| Author | Message | ||
     Mark Taxis Experienced User Username: mark_taxis Post Number: 14 Registered: 4-2007 |
Hi My Mk6 special has an uneven idle, I have just had the caburettors completely overhauled and reset so I am eliminating these as the source of the problem. I am now looking at the possibility that there is some problem in the ignition. The spark plugs were replaced about 2000 miles ago - so I am assuming that these are ok (I have checked the gap and cleaned them) I have checked and reset the points. The only thing left now is the coil (which is about 20 years old) and the condensor. Is there anyway that I can test these items or is it better just to renew them. If renewal is the way to go how do I make sure that I am buying the correct items for this car. I see there are all sorts of different coils on the market, but I do not know which would be most suitable for the MK 6 engine Regards Mark Taxis | ||
| Martin Cutler Prolific User Username: martin_cutler Post Number: 117 Registered: 7-2007 |
Hi Mark, Change the condensor and coil, fairly cheap to do. Bosch GT40 coil works OK. You will know when the coil fails, the car won't start. I doubt either of these things would cause an uneven idle. Have you done a compression test? Are your valve clearances OK? I would check the valve clearances. The exhaust ones are a pain to do, but if a valve is riding, that would cause an uneven idle. Cheers Marty | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 15 Registered: 4-2007 |
Hi Marty I have checked the inlet valve clearance, but not the exhaust, I did a compression test last year , it gave a pretty uniform pressure for each cylinder but will redo before I go further, I am getting the exhaust repaired so I will check the valve clearance when that is removed - makes access a lot easier. I am away at the moment but will tackle this when I get home next month Many thanks for your input Mark | ||
| Laurie Fox Frequent User Username: laurie_fox Post Number: 59 Registered: 6-2004 |
Mark You have seen my reply on the BDC Forum but I am now joining in with the discussion here. My question is whether the lumpiness varies according to whether the engne is cold or warm. As Marty says the valve clearances may be a factor and if this is causing the problem than I would expect a difference in lumpiness according to engine temperature. I find that the clearance on the exhaust valves tends to get less as time goes on and I set the clearance (cold) a thou or two above the recommended 0.012". When the engine is idling does the engine rock in a regular manner or does it vary in a random kind of way? Of course, much will depend on the iding RPM. My idle is probably around 500 rpm or maybe a little less. On a level road, in top gear, I can pull away with a gentle application of throttle from idling speed quite happily without any jerk in the drive train and you can then hear (feel?) whether each separate cylinder is providing the same amount of push as the others. How much does the iding RPM need to increase before the unevenness disappears? I can get lumpy idling if I don't remember to set the mixture control fully back to "run" after the first start of the day so as to raise the jets up to point where they meet the adjustable stop on the carburetters. There is always a slight amount of play in the linkage which has to be allowed for. Regards Laurie (Message edited by laurie_fox on 13 June 2009) | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 16 Registered: 4-2007 |
Hi Laurie I am not sure what the idle rpm is on the car the rev counter fitted is wildly inaccurate (another job on the list) The uneven idle is fairly random and the engine does rock, it is not a regular missfire, I have removed plug leads one by one and the resulting loss of rpm is identical regardless which cylinder is disconnected. The uneven running appears to be worst when the engine is hot, but this is hard to quantify as the engine runs at a slightly higher rpm when cold and using the choke. The carburetors have just been rebuilt and reset so I am counting them out at the moment. Once off idle the engine runs smoother, but there is still the occassional falter . Once on the road the car goes well with no missfire | ||
| Laurie Fox Frequent User Username: laurie_fox Post Number: 60 Registered: 6-2004 |
Mark From what you say it does look like an ignition issue. Not likely to be a problem with the coil but more likely to be connected with the distributor. The strength of the spark from the coil is at a maximum at low revs provided the contacts are opening and closing correctly and are clean. Carbon tracking in the distributor cap could be the cause. Do you have a spare cap which you could try or could you borrow one from a pal? Regards Laurie | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 17 Registered: 4-2007 |
Hi Laurie I renewed the cap about 3 months ago , I had a spare so decided to use it but it did not make any real difference Rgds Mark | ||
| Laurie Fox Frequent User Username: laurie_fox Post Number: 61 Registered: 6-2004 |
Mark I am sorry to say that I don't have any more ideas. Let us know if you do eventually find out. Regards Laurie | ||
| Martin Cutler Prolific User Username: martin_cutler Post Number: 118 Registered: 7-2007 |
Hi Mark, If the valve clearances come up OK, I would then check the valve springs. If you have a weak spring, it may not be closing the valve properly. You need to know the installed height of the spring, and then know what the lbs are for the inlet and exhaust springs at the installed height. I am sure somebody here will know. On the old Italian motorbikes i play with, they also measure the lbs with the valve at the open position, and check for coil bind, but that is probably not necessary in the case of old Bentleys. Cheers Marty | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 1729 Registered: 4-2003 |
Carbutettor overhaul or not, it still sounds like the idle mixture is a bit out: plobably a bit lean, bit adjustment will take less time that to read this post. Over the first 1,000 miles after any carburettor work, the mixture will need adjusting every 250 miles in any case. Weak or broken valve springs: an extremely unlikely cause of the symptoms described. Take it to 4,250 RPM. If there is no valve bounce, very audible if it happens, then the springs are all fine. I have the specs for the inner inlet valve springs if you need them: standard and two grades of stiffer springs in case you go for a high-lift special grind on the camshaft. RT. | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 18 Registered: 4-2007 |
Thank you to everyone for the input, I will check the exhaust valve clearance first and go from there, I am away at the moment, but I will revert back when I have had a chance to work on the car, Mark | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 19 Registered: 4-2007 |
I was reading Richards posting about different Spark plugs,Unfortunately I cannot remember which plugs I have in the car, but when I put new ones in 2 years ago I asked for the ones specified in the handbook, I have some recollection that I got a different make but the equivalent plug.Could these plugs be too "cold" and therefore be contributing to the uneven idle? I know that the valve stem seals need replacing and I have ordered new ones. | ||
| Bill Vatter Frequent User Username: bill_vatter Post Number: 52 Registered: 9-2004 |
"Just had the carburetors completely overhauled" What was it like before that? You know, not everybody knows how to overhaul a carburetor correctly. Also, "Completely" as in completely restored, overhauled, rebuilt etc. is one of the most overused words in the antique car hobby. In the jet assembly, there are two packings that seal against fuel leakage. Your "complete" overhauler may have decided not to renew the packings if they were not leaking, and if he did renew than, possibly the new packings were not correctly installed, or maybe even the packings were defective or incorrect. Old seals or incorrectly installed seals can bind against the jet preventing it from moving up and down properly. I suggest you disconnect the richening linkage where it attaches to the front carburetor leaving the linkage between the carburetors intact. Move the jet mechanism by grasping where you disconnected the rod at the front carburetor and see if the spring will raise both jets completely against the adjustable stops. Adjustment of the rod connecting the carburetors is critical. It must allow both jets to raise completely. The richening linkage may be out of adjustment such that the jets do not move all the way up to the stops. That will give an inconsistent idle mixture. Only after the carburetors are mechanically correct can you adjust them with any certainty. | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 1864 Registered: 4-2003 |
I must agree with Bill. 90% of SUs I have seen may have been overhauled with no expense spared, but run like a you-know-what until someone centres the jets, synchronises them and adjusts the mixture. Especially after an overhaul, replacement or jet and needle job (most so-called overhauls and restorations are just a scrub, float and needle job), they need adjusting at 200 miles and then twice in the next six months. RT. | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 1865 Registered: 4-2003 |
ps: my R-Type has the automatic choke version. Setting the mixture is simpler and more accurate than on the manual versions. In the tool tray is an Aussie 1¢ piece, Crewe special tool RH2,000,001SD, to this day. As a kid when my father drove me on errands, he would stop, lean the bonnet on his head, undo the cap, turn the jet adjustment 1/8 turn, cap back on, and drive off, all in 20 seconds and without stopping the motor. That made it run along smoother than any other, and puff those twin exhausts perfectly on idle. Rear carburettor left exhaust; front carburettor right exhaust. Left to a shop, it would have been back every three months so demanding that my father is. | ||
| Peter Colwell Frequent User Username: peter_colwell Post Number: 75 Registered: 3-2005 |
The best way I know to track down an uneven idle source is to listen *very* carefully to the exhaust note of a fully warm engine. Mainly you can easily pick the difference between fundamental causes like low compression and burnt valves which will cause an even 'uneveness', or random partially missed beats which indicate electrical problem. Carburettor problems usually show up as indicators of lean or rich mixture, the first being a tight sound, the second being an obvious woolly sound. I hope this make sense, hard to describe, but very accurate diagnosis. | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 20 Registered: 4-2007 |
I have just read all your messages posted in august, not sure how i missed them. I will check the linkages, hopefully the carb rebuild was a complete job, they are specialists and the car was with them for a few days, and given the size of the bill then I expect a decent job to have been done. As well as the unevan idle the car is blowing a bit of blue smoke, i have installed new valve stem seals so now i am beginning to suspect that the rings / engine are worn. The compression still reads about 100 psi. I have renewed the plugs with hotter ones, but have not had a chance to check their condition. At present I only do about 1500 miles per year so i am hopefull that the engine will keep going for another few years till I can afford to either buy a rebuilt 4 1/2 or get this one rebuilt | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 1962 Registered: 4-2003 |
What valve stem seal type was installed ? For love of a new motor, smoky engines are almost always due to old-style valve stem seals, even when fitted yesterday, unless the crankcase is almost on fire. Vesuvius crankcases are invariably kissing pistons knackered beyond belief, usually if mandatory full-length liners have not been fitted. RHT. | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 21 Registered: 4-2007 |
Hi Richard I have installed the new blue "plastic" seals, I think I first heard about them on this forum. Having thought more about this I am beginning to wonder if the Carbs are running too rich, will check this when I get home. If I take the car to a garage can they test the exhaust emmission and will this give me a definative answer as to whether or not the car is running too rich. I am loath to fiddle with the mixture as I really do not know exactly what I am doing. I assume that the engine has full length liners but I cannot be sure. It is still fitted with a bypass filter system so the engine may have accelerated wear. I change the oil every 6 months regardless of the small number of miles that the car travels. Last oil change the car had only travelled 1,100 miles since the previous change and already the oil was Black and had a slight smell of exhaust gas. | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 1964 Registered: 4-2003 |
Best send me a private message with your phone number and time zone. RT. | ||
| Mark Anson Experienced User Username: mark_anson Post Number: 15 Registered: 11-2006 |
Sounds very much like my car. I had a full restoration done on my carbs by Burlin Fuel Systems. The job was outstanding but when I fitted the carbs the car ran lumpy. I had set the needle height correctly but the car did not like this. I disconnected all my linkages (except the carb link bar) I loosened the pinch camp first then set the tickover on each carb. I then turned the mixture nut until the engine ran lumpy. I turned the nut back one turn (richened) and the engine evened out. I did the same with the other carb then tightened the pinch clamp. After connecting all the linkage the car ran so smooth. Cheers Mark | ||
| Peter Colwell Frequent User Username: peter_colwell Post Number: 83 Registered: 3-2005 |
Engines fitted with SU carburettors will run spectacularly smoothly when properly tuned. And they do not lose tune easily. It is imperative to have twin carburettors properly balanced. Not difficult but must be done someone who knows what they are doing. SUs are not subject to heat-sink flooding. Because the float bowl is seperate from the carburettor, engine heat does not affect the float level, as it does in others. Flooding at all is very rare. Properly tuned, they also promote instant one-click starting. | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 455 Registered: 6-2006 |
All carburettors coming back from rebuilds - great ones included, will need tuning once fitted. Peter, Although your observation about the SU's not heat sink flooding are right, I think it's less to do with the amount of heat and more to do with the float chambers being vented directly to the atmosphere. If you do experience occasional flooding, especially when the car hasn't been used for a while or in hot soak conditions - it's often caused by the float dropping to the bottom of the bowl and the fork on the lid holding it down. Short term a few little taps with a screwdriver or spanner should jiggle it free. Long term - take off the lid and inspect the float and the fork. You will usually find brown petrol gum coating them both. This gum has a high coefficient of friction and can stop the fork sliding. Get the metal polish out and hopefully that will cure the problem. Also make sure that the centre lever on the fork behind the pivot pin does not let if fall too far and jam. |
