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ASEMaster35yrs, Master Auto Tech
Category: Classic Car
Satisfied Customers: 2095
Experience:  Master Auto tech since 1972,I own a 65 mustang and have built a 57 ford race car from the bottom up
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72 Chevy: with a 350 ci motor, Automatic..carb..miss..runs fine

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I have a 72 Chevy C10 Suburban with a 350 ci motor, Automatic with a Quadrajet carb. It has a miss at idle but runs fine after idle. Is it a carburator problem of some thing else?
First check for vacuum leaks, especially at base gasket. Quadrajets commonly leak fuel from a plug thats between main carb body & throttle plate so you may want to take carb apart to look if you can't get it to run smooth at idle. Get a vacuum gauge and connect to manifold vacuum and adjust idle mixture screws to highest & best idle and re adjust timing & idle speed. Hopefully the idle mixture adjustment has been done before. If not you will have to remove the plugs that cover the mixture screws. Check the vacuum line going to vacuum modulator at trans. If you pull the line off and any fluid comes out, replace vacuum modulator.
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Customer: replied 5 years ago.
Is replacing the carb with an after market carb like an Edelbrock a good option? Can I expect better mileage from that swap? Is this problem definately a fuel issue?

No, this is not definately a fuel issue, but is a possibility. You need to check all the possibilities out starting with checking for vacuum leaks or fuel drips.

First, I can tell you a carburetor from edlebrock is not a good option and no you will not get better mileage from it. Quadrojet is a good carb. It just has some things you need to be aware of if they are original. Rebuilt quadrajets have already solved the problems of the originals. You get much better mileage with a quadrajet since the primaries are small and secondaries are large and are vacuum operated which means the secondaries won't kick in unless the engine demand requires it and won't give more fuel than the engine needs. Another possibility for a bad idle is a leaking valve, or misadjusted lifters. To check this, you need to do a compression test. There should be no more than 20psi difference between the best to the worst cylinder. Check to see that all the spark plugs look identical such as black, burnt, or discoloration. To narrow down what cylinder is causing a problem, you can hook up a tach and watch rpm change as you pull a plug wire from each cylinder. They should all drop about the same amount. The bigger the drop, the better the cylinder. If you find one cylinder that drops little or stays the same, there is a problem there such as valve, spark plug or plug wire. Make sure your vacuum advance is working properly and holds vacuum.

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