Dellorto Idling Frustration - S4 Elan

[rgh0]

Hi Ron and Rohan, thanks for your helpful advice. I have carefully checked the butterfly positions with regard to the progression holes and they do cover them all when shut. The carbs are 40Es' so they have the additional idle bypass circuitry. As you say Rohan maybe adjusting this and the idle screws will cure the problem. The car does also seem to pop a bit at idle. I am taking the car to a Lotus professional mechanic on Wednesday to try and adjust the carbs with proper equipment. I will let you know what transpires. Regards, William

I would try opening the bypasses and shutting the butterflies a little as that should solve the problem

cheers
Rohan

[Concrete-crusher]

I have the same problem with my Weber 40dcoe carbs. I have eliminated the pops and backfire by turning down the mixture screws and advancing the timing. I have also fitted the sytec throttle kit which looks very good and is smooth but does not stop the erratic idling.

What I have noticed is when it's idling fast , if I add some load to the engine by pulling forward slightly in 1st the idle returns to around 1100 rather than racing around 2000

My current conclusion is the throttle plates are not sealing , does that sound correct. I'm about to take the carbs off to have a better look.

What I'm wondering and would appreciate advice , do the throttle plates actually wear , rather than the carb barrel , and how close should the closed seal be.

I guess this would be the same on the Dellortos which is the subject of this thread.

Steve

[William2]

Took the car over to Max (Max 500 at Guildford). Interesting!! He worked on the car for 4.5 hours. He had the carbs off and blew out the idling jets, etc. He has found the following problem. Numbers 2, 3 and 4 cylinders all work fine and idle mixture screws all work as expected. No 1 cylinder is the problem area. If the spark plug cap is removed with the engine ticking over at 500 - 1000rpm it makes no difference to the engine at all (rough idling) but the plug has a good spark. So ignition is fine as is the timing and the dizzy weights are working fine too. If you increase the revs then the engine runs normally and idle circuit fuel works ok but with the adjustment screw opened unscrewed quite a lot. He tried everything else like disconnecting servo/headlamp vacuum circuits and blanking manifold off. He also checked for air leaks around the Misab O rings. Carried out a compression test on all cylinders to check for a sticking valve, etc and all was fine. His educated guess is that there is a blockage somewhere still in the idle circuit of no.1 cylinder which he thinks would also cause the idle rpm to rise. The hole at the bottom of the carb body is clear so it would appear that there could be a blockage between the idle screw gallery and the float chamber.
It's interesting that having driven the car for a 90 mile round trip it pulls well and there is no hesitation between idle circuits and main jet. The problem can be summed up as no.1 cylinder is not getting any fuel at all at tickover rpm.
Max suggested getting hold of another carb from somewhere and swapping it to prove the problem disappears.
At least he has narrowed down the problem area but we were both frustrated that he couldn't cure it.

[Craven]

Hi,
Not familiar with Dellorto throttle linkage, can you swop left to right. If you have a ColorTune check for spark under compression.
FWIW
Ron.

[William2]

I recently purchased a pair of refurbished DHLA40 carbs at not an unreasonable cost. Re-jetted them to suit my engine spec and meticulously checked everything I could think of whilst fitting them to the car. Ran the engine and set the balance using a Syncrometer and adjusted the idle mixture screws in conjunction with my recent Colortune purchase to get the required blue combustion colour. Took the car out for a test drive and everything works as it should. Idle no longer sticks and returns every time to 800rpm and no obvious flat spots or hesitation. Engine idle is also much smoother now that number 1 cylinder is operational. Got there in the end but after an awful lot of head-scratching and much help and advice from the forum. Many thanks, William.