help with throttle problem - high RPM idle

brian-80SC · 05-03-2008, 06:09 PM

Hi everyone,

I need some help with an high-idle problem on my 1980 SC. I haven't driven the car in a few weeks (while waiting on a DIY sway bar repair). Today, after finishing my repair work I drove the car and the engine will does not drop below 1800 to 2000 RPM when I am off the throttle and the car is idling. Jiggling either the throttle pedal or the linkage on the engine does not help, so I don't believe the throttle linkage is binding anywhere.

BUT... before I stopped driving the car a few weeks ago, I noticed that a nut on the throttle linkage was so loose that it was about to come off (see the arrow in the picture) and the linkage plate it secures was wobbling. So I tightened it up and counted my blessings for noticing the problem before it fell off completely and I lost throttle while driving. I didn't start the car after this until today.

So I suspect my tightening this loose nut may somehow have caused my high idle... Does anyone agree and can you explain why this happened or suggest how to put it back to normal? I was thinking about loosening the nut to see if the idle RPMs come back to normal - but I ran out of daylight and thought I'd solicit suggestions here before I look again tomorrow.

If the nut is not the solution (I'm betting it is), I may have to start reading up on CIS components to see what else might be at fault.

Thanks in advance!!
Brian

brian-80SC · 05-04-2008, 08:04 AM

Ok, I loosened the above nut on the throttle plate, and after warming up the engine the idle speed came back down to normal (~1000). With the engine running I began to tighten the nut. At a certain torque level the idle begins to climb, so tightening this nut definitely affects the throttle. I backed it off so it is still a little tight and the idle remains correct, although I think the nut is not torqued enough to prevent future loosening due to throttle operation and vibration.

My questions:
Should the torque on this nut affect the throttle operation?
Is this evidence of a problem? (e.g. worn bushings?)
I can't find the proper torque for this in the Bentley manual - anyone know what it should be?

As a solution, I might try to use a second nut as a locking nut to prevent the nut from loosening and coming off. But I want to address any underlying problem as well if necessary.

Thanks everyone.
Brian

Flieger · Flieger's Garage

+1 on the second lock nut idea

ossiblue · 05-04-2008, 05:46 PM

I suspect that if you removed the nut you'd find the hole in the throttle plate through which it passes has become enlarged, allowing the threaded shaft to move as you tighten the nut. The hole should have two flat sides that fit snugly against the flat sides of the shaft. If they have become distorted or enlarged due to wobbling, then the shaft will turn as you tighten the nut and cause the butterfly to open a bit--high idle. Check it out and see.

If that's the case, you might be able to repair it with a small metal shim driven between the flat side of the shaft and the plate that will wedge the plate firmly against the shaft. Then you should be able to tighten the nut without turning the shaft, and the nut will not loosen.

brian-80SC · 05-06-2008, 06:13 PM

Thank you for the suggestions. Ossiblue - your explanation makes sense to me. I'll take a look next time to see if there is evidence of wear as you suggest.

A question: if I tighten the nut (with or without a shim as ossiblue suggests) and the idle is elevated, can I appropriately adjust the idle back down using the (bypass?) adjustment screw located just above the arrow?

I don't want to make inappropriate corrections/adjustments to the CIS system, and its certainly unclear to me how my particular situation might be affecting the operation of the rest of the system.

Any CIS guru's with advice on this one?

Thanks!!
Brian