Erratic Idle on 72

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Oct 10, 2004 | 08:09 PM

newbe65

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Heel & Toe

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From: Aiken SC

Erratic Idle on 72

I have a 72 automatic with a 350. After Lars recently gave my not so stock 72 qjet its last rites, I purchased a rebuilt qjet from Ecklers. At first it didn't want to idle below ~1000 rpm. After looking down the throat of the primaries I realized that the idle circuit wasn't functioning. Took the whole thing apart and to make a long (and painful) story short I now have the idle circuit working again.

However, I now have an erractic idle when the car is in park (600-900). It steadies out to about 600 when I put it in gear but is still slightly erratic. I also believe I am running it a bit rich to keep it from totally stalling out (e.g. no change when I remove one of the manifold vacuum hoses without plugging).

I know this could be a number of things (vacuum advance, vacuum leak, etc.) . What I'd like is some help in narrowing down the most likely sources of the problem and the easiest way to confirm or eliminate it as the source.

My manifold vacuum is running ~20 @ idle. Also the prob seems worse when the engine is fully heated up.

Also could bad spark plug cause this problem as well??

Any and all advice will be greatly appreciated.

Thanks!

Oct 10, 2004 | 08:42 PM

Jack71

Drifting

Joined: Sep 2001

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From: Boxford Mass

You could try plugging both ends of the vac advance at the carb & dist and see if that evens it out. I think a vacuum leak would cause a fluctuation like that. As for the carb, I'm the wrong guy to answer.

Oct 10, 2004 | 10:07 PM

bobs77vet

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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod

i'd start by disconnecting all vacuum hoses and put a vacuum gauge on the engine and see if the vacuum drops when it warms up. if it does it could be a distortion of something intake manifold, carb base plate etc...

Last edited by bobs77vet; Oct 10, 2004 at 10:16 PM.

Oct 11, 2004 | 06:08 PM

newbe65

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Heel & Toe

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From: Aiken SC

Erratic Idle Part 2

Ok I took all the vacuum hoses off and plugged them. It not as erratic but I still wouldn't call it great. May vacuum was reading right around 17psi off the manifold.

As I hooked things back up nothing changed until the vacuum advance. I may have it on the wrong port as I've read on at least one post that it needs to be on a port that has zero vacuum at idle. Any thoughts on this?

I also noted that when I took the #8 cylinder wire from the dist it changed nothing! I'll be putting new spark plug wires on this weekend but since it did nothing to change the idle problem I'm not sure that is the root.

Any help is always greatly appreciated.

Thanks!

Oct 11, 2004 | 06:20 PM

bobs77vet

Race Director

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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod

as it got warmer did the vacuum change? if there was no change you probably don't have any metal distortion caused by heat... this is a good thing.

don't know about the correct port although mine is hooked up to a ported spot..

when you took off number 8 wire , something should have happened. next time start the car with the all wires on and touch the exhaust manifold next to the wire you suspect is bad if the exhaust manifild is noticably cooler you do not have a spark in that cylinder, could be wire? could be spark plug?

Oct 11, 2004 | 07:39 PM

newbe65

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From: Aiken SC

Thanks

Thanks for the advice. New spark plugs are already in. The old ones were nasty. Some oil on the sides but a lot of carbon buildup. I bought her in August and have no idea when these things were last changed! I'll post how things turn out after that.