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Old 01-31-2013, 07:41 PM
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backfireing through carb at low rpm

i have a 355 chevy that is backfireing through the carb under 3000 rpm. i have a 770 holley carb, i have had the valve covers off there is not a flat spot on the cam or a hung up valve. good compression. i do not know what the cam is though. it also has low vacume coming off the carb. anyone have any suggestions where to go from here. it runs great over 3000 rpm and idles fine. has new plugs and wires. just pops when i get on it at low rpm.

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Old 01-31-2013, 09:17 PM
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What's your timing set at?
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Old 01-31-2013, 09:32 PM
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Sure sounds like a cam lobe..
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Old 02-01-2013, 01:49 AM
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backfireing through carb at low rpm

Check your distributor to make sure that the weights are operating correctly. Check all the bushings for wear. Use a timing light with advance to check timing from idle to 4000 rpms. Use a vacuum gauge to run test on engine. Test are in the booklet that comes with vacuum gauge. Check for weak valve springs.
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Old 02-01-2013, 04:48 AM
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Lean fuel mixture or retarded ignition timing makes it back fire out the carb.

Does it do it just when you press on the gas or does it do it all the time under 3000 rpms?
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Old 02-01-2013, 06:51 AM
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it does it just when i get on the gas under 3000. i had the valve covers off everything looks fine moves as it should. i adjusted the valves properly. maybe i need to jet the carb down a bit or does it sound like more of a timing issue. im pretty sure the timming is set as it should so idk where else to go.
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Old 02-01-2013, 08:14 AM
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Timing set as it should means nothing. Get a timing light and check it. Start at 12 degrees initial. Disconnect and plug vacuum to the distributor when you set it. You might need to go up to 16-20 degrees too.
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Old 02-01-2013, 11:04 AM
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Try a bigger accelerator squirter and more timing.
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