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Feb 20, 2002 | 10:42 PM

larbuck

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Joined: Mar 2001

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From: albuquerque USA ALL THE WAY

back firing

hay guys i have a question for you my buddy has a 1979 f150 with a 302 the motor has about 6000 miles on it so far and it has developed an exhaust backfire we played with the carb a brand new holley 600 cfm
and that did not work we checked for vacume leaks and found only one fixed that and it helped but it still pops when you let off the gas and the motor comes down to about 2000 rpm any ideas??
here is the specs
302 bone stock
holley street dominator intake
holley 600 cfm carb
stock distrubitor

here is what we have done
checked for vacume leaks found one still pops only not as bad
replaced carb gasket no improvment
checked for bent push rods all fine
moved distrubitor no improvement
adjusted carb no improvement

please help this truck is driving me nuts

larbuck

Feb 21, 2002 | 12:35 AM

Franklin2

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From: Virginia

back firing

Check your float levels. Does it idle a little rich? A rich mixture will cause a backfire in the exhaust.

Feb 21, 2002 | 08:27 AM

larbuck

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From: albuquerque USA ALL THE WAY

back firing

the float levels are perfect and it idle's fine it dosent seem to be rich i adjusted the card and it seems to fine no bad exhaust fumes any orther guesses

Feb 21, 2002 | 12:19 PM

TorqueKing

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From: Atlanta, Georgia, USA

back firing

Don't forget the vacuum advance on the distributor, if it is plugged into the wrong port on the Holley, it will backfire, probably through the intake manifold though. THe Holley has two advance ports, one for full vacuum at idle, the other is for partial vacuum at idle. My '77 302 powered F100 makes 21" of Hg at idle, so I suggest verifying that it's in the full vacuum @ idle port. Maybe that didn't help cause youve already got it hooked up right, but forgive me, I'm an Edelbrock Carbs guy. If it keeps giving you fits, allow me to inform you that AutoZone keep 600 cfm electric choke Edelbrocks in stock for around the $220-230 range. Happy wrenching, John

Feb 21, 2002 | 12:42 PM

slikness

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From: Durham

back firing

Don't forget to check your power valve...It may have been blown out from the backfiring.

Feb 21, 2002 | 11:47 PM

TorqueKing

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From: Atlanta, Georgia, USA

back firing

Power Valve? All the more reason to switch to the Edelbrock/Carter style carburetor, no more power valves, much better gas milage and low end, and no "moods" that Holleys have. Just day in/out consistency. John

Feb 22, 2002 | 09:49 AM

slikness

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From: Durham

back firing

Don't let airharley hear you!!! Anyway, I agree the Edelbrocks (old Carters) are a lot easier to setup and work on...but the Holleys will tune a lot better to specific conditions, especially if you're looking for that last horse out ther on the drag strip.