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Old 06-11-2019, 09:30 AM   #1
brown7373
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Erratic engine idle 72 350

I installed a new 350/290 crate engine. It starts and runs great, but has one issue. I can't get it to hold the idle at 600. Sitting at a light, it just starts increasing, almost like I am depressing the gas pedal. It will not kick down.

I cleaned the weights on the distributor and disconnected the vacuum advance. But I think the distributor is advancing on it's own. I installed 2 heavy springs with no help.

Question. Should I lubricate the area under the weights where they rub? If so, I think I would use a lube intended to use on the rubbing block of points distributors. I run a stock distributor with a Pertronix 1 installed. I had no problem with this distributor on the old engine, and I had no issues with the idle.

I cannot find any obvious vacuum leaks, but would one cauth the engine to race like that? I am at a loss for what else to check.
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Old 06-11-2019, 10:16 AM   #2
Steeveedee
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Re: Erratic engine idle 72 350

Try a stronger throttle return spring. Also, check the throttle shaft for slop. If it's an Edelbrock, you even have to check the passenger's side of the throttle shaft.
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Old 06-11-2019, 10:18 AM   #3
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Re: Erratic engine idle 72 350

I know this is probably a dumb question but did you reconnect the return spring on the carb?
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Old 06-11-2019, 10:52 AM   #4
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Re: Erratic engine idle 72 350

Yes the return spring is attached. It is a know good q-jet 7042210. This happens when it is off idle, either while driving or if shifted into neutral. It just slowly increases speed, up to about 2000 rpm. I will have to check, but I think the throttle linkage is resting on the stop, and I think the timing is advancing. At first I thought the advance springs were too loose, allowing it to speed up, and as it sped up the weights would keep it going up. That is why I switched to heavy springs, even though it should run best with something less stiff.

The 72 is supposed to have transmission controlled spark, but I don't think I have all the components. So I disconnected any vacuum advance and plugged the fitting.
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Old 06-11-2019, 11:43 AM   #5
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Re: Erratic engine idle 72 350

Check that the choke lever isn't lose and allowing the fast idle cam to rotate around, and that the fast idle screw isn't adjusted in too far.

Could it be a vacuum leak at your booster and when you are pressing on brake its getting more air? And in reality, its running too rich?

Both of those would go together BTW.

Just some thoughts.
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Old 06-11-2019, 11:58 AM   #6
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Re: Erratic engine idle 72 350

This doesn't really sound like advance to me, it sounds like a vacuum leak. I think I would plug up everything I can at the carb or intake to see if that makes a difference. If it doesn't, spray carb cleaner around the gasket surfaces to see if it makes a difference. Make sure the choke is coming off, and none of the linkage is binding up.
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Old 06-11-2019, 12:43 PM   #7
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Re: Erratic engine idle 72 350

Thanks for the ideas. I am going to look for possible vacuum leak, and also the choke/fast idle.
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Old 06-11-2019, 08:01 PM   #8
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Re: Erratic engine idle 72 350

I've seen the brake booster as a source for the vacuum leak with your symptoms. As the diaphragm move the leak comes and goes.
Easy to check the timing is the cause. Put a timing light on the engine and watch it when the RPM changes.
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Old 06-12-2019, 09:38 AM   #9
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Re: Erratic engine idle 72 350

If it's the booster, you could check it by just plugging the vacuum line, if it happens while you aren't moving. I know how much fun it is to stop with no vacuum to the booster.
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Old 06-12-2019, 07:17 PM   #10
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Re: Erratic engine idle 72 350

running lean can cause a surge in rpms. Too little fuel or too much air ( vacuum leak)
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