostView Post/Check IP

James 2 degrees @ 500rpm.

The dwell is taken on all lobes of the distributor cam with one set of contacts blocked.
the dwell is carried out by cranking the engine on the starter motor.

Power balance is after all is ok with engine running on idle, removing one ht lead at a time watching for any variation in RPM.

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