Back Fires At High Rpm's
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Joined: Feb 2005
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From: TUCSON ARIZONA
Back Fires At High Rpm's
My 78 F 250 Has A 400 {NO M} In It And It Back Fires At High Rpm's When It Is In Gear But, Rev's Fine In Neutral. I Just Put A Newer Carb. On It. Before The Carb It Backfired Even At Low Rpm In Gear. Any Help Will Be Greatly Appreciated.
Cargo Master
Joined: Dec 2004
Posts: 2,232
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From: NE Texas
It may need a little more advance or some modification of the advance curve, OR the power circuit in the carburetor is not richening up at vac level that is too low. As RPM's increases, vacuum increases. If the power system leans out at a vacuum too low for the truck then there is an rpm/load condition that wants more fuel and is not getting it. If it is a Holley you might try a power valve a few points higher. This would be true if slight acceleration keeps it from backfiring. In that case the slight acceleration is dropping the vac enough to tip in the power system and richen it up.
If you want to be more scientific about it, hook up a vacuum gauge such that you can see your vacuum reading as you drive. Get the truck to a good cruise speed and see what the vac reading is. Get a power valve about two inches higher than that reading.
Example. If the truck cruises a 11 inches, put an 85 or 95 power valve in it.
Good luck,
Doc
If you want to be more scientific about it, hook up a vacuum gauge such that you can see your vacuum reading as you drive. Get the truck to a good cruise speed and see what the vac reading is. Get a power valve about two inches higher than that reading.
Example. If the truck cruises a 11 inches, put an 85 or 95 power valve in it.
Good luck,
Doc
Last edited by MBDiagMan; Feb 14, 2005 at 02:40 PM.
