why carb loading and backfiring when let off throttle on my 85 f-150 300 6cyl.

Asked by Jul 07, 2013 at 12:08 AM about the 1985 Ford F-150 STD Standard Cab SB

Question type: Maintenance & Repair

runs good when accelerarting when letting off fuel throttle carb loads up and backfires.
when idling rpm revs up and down.and starts anytime!

1 Answer

21,805

Typically, if it's backfiring out the exhaust on deceleration, it's because of a diverter valve for the air injection that is stuck or not receiving the proper vacuum signal. It could be dumping fresh air into the exhaust when it shouldn't be. The Idle: some possible causes are an idle set too high causing the EGR to open and close and perhaps float settings. It sounds as though it needs a little TLC. Often, the exhaust manifold warps causing the intake to warp with it and creates a vacuum leak along with the exhaust leak. Both manifolds are bolted together and are quite heavy to hang off the side of a motor without checking the torque of the nuts a couple of times a year. Someone sets the idle high to try to keep it running with a vacuum leak and the EGR activates. Kind of a cascade effect.

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