I have a 1985 F350 with 351. I have had this continous problem with it stumbling and backfiring through the carb. I put new carb on it, switched out the plugs and changed some of the plug wires. Ran awesome last few days and then yesterday after running for a 1/2 hour, started to do the same thing. I also have adjusted the timing and fuel mixture. Running a Holley 4 barrel. It is very perplexing. Any ideas?? Thanks.

 

When out and started it and warmed it up. Adjusted the fuel mixture a little and test drove it. No back firing. I don't think it is the fuel mixture though. It seems when it is driven for a while or gets really warm is when the problem occurs. Yesterday I drove it for 30 minutes to a job, then it started to back fire. It then set for 5 hours, started right up to cime back home and still was back firing through the carb. I think it is heat related some how, but not sure what. It is a gremlin. I hate those kind of problems.

 

How new is the carb and maybe float adjustment or something sticking give it look, other wise bad distributor possibly

 

If it's heat related, it may be the ignition. Does it use the TFI module that's bolted to the distributor? That's known to have problems with heating, and your symptoms sound similar to what others have experienced with it. See if you can find a replacement from your local salvage yard just to see if it helps. If so, the old module is probably going to die, so it needed replacing. I remounted mine onto a cooler location under the hood:

https://picasaweb.google.com/1155523...7MustangEngine

 

Changed some of the plug wires ? Not saying this is your problem, but backfires thru the carb can be either ignition or a lean condition. If you have too much slack in the carb's accellerator pump linkage, that'll do it everytime. Remove the air cleaner cover and while looking into the carb bores, slowly work the accellerator cable, you should see fuel squirt from the pump nozzles the instant you move the cable/lever. Timing can also do cause a back fire, what did you set the base timing at ?

 

My dad had a car back in the day that did this and it was a wore down cam lobe. It too had to be warm before it would back fire.