Revving at idle

[LaTwatta]

Guys -

My '71 411 is revving at idle. I'll start the car - it fires up first time, every time - and for the first couple of minutes it runs at a fast idle. Once it gets warm, the RPMs slow down, then the engine starts to surge, sort of like someone's quickly depressing the accelerator avery 4 seconds or so. It will rev, then back down to the point where you think it might die, then suddenly zoom back up to higher than fast idle RPMs.

Thoughts?

Thanks, Twats.

[vwbill]

Hey, have you checked the aux. air regulator? Sounds like maybe a vacuum leak? I think Ray had a post about the pvc looking thing at the right rear near the heat flap rod that has the flame trap and if the air hoses are hooked up wrong there it can cause a weird Idle? Maybe Ray can tell you more! Bill

[raygreenwood]

Several things couldbe doing this. As bill noted, it could easily be the Auxiliary air regulator not closing all the way. Either way, it is a vacuum leak. The PCV valve can do this. The o-ring behind the throttle body can leak, the hoses from the heads, the valve cover gaskets, the cork seal at the base of the oil filler/breather. To a lesser extent, the runner boots, the injector seals. Really important ones are the funky little boot that connects the plenum to the auxiliary air regulator....AND...you must do a vaccum test on the MPS. It should leak down not at all. If it does, it has a cracked vacuujm diaphram. If you have an automatic, a leaking vacuum pot at the accelerator dashpot...that big gold round diaphram with the vacuum line, that pushes on the throttle lever, or the vacuum line that feeds it. The vacuum advance leaking can cause this. Also check the hose to the modulator valve and the valve itself on the automatic tranny. If you check all of this...you will find it. Ray

[LaTwatta]

Thanks for the tips. I will "just" check that exceedingly long list! Actually, it's usedful to have so that I can work my way through methodically.

Thanks, Ray.