High idle and elevated timing

[71rehab]

Just acquired a 71 C20 and the plug wires were crumbling apart. Changed plugs and wires. PO had already added HEI.

I set the timing the old school way. Retarded the timing until when I romped on it, there was no pinging.

Newbie question: It *sounds* like its idling a little high. When I put a timing gun to it, it reads 32 at idle. Now, I've read the total timing should be around 32. Those two things combined make me think its idling far too high. My idle speed screws are turned all the way out.

Can anyone point me in the right direction?


71 C20
350 and quadrajet are original

[geezer#99]

Old school, eh!!
Nobody 'old skool' ever did that.
We plugged the vac pot off, drove uphill in high gear at 30 mph and advanced until it pinged then pulled it back a hair!
Plugged the pot back in, turned down the idle, adjusted the mix screws and went out to find some fords or mopars to blow off the street!
old skool indeed!

[71rehab]

Originally Posted by geezer#99

Old school, eh!!
Nobody 'old skool' ever did that.
We plugged the vac pot off, drove uphill in high gear at 30 mph and advanced until it pinged then pulled it back a hair!
Plugged the pot back in, turned down the idle, adjusted the mix screws and went out to find some fords or mopars to blow off the street!
old skool indeed!

Well, thats exactly what I did. Except for the racing bit.

[geezer#99]

Originally Posted by 71rehab

Well, thats exactly what I did. Except for the racing bit.

Not quite!
We advanced it, you retarded it!
Big difference.
What do you have for a timing light?

[webfoot]

Take the reading at idle with the vacuum advance plugged. Is it plugged into ported or manifold vacuum?

[71rehab]

Originally Posted by webfoot

Take the reading at idle with the vacuum advance plugged. Is it plugged into ported or manifold vacuum?

Yes the vaccum advance was plugged.

[HO455]

Yes the timing is way to high at initial. Should be in the 8 to 12 degree range with the vacuum advance disconnected and plugged. Bringing the timing down will mean readjusting the idle speed down too. Ideally when you get the initial timing correct you will have backed the idle speed screw out closing the butterfly's enough for the idle mixture screws to work again. Find a tach and dewel meter to check your idle speed. They are easy to find cheap and used. Nobody seems to want them any more. At least around here it seems that way.

[71rehab]

Is it possible that the HEI was installed incorrectly such that I can still get a reasonably well-running truck without an accurate timing reading?

[webfoot]

Originally Posted by 71rehab

Is it possible that the HEI was installed incorrectly such that I can still get a reasonably well-running truck without an accurate timing reading?

Nope.

As mentioned previously plug it and for a stock-ish cam shoot for 10-12 at idle, removing some advance should bring the idle down some too.

[71rehab]

Originally Posted by webfoot

Nope.

As mentioned previously plug it and for a stock-ish cam shoot for 10-12 at idle, removing some advance should bring the idle down some too.

Yep that worked. Just had to move each of the plugs over by one post to give me the clearance to retard to 12 and now its starting and running much better. Thanks!

[Wrenchbender Ret]

If you are running a Q-jet the throttle shafts get worn & cause a high idle even with the screw backed out. Check the mechanical advance in the HEI unit. They are known to get dry & freeze up.

[71rehab]

I tried covering the carb with a rag and the truck kept running. I've read this means that I have a vaccum leak. I sprayed WD40 around the gaskets and didn't see any sign of a leak.

My should this choke valve be completely closed at idle when the engine is warm? Sorry for the beginner questions.

https://drive.google.com/open?id=0B9...3pncllWVmhLQWM

https://drive.google.com/open?id=0B9...TFwLVEyUWZPbEk

[71rehab]

Originally Posted by 71rehab

I tried covering the carb with a rag and the truck kept running. I've read this means that I have a vaccum leak. I sprayed WD40 around the gaskets and didn't see any sign of a leak.

My should this choke valve be completely closed at idle when the engine is warm? Sorry for the beginner questions.

https://drive.google.com/open?id=0B9...3pncllWVmhLQWM

https://drive.google.com/open?id=0B9...TFwLVEyUWZPbEk

I just realized the clutch valve is resting in the open position and it is supposed to rest closed? Can anyone point me to the adjustment I need to make here?

[HO455]

Originally Posted by 71rehab

I tried covering the carb with a rag and the truck kept running. I've read this means that I have a vaccum leak. I sprayed WD40 around the gaskets and didn't see any sign of a leak.

My should this choke valve be completely closed at idle when the engine is warm? Sorry for the beginner questions.

https://drive.google.com/open?id=0B9...3pncllWVmhLQWM

https://drive.google.com/open?id=0B9...TFwLVEyUWZPbEk

If your looking for a vacuum leak you should also try disconnecting and plugging the brake booster and any vacuum lines to the transmission. You may or may not have either of these. Also the vacuum advance canister on the distributor can leak. If you sprayed the throttle shaft and did not hear any change in RPM then most likely your throttle shaft bushings are not worn.

The Choke flapper should be completely open when up to operating temperature. The only time it should be completely closed is at cold start up after pumping the accelerator pedal prior to start up. Then it should be about 1/4 inch open. It will then slowly open fully up as the engine warms.

[LongBox]

Just a related question here: If you run 10 - 12 degrees initial advance (vacuum plugged, engine idling around 650), is it necessary to run premium fuel, or to limit the total centrifugal advance in the distributor?

[HO455]

Originally Posted by LongBox

Just a related question here: If you run 10 - 12 degrees initial advance (vacuum plugged, engine idling around 650), is it necessary to run premium fuel, or to limit the total centrifugal advance in the distributor?

It is quite possible that you may need to run premium fuel. It would depend on the compression ratio of the engine. If your trying to run a high compression engine on low octane fuel you should retard or limit the available timing to prevent detonation. Timing is set in accordance to compression ratio not the other way around. Engines with low compression, a large bore, and large compression chamber will always need more timing than a high compression, small bore, small compression chambered engine. Largely due to the greater distance the flame front has to travel.