Posted by Bob M on January 11, 2021 at 15:46:12 from (172.101.186.71):
In Reply to: More backfire question posted by svcummins on January 11, 2021 at 14:39:08:
A few things to check:
1 - Ignition timing. Make sure timing at idle is set to spec. Also that BOTH the vacuum and centrifugal advance mechanisms work properly, and that the spark timing doesn't randomly "wander" - indicating worn distributor bushings.
2 - Idle RPM is set to spec. If the carburetor has an idle "kicker" or dashpot, make sure it is working and adjusted.
3 - Carb idle mixture is correct. A LEAN (not a rich) mixture promotes backfiring on deceleration. On the Rochester carburetor (both 2 bbl and Quadrajet) idle mixture needles screw horizontally into the throttle body at the bottom of the carburetor. Depending on emission laws in effect where the truck was initially sold, they may be covered by plastic "anti tamper" covers or hidden behind hardened steel plugs pressed in over the screw heads. Plastic covers can be broken away to reveal the mixture screw head. Steel plugs must be worked free then a special "double D" socket used to turn the screw.
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