I have a 77F250 4WD with a 400. I drove from Wisconsin to San Diego a few months ago and had no issues with the engine. Recently, it started to backfire through the carb when accelerating. I checked the timing and it was was running at 2 BTDC. I adjusted it up to 10 BTDC at idle. This helped. Instead of backfiring all the time when accelerating, it is only doing it when I accelerate hard. I checked the vacuum advance next and the diaphragm was shot, so I replaced the vacuum advance. This seemed to help out some, but it still backfires periodically on hard acceleration. It also seems to be worse after the engine warms up. When it is warming up, I can accelerate any way I want with no backfire. Has anyone else had similar problems?

 

check your plugs?

 

Sounds like the accelerator pump stroke is too short.

 

I would try changing the acc pump diaphragm and double check the stroke like fmc400 said. If that doesn't help I would check the color of your plugs. If your on the lean side they will backfire through the carb more. I had that problem before.

 

Just fixed that same prob with mine. Replace spark plug wires and make sure the firing order is correct. Someone had my spark plug pluged in in the wrong order. IDK how it started and ran. If u need the correct order, let me know.

Mine rund really well now

 

Thanks for the info. I replaced the plugs, wires, cap and rotor and the problem has gone away.