So i just got done rewiring and replanting all of my ignition parts because i was having trouble with my motor cutting off. It ran nice for about a week. But unfortunately my gas station quit selling non ethanol gas. Ever since i have been running ethanol gas i have had a nasty exhaust backfire and the occasional carb backfire. It only does it when vacuum changes. So when i start giving it gas it backfires until vacuum rises back to it's normal position. I can rev it in park and it doesn't really backfire but if you put it in gear and hold the brake while accelerating, which puts strain on the motor then it will be a constant backfire. I'm leaning towards it being the gas but I'm open for suggestions.

 

It sounds like your engine is running excessively lean, like maybe a vacuum leak. Do you see normal vacuum at idle?

Otherwise, what kind of carburetor is it? Try blocking off the air inlet a little and do your load tests to see if enriching the A/F would help.

 

I just figured out i have one bent pushrod on a exhaust valve and one bent pusrod on a intake valve. Time to pull the valve covers and replace them. also it ticks like crazy and you can tell it backfires off the same cylinder everytime.