The V8 Owners Forum -

I have a 3.5 with SU carbs, which idles at around 750/800 rpm when cold. but once upto operating temperature it rises to about 1000rpm.

Has anyone any ideas what could be causing this?

I've done a bit of research but can't really find anything, other than possibly a problem with the bi-metallic temperature compensator assembly.

Any thoughts, suggestions most welcome.

Mark

Hello Mark,

How long has the current seal kit been in your carburettors?

It is my understanding that worn butterfly spindles along with air leaks around the spindle seals can result in large variations in idle speed.

Do note though that once warm the engine will always idle with a slightly higher rpm than when cold.

Ron.

Hi Ron,

I overhauled the carburettors in April this year, the spindles didn't look worn and new seals were put in upon reassembly. I have also checked the spindle/seal area for leaks by spraying easy start around them and listening for any increase in rpm - something I read somewhere, not sure how reliable it is.
If the spindle was worn or the seals were leaking would the idle be un-unstable?

RoverP6B wrote:Do note though that once warm the engine will always idle with a slightly higher rpm than when cold.

Ron.

Would the difference be as much as 200-250 rpm between hot and cold?

Cheers

Idle speed should always be set when engine is hot so try adjusting it to what you want and see how it works for you when cold. Some cars hardly idle at all when cold so what your seeing is nothing unusual.