Back fire problems

Explain the timing, you talking about total advance, or what?

What do you mean "Bacfire," is it puking back out the carb?

Under what conditions, high RPM lots of throttle, or initial off idle?

If off idle, could be the accellerator pump is not working, not enough initial timing

If high RPM you could have a bad plug wire, two crossed, or low spark voltage, or even a flat cam, or valve hung open.

We need more details on how you set the timing

Does it idle OK?

Does it "rev" OK with hood up, in neutral? Have you visually checked the accellerator pump shot?