rough idle and backfire - Team Camaro Tech
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post #1 of 3 (permalink) Old Mar 13th, 00, 04:36 AM Thread Starter
Join Date: Mar 2000
Location: doylestown, pa
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The engine is a 1980 350 in a 1970 camaro with no pollution controls on it. It runs rough at idle and backfires through the exhaust after you rev it up and lay off the throttle. I have check the timing, new plugs, cap and rotor. I have an HEI dist. on the engine. Can a bad carb cause this or do I have a bad cam? How can I tell if a have a bad carb vs bad cam? The motor is stock with the exception of aftermarket manifold and carb. Edelbrock duel plane manifold and 600 cfm carb. Thanks in advance.

[This message has been edited by Tortoise70 (edited 03-13-2000).]
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post #2 of 3 (permalink) Old Mar 13th, 00, 06:34 AM
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Join Date: Dec 1999
Location: Yorkville, Illinois USA
Posts: 22,121

Best guess is a vacuam leak. Put a vacuam guage on it, and look for a bouncy needle at idle. Try spraying a (Little bit) of starting fluid around the manifold intake ports, one at a time, if it speeds up, it's leaking. Pull the pvc valve and check for blowby.

[This message has been edited by JimM (edited 03-13-2000).]

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post #3 of 3 (permalink) Old Mar 13th, 00, 10:02 AM
David Pozzi
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Join Date: Nov 1999
Location: Central California, USA
Posts: 10,080

Try spraying the carb inside with carb cleaner.
When a car sits, the fuel can dry out and form deposits on the idle air bleeds, plugging them. They are the smallest holes in a carb.
When fuel is drawn thru a carb, it is siphoned out of the bowl, mixed with air into a froth. The froth is more easily vaporised in the engine.
This is done on both the idle circut and main circut.

This is accomplished by the fuel entering the idle circut, and there is a T connection where fuel is drawn from a fuel jet, and air is drawn thru an air jet and mixed. If there is plugging in the air jet, fuel can keep siphoning out of the fuel bowl and into the engine.
Normally the air jet would cause the siphoning to stop by breaking suction when the throttle is closed.
Also the air fuel mixing does not happen properly, so raw fuel droplets enter the engine, and pass right through into the exhaust system unburned.
If there is any air in the exhaust system, the fuel will now vaporise and burn causing popping.

Also, unscrew the idle screws and squirt carb cleaner in the holes.

My best trick is, with the engine running, rev up the engine to 3000 rpm and close the choke for about 3 seconds, do it a couple of times.
This pulls a vaccum on all primary circuts of the carb. It will pull fuel thru the air bleeds and hopefully unclog them.
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