simple idle problem
 

1979 sc - the accelerator cable seems to be getting stuck somewhere; when i start up idle is ok, sometimes high, but then when shifting, idle does not drop (stays anywhere between 3 & 5k) unless I blip the throttle hard a few times.

I was driving a few times in the rain, and after the car was wet, it dropped right down to 1k...ie lubricated from the rain, and worked perfectly fine until the weather cleared up.

So, where should I start looking? (and I do have a Bentley manual, but it's in a storage unit temporarily, so pardon this question).

Thanks,

this sounds to me like a throttle linkage issue although i'm no expert on the subject. I had mine redone because I wasn't attaining full throttle at all and the part that could be causing your problem I think are the bushings at the bottom of the throttle rod which is right next to the road on the bottom of the engine. This would make sense with the rain making it work better..... I would start by checking out the linkage at the each end of the throttle rod and watch it as someone else depresses the gas pedal to check for hangups......hope that helps a bit and maybe someone else has had this exact problem and will chime in...

There are a number of different throttle return springs which "could" cause this. A quick visual check should be able to tell you if they're in place and whether the thottle is returning completely after you step on the gas.

However, since it idles fine at startup, I suspcet an air leak. My first guess would be the deceleration valve is sticking open. This little thingey keeps revs up a little when you shift, and lets additional air in when you step off the gas at high revs. It can stick open or fail, and let additional air around the throttle plate and keep your idle high. Do a search on deceleration valve. The quick test is to plug the vacuum line going in to it and see if this changes things.

Good luck!

I've disconnected the decel valve, no change (so I reconnected it). All return springs seem to be ok.

Note that it only happens when the engine is hot, ie at operating temp. When first starting, and then for around 10 minutes of driving until it warms up, the revs drop down to 1k as they're supposed to.

And if I blip the throttle in neutral to 4-5k it drops down to 1k just fine (it will "stick" anywhere between 3 & 5k)

Any further thoughts?

does the throttle linkage hang up on the end of the engine mounted heater blower? disconnect and plug the hose to the deceleration valve temporarily. pull the mats out from under the gas pedal. be sure the clutch cable bowden tube is not tight against the top of the throttle bellcrank on the side of the transmission. it can tighten against it when the clutch is depressed, and hang the throttle.

I don't have a manual handy to check, but isn't the idle supposed to be adjusted at operating temperature? Seems like your idle is actually set too high in order to allow the car to start. You probably actually have a cold start problem that's masked by a rich mixture.

Thank you for your replies. I will look at the John Walker suggested items, and check back...op temp = heat = expansion of heater blower = hangup? That's a great lead, thank you.

Re idle speed set too high...it was idling fine (ie for 2 years without being changed); and when the throttle is blipped, idling is fine - I think this is not the case. But thank you (and great year on your SC!)