Timing can do it, but so can a lean condition. Lean (insufficient fuel) can also cause a miss. A lean mixture burns slower which means combustion can still be underway in a cylinder when the exhaust valve opens. This ignites the charge in the intake manifold intended for the next cylinder in the firing order - and you get a backfire through the carb. So check your distributor/timing as suggested, but also look for something that is causing a lean mixture (clogged jet, low float level, etc)
