That was my off the cuff diagnosis too until I read it was Qjets. I have never seen a coil cause an intermittent problem. Got a bunch of coils laying around from everytime I convince myself it's the coil. Never is.

I've had intermittent coil problems in the past. Boat would run good for 45 min, then the motor would start back firing and breaking up at higher rpms. Replaced coil, ran perfect.

Anyway, since you just got finished working on the fuel, I'm thinking it is fuel related. Swap carbs, check fuel filter, if still the same it's gotta be spark. Ignition modules will have the same effect so you may want to swap modules if you still can't figure it out.

Is it backfiring out the exhaust or carb? Also this filter you replaced; Where was it's location? Was it the one in the inlet of the carb? If so did it have that little rubber springy thing in it that is suppose to prevent backspillage should the line be severed? If so there is your problem.

I have learned here that some coils do go bad. They can breakdown and cause an on/off effect with all of the secondary ignition. That will certianly cause a backfire. If the connections are clean then I believe he should stick to looking at a possible fuel problem since he was loading up.