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Thread: Exhaust backfire

  1. #1
    mesully123 Guest

    Default Exhaust backfire

    I have a roller 351C with a 8162 holley 850 Double Pumper. The engine revs very smooth to 7500 but if I back out of the throttle quickly it back fires like a cannon out of the exhaust. It only back fires once per lift but it very violent. If I back out of the throttle easy it doesn't back fire at all. Anyone every have this problem.

  2. #2
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    An exhaust backfire when decelerating, is usually caused by:
    a lean condition (especially when the throttle blades snap shut)
    exhaust header gasket leak (failed exhaust gasket allows additional air into cylinder during the camshaft's overlap period)
    retarded ignition timing (use a vacuum advance distributor or adjust for more lower RPM timing - lighter centrifugal springs)
    vacuum leak (which may only leak when decelerating - a high vacuum condition)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: Dominator MPFI & DIS, A/C, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/X-pipe/shorty headers, 100HP progressive dry direct-port NOS, 160A 3G alternator, Optima Red battery, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  3. #3
    mesully123 Guest

    Default Thanks Danny

    Danny, I put a vacuum gage on the carb and at idle it showed no vacuum. I reved the engine slightly and it came to 12". With the engine idleing I measured the vacuum at the back of the manifold and it pulled 12". Reving the engine to 4000 rpm slowly it went to 20". Oddly, as I reached around to pull off the vacuum gage, my hand touched the rear float bowl vent and the engine quit. I fired it back up and covered the rear float bowl vent again and it immediately shut off. I refired it and covered the front fuel bowl vent and it kept running. Does that mean anything to you?

  4. #4
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    That's strange. Maybe that bowl's needle & seat was filling the bowl at the time, so by blocking the vent, fuel displaced air couldn't escape and was forced into the carburetor (rich condition - stall). Look to see where fuel comes out when you block the bowl vent.
    http://speedtalk.com/forum/viewtopic...117036#p117036

    Is the vacuum advance hose connected to the timed spark port or the full manifold vacuum port? The timed spark port is only for emissions/EGR engines.
    Last edited by Danny Cabral; 05-05-2010 at 08:05 AM.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: Dominator MPFI & DIS, A/C, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/X-pipe/shorty headers, 100HP progressive dry direct-port NOS, 160A 3G alternator, Optima Red battery, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  5. #5
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    Quote Originally Posted by mesully123 View Post
    Danny, I put a vacuum gauge on the carb and at idle it showed no vacuum. I revved the engine slightly and it came to 12". With the engine idling I measured the vacuum at the back of the manifold and it pulled 12".
    The reason you did not get a vacuum reading off of the carb until you revved it is because you had your vacuum gauge hooked to a timed spark port (for the distributor vacuum advance).

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