Missing, hesitant at constant rpm

[vee12eman]

Hi all,

My Series 3, since completion has never been very smooth at idle or at constant rpm. In 4th gear at 60 km/h, or 70 km/h in 5th (Getrag gearbox - rpm at around 1300 1600), the car is sometimes quite jerky, but put your foot down and the pick up is smooth and progressive, any jerks or hesitancy disappear completely.

At higher constant rpm, there is sometimes a very slight hesitancy, but not bad and not every time.

The engine was rebuilt to standard specification and seems to have plenty of power, but not having driven other similar cars it is difficult to be certain how good it is. The ignition system is a ReOpus, with new spark plugs fitted at the build - I haven't changed these yet. I believe the rotor arm is a NOS Lucas item and the distributor cap was replaced. The distributor was stripped, cleaned and all the weights/springs are free to move and well lubricated. The vacuum unit was replaced and new leads were used with the original coil.

Carbs were rebuilt with kits from Burlen Fuels, little wear was found on the spindles, and all seals, gaskets were used with new diaphragms etc.

So far I have fitted a replaced the original coil with a new one and another new ignition lead set was fitted as I happened to have one, but nothing has made much difference. The carbs were tuned and balanced and this appears to have held; when checked today, the balance was still good and a Colortune showed a good flame colour.

Any suggestions?

Regards,

Simon

[ChrisC]

Oil level in dashpots?

[christopher storey]

It sounds to me as though all 4 carbs are not synchronised . I would

1. if you have an airflow meter, check that it is the same through all carbs at say 1500 rpm - if you have not got a meter, a rubber tube in the ear is an acceptable substitute

2. Check to see that the carbs ( I am assuming they are CDs and not an SU conversion ) all open at the same time

3.Check to see that the rich mixture and /or fast idle control is not sticking on on one carb, or indeed one bank of carbs

4. A good guide to simultaneous opening is if they all reach full throttle together ( with engine off obviously )

[Heuer]

Putting your foot down to cure the problem suggests it could be a mixture problem especially as it is jerky on what is in effect a trailing throttle at low cruising speeds. Do you have a Colourtune or access to an AFR meter?

[PeterCrespin]

What you are describing is a misfire at high vacuum which is a lean running condition with borderline ignitability. I'd be looking for an air leak or some other mixture-related issue. If you've taken the emissions gubbins and vac retard off, did every little pipe get blanked off? What about the overrun compensator valve thingies on the sides of the carbs and their vacuum connections? See if putting a bit of choke on improves the hunting.

Pete

[vee12eman]

Thanks for the replies guys, guess I will have to get out and check all the mixtures again with my colortune. I had used two colortunes together in sync to compare mixtures, but I can do that again.

Pete, pulling the choke does make the problem go away and from this and general knowledge, the idea it was mixture related was prevalent in my mind. I think all the air inlets and vacuum pipes are blanked, but I will have another good look, if not I guess it's another day in the garage sitting over the hot engine and breathing in occasional petrol fumes!

Thanks agsin, I'll report my findings when I get chance to investigate again.

Regards,

Simon

[PeterCrespin]

If it stumbles at idle as well as low rev cruise, try listening at the exhaust to see which bank (ignore cross-over pipe). if you can't quite tell, hold a sheet of paper near the tip and it will amplify any stutter effect by flapping differently (see me waving my hands?). Try pulling one plug lead at a time and see how much the revs drop (ideally with a digital timing light tach but the car's tach and your ears may tell you. Any cylinder(s) that makes little or no difference is suspect. You may find three on one carb are iffy in which case check float height or other parameters. If it's one of the two carbs with choke try applying choke just to that carb - maybe by pinching or capping off the dog-leg choke fuel hose to the adjacent non-choke carb. If you still can't find what's up with a suspect carb try swapping sides in a last check to see if the fault now affects the new cylinders it feeds.

Don't forget the little hose spigot at the left rear carb and the transmission vacuum pipe for splitting or end leaks if it's an auto.

[AussieEtype]

Just check that you haven't got one dud sparkplug - I had a similar issues and it was a dead spark plug - one out 12 does not make a lot of difference and when accelerating etc the noise of the engine can hide the slight miss but when cruising along where the engine is a bit quieter it can be heard.

Garry

[vee12eman]

Thanks again to all respondents, I will be out there in the next couple of weeks and will report the results as I find them.

Regards,

Simon

[MarkG]

Simon. You might remember my carb issue from a couple of years ago. I was actually trying to suss out the choke operation but I had a long standing problem with slight hesitancy at steady higher RPM. I had changed goodness knows how many parts and thought I'd try balancing the carbs. So I bought an synchronometer airflow meter (the sort you hold in each of the carb inlets one at a time and is not expensive) and whilst at idle they were nicely balanced, at slightly higher steady RPM, one would consistently read low. When I removed it and started stripping it down, I spotted a small hairline crack in the body. I sealed the crack from the outside using mastic sealant as a temporary measure and hey presto, I got them all to balance across the rev range and the car has run perfectly ever since. That 'temporary' repair is still in place!

Like others have said, also check vac pipes are blanked off properly and there are no cracks in the rubber causing leaks.

Mark

[Woolfi]

Hello Simon!
Maybe ( ? ? ?) the problem is caused by a lean mixture. If you turn all 4 carb-needles 2 or 3 full turns higher, the mixture will be less lean. If the problems have disappeared, it (maybe? ? ?) was a problem of lean mixture.
I don't believe, that your problem is caused by lean mixture, because the chocke doesn't change the problem. But the test with the lifted needles is done very quick. If there is no change , you can be more shure, that it is not a problem of lean mixture.
In my EV12 I sometimes had a combinated problem, "ignition problem" + "carb problem". If a mixture is a little bit lean, lets say at the "igniting border", an additional problem with a spark plug, an ignition cable or something else, will lead to a non igniting cylinder. With another throttle position the mixture will differ and the problem will disappear. Such a doubble caused problem is difficult to find.
Regards Wolfgang Gatza

[PeterCrespin]

I don't believe, that your problem is caused by lean mixture, because the chocke doesn't change the problem.

He said it DOES change the misfire Wolfie, so it probably is a high vacuum lean running issue. As you say, this kind of running is at the margins of mixture ignitability so one thing can lead to another.

Pete

[Woolfi]

1. If there would be a small air-hole in the system, the idle would be worse than driving at part throttle. The lower the "throttle" the worse would the motor run.
2. He said, that the colortune test-sparks showed a good colour. At all 12 cylinders ? If the mixture at iddle at all 12 would be okay, the idle must be very good.
3. Still I would "lift" all 4 needles by 2 or 3 complete turns and make a test drive. The idle should be bad, because of much to fat mixture. If the problems at low part throttle would have disappeared, the mixture is too lean.
4. I would drive with this car when the motor is warm, 10 miles in staedy state and shut the motor immediatly. Maybe by checking the colour of ALL 12 spark plugs, the bad / lean running cylinders could be identified.
- 1 or 2 white sparks ? Air holes at the border between inlet pipe and cylinder head.
- 3 sparks plugs of one carb white ? Maybe this carb is runing too lean.
- 6 or 9 cylinders show white spark plugs. More carbs are too lean.
- all 12 spark plugs are white. Caused by air leak at the center air pipe between the two banks or 4 wrong carb needles.
I would bet, that 3, 6, 9, or alll 12 cylinders are running too lean and not an ignition problem.
Regards Wolfgang Gatza

[vee12eman]

Hi again,

Many thanks for all the input on this. I have been away on holidays and also working on a new project ? totally un-Jaguar related, an Austin A 50 Utility (can?t afford another E-type!), so progress on the E-type has been delayed until recently.

I changed back to the Resistive wires ? no change. I took out the temperature compensators and spent an evening adjusting them to open at 40degrees C as per the recommendations on the ReOpus site ? no change. I researched my plugs, the Unipart plugs appear to be resistive plugs (as mentioned earlier) so I didn?t bother replacing them just yet. Finally, I followed Woolfis? suggestion, and enriched the mixture, albeit only one turn and only by a half turn each time. This time the problem is progressively decreasing, but I haven?t had chance to colortune and check, plus the other changes are likely to have an effect too, so it looks like I have to look at the tune again and perhaps revise my opinion as to the exact colour of flame which constitutes the correct one, plus balance all carbs. All very interesting stuff, but time consuming with four carbs and very warm working over a heat source the size of a V12, so I have not finished yet, but feel I am on the right track. I initially though it was a lean mixture, but somehow I think I messed up my initial diagnosis actions; hence the questions, which appear to have at least got me going the right way.

Thanks to all contributors, I will have another go this week and report how I finally fix it when I do.

Regards,

Simon