Answer:
Start by setting the timing. Make sure it's right before you waste a lot of time on a non-problem.
Next, examine the distributor cap. You shouldn't see any cracks or black "spark traces" anywhere on the inside of the cap. It should be clean, no moisture.
If that's not the cause, check the carburetor's accelerator pump. If that's not functioning right you can get some backfiring.
Check the compression. Sometimes a sticky intake valve can cause trouble.