Need help with a carb question !, backfiring when I let off the accelerator |
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Need help with a carb question !, backfiring when I let off the accelerator |
pin31 |
Apr 8 2007, 06:52 AM
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#1
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Member Group: Members Posts: 398 Joined: 30-January 07 From: Newport, Rhode Island Member No.: 7,492 Region Association: North East States |
Here's the data:
PO owner modifications: 1974 2.0L w/dual weber 40s stock fuel pump with pressure control unit (w/return line clamp backed off). pressure control unit = guage with a clamp on the return fuel line (PO installed for autoX). The idea is to restrict the return line to increase pressure from street driving 3-4psi (clamp backed off) to 7-8 psi (clamp restricting return line). My modifications: Removed points and installed Pertronix II electronic ignition. Have not changed/adjusted the timing Observations: Car starts fine, idles great (after initial 5-10 sec of throtle play when starting cold) Car drives great. Plenty of power in all RPM ranges. Running a bit rich (plugs are black) The car backfires when I let up on the gas (when driving more aggressivly in higher gears) not so much (or at all) in 1st or 2nd or when I'll taking it easy. Because the car runs great (except for the backfiring) I did not want to start blindly messing with the carb adjustments if it could be something else. Could it be: carb mixture? floats? pressure control unit? stock fuel pump instead of lower pressure carb pump? timing? etc....... What do you suggest ???? Thanks !! |
rhodyguy |
Apr 9 2007, 01:56 PM
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Chimp Sanctuary NW. Check it out. Group: Members Posts: 22,093 Joined: 2-March 03 From: Orion's Bell. The BELL! Member No.: 378 Region Association: Galt's Gulch |
in the side view picture, the idle jets are beneath the 2 brass screws adjacent to the fuel inlet, actually they are the jet holders. on the bottom side of the carbs the idle air mixture needle screws are the 2 center ones, they should have a spring on them. the 2 outer screws are the air bypass/bleed screws. note, there should be a little o-ring under the idle air mix screws. inspect them closley for cracks and deterioration.
in the top view, the main jet/ac/emulsion tubes are the brass items adjacent to the throats. if you think you ned to blow out any of the circuits of the carbs, be very care with the air pressure you use. the little cans of compressed air work well for this. the cb manual has a very comprehensive trouble shooting guide. k Attached image(s) |
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