Engine Timing

It would help us to help you if you explained HOW you set the timing. It might also help if you mentioned the model year of your Mustang.

But in answer to your basic question. Timing is one of the possible causes of back fire. Other possible causes is mis-routed spark plug wires.

Note, a common mistake that is sometimes made is confusing the cam sensor bump for the left hand cam timing dot. If this mistake is made, then the left side will be about 180 degrees out of time to the right side. The motor may run but won't be smooth or make good power. If you note a BIG temperature difference between the right/left sides, think 180 degrees out of time as a possible cause.

If this is a coil pack model year (98 older), then double check the spark plug routing before digging any deeper.