More backfire question

SVcummins · Jan 11, 2021

Now since I put the non fouler on I get a backfire when I let off the gas . 1972 gmc 5500 350 engine

old · Jan 11, 2021

Air clean good and clean?? Choke wide open. ? Sounds like it is running a bit rich so when you let off raw gas get into hot spots in the exhaust

SVcummins · Jan 11, 2021

New air cleaner .. I will check on the Choke anybody know what screw to unscrew to lean it up a bit Rochester carb

rustred · Jan 11, 2021

the 2 mixtures screws at the base of carb. set for the most rpm then can screw them in 1/4 turn. set idle at 650 as 700 is the common setting. make sure timing is set to spec. first.

rustred · Jan 11, 2021

i thought you were setting timing before. but you must make sure the advance is unplugged . you might have retarded timing doing that backfire.think the spec is 8 degree's btdc. on an old engine go 2 more degree's due to wear in timing chain . should be starting with a touch of the key each time. like no engine cranking soon as i piston fires it will be running.

rustred · Jan 11, 2021

and the mixture screws out 1 1/2 turns you are real close, going by ear, without hooking up a tach. everytime i set a carb i count the turns out from seated.

Bob M · Jan 11, 2021

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.

gbs · Jan 11, 2021

if you hold the the gas peddle down just enough to keep it above idle while slowing does it still back fire? if not then check for a leak in the exhaust system near the engine.

UncleTom · Jan 11, 2021

Does it smell rich or any black smoke from the exhaust?

David G · Jan 11, 2021

Ya got an exhaust leak somewhere.

ASEguy · Jan 11, 2021

To be clear, don't adjust the choke unless you want to change cold weather starts.

grizz02 · Jan 11, 2021

Indiana Ken · Jan 11, 2021

1972 is getting into the beginning of emissions equipment. One of my motorcycles (1987 Kawasaki) has a diaphragm operated valve that prevents back fire when the throttle is closed. I don't recall how it works off the top of my head however, when the diaphragm gets a hole in it the bike pops and backfires. I don't know if your truck uses such a system however, I thought I would mention it. Other wise I agree with those who posted to look for an exhaust leak.

4520BW · Jan 11, 2021

Vacuum lines all on? Had an 84 chevy and of you removed the Vacuum line in back of carb, kicked the 4 barrel in the let off the pedal ot wod shoot 2 foot flames out the exhaust. Use to do that on the wrong side of town for fun end scare the hood rats.

wore out · Jan 11, 2021

coonie minnie · Jan 11, 2021

I agree.

muleman51 · Jan 11, 2021

Just showed this to my wife and she said "what the ----"

Traditional Farmer · Jan 11, 2021

With the anti foul extender its not burning all the fuel mixture in that cylinder.

SVcummins · Jan 11, 2021

No black smoke

SVcummins · Jan 11, 2021

Did she say how to fix it

juniur · Jan 12, 2021

either its running to lean or the timing is slow. if it has exhaust check valves for the emissions those will cause backfires as well. is it popping in the exhaust or the intake?

CaseChev · Jan 12, 2021

Had a very similar problem on my 350 truck engine. Chased a lot of stuff before I replaced the base gasket where the carb bolts to the manifold and also when the carb was off, I tightened the base to bowl bolts. Good luck

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