[kcjones]

You need to time it with a light if at all possible. If it's too retarded or advanced, yes, it will cause backfiring, especially if there is also some carb tuning needed. How did you time it without a light?

To answer your carb question, basic carb tuning probably isn't going to help with the problem you're having - full throttle acceleration at low rpm/heavy load (I assume this is what you mean - get into 4th or 5th gear, step on it...). Generally, if you're having problems immediately upon dropping your foot, it's in the accelerator pump system: cam, lever, pump diagram, pump nozzle. If it runs ok if you feather the throttle a little, maybe it is running lean at heavy loads...in this case, maybe your secondaries are opening too early? What type of carb? Is the backfiring at a specific rpm? Maybe since you changed your coil to a stronger coil, there are other things in the ignition system to look at? Wires, plugs, plug gaps...