Hey guys, I recently swapped a virtually new 300 into my 73 f250. I bought the engine out of a school bus from my high school and i saw paperwork proving the engine was bought new from ford ~1100 miles before I got it. It is an industrial variant as it has the HD exhaust manifold and the cast iron thermostat housing. I rebuilt the original Carter YFA, added new plugs, ran a 2 inch exhaust through a flowmaster i had laying around, HEI distributor and it fired right up and purred like a kitten. The only issue was that it would backfire badly out the carb and the tailpipe under load and when blipping the throttle in neutral. if I brought the rpm up slowly un neutral it would be okay til a certain point and would start popping out the back. I could see a stream of gas with the accelerator pump. Having little faith in my rebuild i bought a brand new carb. Dropped it on today and i still have the same issue although it did get marginally better. I have tried tuning the carb and verified timing. I have no vacuum leaks. I know my vacuum advance is working because i tested it with a vacuum pump. I dont know what else to do here. Its very frustrating because it runs perfect, and I mean perfect, at idle and just off, but once i try to drive it, it just blows out the carb and the tail pipe. If i floor it quick in neutral it also backfires out the carb. Hopefully you guys can give me some direction here. Thanks!

 

I think you should look at the valves. One may be hanging open. Put a vacuum gauge on it and see if it is steady or quavering at idle.

 

It could be weak spark.

 

I did some more investigating today. Started pulling plug wires while it was running and #1 wasn't firing. Pulled the plug and it looks not to bad. a little oily maybe but not fouled. I verified the wire is good with a spark tester. I pulled the plug and switched it with number 2 which was firing and there was no change. #1 still not firing. I pulled the plug and started the motor to listen to compression. Sounded okay. Compared to #2, #1 is a little weak but not by very much at all. I will do a proper compression test later this week to verify. I also pulled the oil cap to check for blow-by and there is none at all. I can see the valves moving fine on #1 through the oil fill hole. I will take the valve cover off to make sure later this week. Something I didnt think was much of a problem before was mud dobber bees. When i pulled the motor there were mud dobber bees nests in other places and they had gotten into the cab bowl vent. I think its possible that the runner for #1 is plugged causing it to get no fuel. The plug out of #1 did not smell like unburnt fuel like it normally should if it wasn't firing. The truck also doesnt sound like its running rich. I will try to look down the manifold with a borescope when I get time to see if anything is plugged.
Thanks!