chevy 350 backfires out intake. what causes this - Hot Rod Forum : Hotrodders Bulletin Board
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Old 05-14-2010, 03:26 AM
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chevy 350 backfires out intake. what causes this

1978 chevy 350.........edelbrock performer with a 625 demon carb. everything else stock. hold 18psi vacuum. compression even and normal on all cylinders


timing is set at 7...........idles really rich has lots of power......backfires out intake. lean out with carb and motor just dies.


timing set at 4.......idles leaner has no power still backfires out intake.

car starts fine.......fuel pressure between 5 and 7 consistently fluctuating between the two

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Old 05-14-2010, 07:46 AM
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backfiring

What started this? sounds like a crossed plug wire. Its firing on a cly that the intake valve is open on.
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Old 07-07-2010, 12:22 PM
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First off, disconnect the vac advance and plug it. Move the timing up to 12 degrees before top dead center. Lock distributor. Reconnect vac advance to a full time manifold vacuum source (one that pulls vacuum full time including idle) recheck the timing, it should be around 20 - 24 degrees. After checking the firing order, if the problem persists check it for vacuum leaks. Pop the valve covers and start the car, watch all the rocker arms, if you see one barely opening, or not opening at all, you have a bad cam.
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Old 07-08-2010, 03:37 AM
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i moved the timing to 15 degrees and it fixed most of the problems.
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Old 07-08-2010, 04:03 AM
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That`s why I recommended setting the timing the way I said, it`s almost universal on SBC`s. Back it down to 12, then connect the vac advance to a manifold source, this will place it around 20 to 24 degrees BTDC at idle, right where it should be. It`ll have much better low end power and throttle response.
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