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Old 03-30-2011, 06:14 PM
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New 350 backfire

Hi all, I am having a problem with backfiring on a new gm crate motor. It happens often once the car comes up to temp and applying light to medium throttle. For some reason wide open it seems to run good. Heres a little info on the motor and if I leave something out let me know. Base 260hp gm with a comp cam 268he and heads shaved .030 (882 I know junk), 650cfm thunder carb (edelbroch) and full tube headers. New plugs, wires, hei and fuel pump. timming seems to be set good, starts up great hot and cold and no backfiring until motor is warm. Not sure where to start next any help would be great. Thanks to everyone in advance.

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Old 03-30-2011, 08:45 PM
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Sounds like you need to reset the valves.Could be that a lifter is pumping up when warmed up and not letting a valve close all the way.
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Old 03-31-2011, 09:56 AM
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Quote:
timming seems to be set good, starts up great hot and cold and no backfiring until motor is warm.
Ayuh,... I'm guessin' the idle circuit is Lean...
Considerin' it only happens after the choke comes off...
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Old 03-31-2011, 07:11 PM
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OK checked the carb. Found the car has no fuel filter. I took off the carb and cleaned it, put a fuel filter in-line and also found a vacuum line on the carb that was not plugged. Any idea if the timing will need to be redone with the vacuum line plugged now. It was the center large front port on the carb that was unplugged. Thanks old school and bondo for getting me in a direction
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Old 03-31-2011, 09:34 PM
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Yes, re-set the timing with everything prepared the normal way to set timing.
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Old 04-01-2011, 02:49 PM
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found another problem with the vaccum lines, the wrong port was used for the distributor. Thanks for your guys help. Any guess roughly what degree timming I should be aiming for. The cam that is in it is a .454 lift 218 deg comp cam. 268h. I used a very thin gasket and had the heads shaved to help bump the compression a hair. They are coming off next year and I am going to go the vortec route. But for lack of money they are staying on this summer. For this year it is simply getting it on the road with fresh paint.
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