Running rough at idle

[collins_dan]

Feels like my fillings are going to fall out. Holds idle well enough and accelerates fine. Checked the engine mounts, which are new, and they seem fine. I did take quite a thrashing a few weeks back at an Porsche autocross event as I felt obligated had to uphold the reputation of the marque. Any thoughts of possible causes? Thanks. Dan

[Elanintheforest]

Something on the engine / box touching the bodywork? Check the manifold & length of the exhaust for hitting / rubbing against the body / chassis, and just check that everything else is bolted up tight. Then have a look at the gearbox mounting.

Mark

[RobL7/Elan]

My experience has been that rough idle, but runs OK on applying throttle, is often related to Carb balance, especially on Weber applications. Suggest to re-check sync on the carbs first, then look for clogged jets etc. Often after running hard at full throttle the carbs can go out of sync with two cylinders not receiving fuel at idle, but coming on line as the throttle is applied.

Rob

[Bruce Crowthorne]

Try setting up the idle stage with a colourtune.
I tried doing it by ear and using a 4 column manometer (I have vacuum take offs in each inlet tract), but the best way by far was the colourtune.
Just my 2p worth
Bruce

[bill308]

I'm with Bruce and Rob on this one but I'd also add that checking the plug conditions, the spark at each plug, plus the ignition timming and advance with a strobe, would be a very good thing to do first. Float levels should also be checked along with the idle jets.

Rob's experience with balance/synch mirror my own.

Bruce's tool kit is the one to use for a DIY'er. Manometers (I like the mercury type) reading each throat's vacuum is the best way to measure balance and see the immediated effects of adjustments, cylinder to cylinder and as a whole. Colortunes can be effective, but in general I've found one needs to be well into the yellow to get reliable combustion. I'm inclined to make the idle mixture as lean as possible yet maintain a reliable idle. I always make this adjustment when the car is stinking hot. I don't want the plugs to load up at idle, that's a driveability issue I'd rather avoid. Another thing with colortunes, if the airflow through a particular throat is too low, you will not get a meaningful burn in the combustion chamber so the colortune will give strange results. If there is no burn at all, there will be no color to be seen. Sometimes there is an intermittant burn where the light color flashes but is not a solid color and does not repeat on every cycle. The thing to do here is to increase the idle speed for the affected carb. This is a problem with older Webers because the throttle shafts are relatively weak and over time take a twist set where one throat of the carb is flowing nicely and the other one isn't. The manometer system should pick this up and agiain a flat or more of a turn on the idle stop and/or balance screw should make things better.

I hope this helps.

Bill