This problem only occurs after the truck has warmed up. My 1980 f-150 with a 351M backfires when I let off the gas. Actually it cackles and pops numerous times. The truck seems to run fine, I have tried the typical ignition cures but no luck.

 

Check for an exhast leak around the exhast manifolds.

 

Did the typical ignition cures include a timing check? Also see if the timing pointer has bent causing you to mistakenly set the dizzy too far advanced. Another thing to check is the vacuum advance. If it's connected to manifold vacuum you could get too much advance when you let off the throttle.

 

My guess is it is the smog pump. If your not getting vacumn to the GULP Valve on the pump that is most likely it. To check just disconnect the belt and drive it and see if it stops. If so then you need to fix your smog system. Don't run it too long without the pump being hooked up.

 

check the timing

 

how do you check the timing? I know you have to have a working timing light.

 

Timing check: Find timing setting (degrees BTC) on the vehicle emissions sticker (should be affixed to upper radiator support) or other source. Find timing marks on harmonic balancer and highlight proper mark with a white or yellow pencil. Remove vacuum hose from distributor advance and plug hose. Attach timing light per manufacturers instructions to #1 spark plug wire, battery and ground. Start engine and let it idle. Aim timing light at timing pointer adjacent to harmonic balancer. If timed properly, the highlighted mark should line up with pointer. If not, loosen distributor clamp bolt and rotate distributor a little at a time until mark and pointer coincide. Shut off engine, tighten clamp bolt, remove timing light, unplug vacuum hose and reattach hose to distributor advance.
P.S., In addition to retarded timing, backfiring through exhaust can be due to excessively rich mixture caused by choke partially closed, dirty air filter, bad power valve, main jets too large or other carburetor problems. Pull spark plugs and check for wet or black, sooty deposits on insulators indicating overly rich combustion conditions. Plug insulators should be tan or light gray. White or blistered means too lean.

 

I appreciate everyone's input. Actually one of the first things that I checked was the timing, it seems to be fine. It is hard to check as you can't see the pointer from the top and you have to lay on your back and hold a mirror and get your neighbor to turn the distributor. The other day I checked every vac line and checked carb mounting and intake mounting bolts. It acts like it could be a vac leak but if it is I can't find it. I am thinking though that the problem has something to do with vac as the problem only occurs after the engine has run for a time. Maybe the vac advance or the carb dash pot, what do you guys think? Thanx

 

There should be a emissions valve on the AIR tube (and I believe on the back of the heads) to the CAT. Sometime the valve will fall apart and cause the AIR pump to pump fresh air into the exhaust causing backfiring...

(as kermmydog said)...

 

I’m very interested in hearing the solution to this. I have the exact same problem. Although I just had new exhaust put on so I don’t think I have cats, nor does my state have emissions.



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I have a gurgle/rattle noise when I get in the gas as well. Do you have that? It almost sounds like the carb is gargling or something. I can’t help to think this is related to my back fire problem because I will get into the gas and hear the gargling, get the Rs up and BOOM pop pop pop pop...... Any answers on this? <O</O

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