Backfiring - Pontiac GTO Forum
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post #1 of 7 (permalink) Old 08-26-2011, 11:35 PM Thread Starter
 
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Backfiring

Hey guys, my 68 gto 400, 4 speed, has began backfiring when I put my foot into it. It is backfiring right underneath me, not at the end of the exhaust. I checked the timing and it is good (14 advance at idle) that is where I have always ran it. It also began surging when coming to a stop. Are these problems related? they both began around the same day. Thanks
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post #2 of 7 (permalink) Old 08-27-2011, 06:52 AM
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Idle mixture screws not properly set.
Spark plug issue not firing correct
Spark plug wire issue

Sounds to me like a carb issue.
When was the last time the carb was cleaned, or worked on?

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post #3 of 7 (permalink) Old 08-27-2011, 11:29 AM
 
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Is it only on driver side? You could have a dead cylinder, ignition related. The fuel/air mixture igniting in the header/manifold. Get an infra-red temp gun(very useful), and check the temp at every exhaust port after running the engine for a while. If you have a dead cylinder, it be cooler than the rest. Then check the plug and wire on that cylinder, or cap and rotor.

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post #4 of 7 (permalink) Old 08-27-2011, 11:30 AM Thread Starter
 
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backfiring

Carb is two years old. Never had any trouble. Its an edelbrock 750. Is there a safe idle setting I can do? I'm not a carb guy. I think the book says 1 half turns from lightly seated. But I haven't tried it
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post #5 of 7 (permalink) Old 08-27-2011, 12:34 PM
 
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Garage
Check/change your spark plugs



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post #6 of 7 (permalink) Old 08-27-2011, 03:28 PM
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Sounds too lean to me. I'd check for vacuum leaks/carb mixture. Try this: at idle, take the air cleaner off and mist a small amount of carb cleaner into the carb. If it speeds up and smooths out, it's too lean----caused by a vacuum leak or the carb itself.
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post #7 of 7 (permalink) Old 08-28-2011, 03:39 PM
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Check it with the air cleaner off, it may be poping through the carb. If it does it consistantly through the carb under throttle, it could be a bad cam lobe.
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