rough running help needed

[sprint4dm]

I am having a few problems with rough running at low revs with my Elan Sprint. The engine starts easily and ticks over without problem but is slightly reluctant to pick up from low revs once on the move it runs and revs as it should.
With the engine just ticking over I can remove number 3 plug lead without any drop off in revs.
I have changed plugs, leads, distributer cap, rotor arm and coil, there is a strong spark to each plug so I assume it's a problem with the carbs I have cleaned all the jets and adjusted the slow running jets but no improvement can anyone suggest any other course of action?
I should add the car is running a luminition system.
Thanks in advance
Dave

[john.p.clegg]

Colortune No 3,Compression test No 3...?

John

[billwill]

Have you tried a ColorTune plug, so that you can SEE if the idling mixture is varying.?

Check that you are not getting air leaks around the O-ring mounts for the carburettors, but do not do them up to tight!

You need to tell us whether you have Webers, Delortos or Stromberg carbs.

[billwill]

I have the same problem, but on number 4 piston at present.

My next step will be to take the Dellortos off and air blast the passages, not just the jets themselves.

[sprint4dm]

Thanks for your suggestions,

The carbs are webbers. I have tried a colortune and can see a very slight colour change but it does not seem to increase or decrease the engine revs even if I tighten right up or remove the slow running jet completely.
The spark plugs are sooty suggesting running too rich. The car seemed to be running really well last week so this problem as occurred over quite a short period of time

Dave

[elanman999]

Dave,
I would also try the carb throttle balance. If you are going to take the carbs off then the accelerator pump passages need checking, these do block up if the car has been stood for a very long time and/or if the fuel has been dirty.
Cheers
John

[Chancer]

Check balance and then check for a twisted throttle spindle, both butterflys should pass the same progression hole at the same time.

recheck mixture

check cylinder compressions

as suggested look for induction air leaks on that choke.

Could also be a partially fouled plug or HT lead fault.

[billwill]

Anyone know what variation in Compression pressure reading is reasonable?

Would a lower pressure actually prevent the chamber from firing properly?

[oldelanman]

According to W/S manual pressure should be in excess of 160 psi and each cylinder within 20 psi.

[Chancer]

Lets say you have 100 psi on the offending cylinder and 200 on the others, you can see that removing that lead will have much less effect on the idle speed than the others.

Engine analysers use a cylinder balance test to check compressions, saves removing the plugs so yes low compression would have the effect but my money is on air leak or mixture setting on that choke, twisted butterfly, carb balance in that order.

Carb balance would usually create a pair of weak cylinders but if one has too weak a mixture also.......................

[billwill]

I wish it was easier to swap over the two carbs, exchanging front one for rear one. It would be a really useful diagnostic capability that.

~~~~~~~

What's the best method for getting the two carbs balanced against each other?

[john.p.clegg]

The best...a four tube dampened Manometer to equalise the flow,followed by four Gunson Colortunes
The easiest...a piece of tube stuffed up each venturi in turn,the other end stuffed in ones ear...

John

[sutol45]

Lumenition...

Sorry to say but I've had lots of problems with Lumenition causing the problems you report.

DO everything other guys suggest, but don't take your eye off that Lumenition...

I changed my (old) Elan to the Tony Thompson set-up (dizzy with internal module, coil and leads) out of sheer frustration and the difference was just amazing good - and I'd been doing those other checks too - carbs, compressions, timing, etc.

Hope I'm wrong and it's something simple, but don't take your eye off that Lumenition...

[MyLotus]

If what you are describing is a bad stumbe just off idle, then it is similar to what I have experienced with my 1970 S4 with Weber carbs. The solution is spelled out in the Shop Manual, under Fuel System, p. 54:
1. Change the carburator choke tubes from 33 to 30
2. Change the Air Corrector jets from 155 to 200.

Make sure the carbs are then well balanced and there you go. Problem solved! Yes, as the Manual says, you will lose some power the top end. So what?