C3(72) Fast idle will not kick down

[Rob C.]

My 72 LT1 will not come down off of high idle. When I disconect the carb solenoid wire it comes down but when I hit the throdel it comes back up. I do not have a trouble shooting manual and I want to take it out tomorrow. Any sugestions. Thank you.

[Patrick H.]

Re: C3(72) Fast idle will not kick down

When you hit the throttle and the idle goes back up, is the carb solenoid still disconnected?
The solenoid, when it is off (or disconnected) "should" close the throttle plates enough to essentially kill the car. (Of course I say this, but my 72 base motor car will not die this way and idles at 400rpm...)

Also, have you actually gone to the passenger's side of the car and watched what the choke linkage is doing when it comes down, then again watch it as you reach over the carb, blip the throttle via the throttle arm, and see what happens?

That info would be useful.

Patrick

[Rob C.]

Re: C3(72) Fast idle will not kick down

Patrick
The throttle dose not go up. It dose not kill the car it iddles about 4-500 rpm. The choke and all the linkages are all working fine. Is there some thing that tells this to back off a little more (the solinoid)?

[Patrick H.]

Re: C3(72) Fast idle will not kick down

No, when the solenoid is connected, it sets your idle speed. Your idle speed is NOT set by your carb, unlike other years. Follow the directions in your service manual or even the sticker on the firewall. Basically, warm the car up, follow the instructions as printed, and use the SOLENOID to set the idle speed. There's a reason the end of it looks like a hex head - that's so you can turn it in and out to adjust the idle speed.

As I noted above, when it is disconnected (or the car shuts off) it is supposed to drop down, close the throttle plates and therefore keep the car from dieseling upon shutdown.

Go ahead and start your car, then let it run for several minutes until warm. Don't touch anything yet. Once you think it's at operating temp (a couple minutes is usually fine with mine; yours may differ) then blip the throttle. Watch the choke linkage to make SURE it is "letting loose" even if you car still idles high. Now walk around the car and check out the idle solenoid. If it is extended AND touching the throttle arm, it's likely set too high. Screw it in until the desired idle is met. Of course, the specs on the firewall emissions sticker also talk about tming, mixture, etc. but this will at least get you a lot closer.

Did that help?

FYI, I never find that my solenoid has enough power to kick up the throttle arm if I start the car without first stepping on the gas pedal with the key in the ON position. Doing this with the key on allows the solenoid to be activated and then extend into place as I step on the gas pedal and set the choke. If the engine is warm (i.e. choke not needed) I do it with the key off, the solenoid stays down (does not extend) and the car will start at 400-500 rpm.

Patrick

[Jack H.]

Excellent point, Pat!

A minor tidbit some miss when doing Operations Check of the '71 and later cars equipped with a carb solenoids... The solenoid was primarily intended to fire when the car was in motion (underway on the road shiftng into high gear). In that case, the accelerator pedal would have already moved the throttle off/above curb idle and the solenoid wouldn't have to overcome the force of the throttle linkage's return springs!

When I do an Op's Check on these cars, I ask the owner to turn the ignition on (but NOT crank the engine) depress the accelerator pedal to set the carb's choke & high idle linkages and THEN key the starter. That way, the emissions control solenoid has electrical engergy to activate it AND it doesn't have to overcome the throttle linkage's return spring to fully extend.

Same goes for 'verifying' the CEC/TCS solenoid when we have the owner shift in/out of a high gear with the car idling... Depending on how fresh the throttle return spring is and how new/used the carb solenoid is, we can see differences in momentary curb idle RPM boost with the car sitting and the driver's foot off the gas pedal.

Basically, ANY appreciable RPM boost is a PASS of this test, in my book, eventhough the factory specified an RPM differential that might be significantly higher. The factory specified RPM delta is only guaranteed to happen when the solenoid fires and encounters no physical resistance as the plunger shaft opens to its fully extended state.

[Rob C.]

Re: Excellent point, Pat!

Pat and Jack
I want to thank you both.

[Patrick H.]

Re: Excellent point, Pat!

No problem.

Does that mean it helped? What did you find out?

Patrick

[Rob C.]

Re: Pat

It did help. What i did notice is that when it warms up the idel drops then after driving it creeps up to 1000-1100RPM. So when it gets real hot I have to set the idle.
Rob