The glow model airplane engine whether it be 2 cycle or 4 cycle runs in a semi diesel manner. The ignition timing is created by the heat in the glowplug plus the rate of burning of the fuel. More heat - sooner combustion. Less heat - later combustion.

A rich fuel mixture will cool the combustion process thereby retarding the timing.

A lean mixture creats more heat and therefore advances the timing.

Keep in mind that the combustion process is somewhat more efficient in a 4 cycle engine. Therefore there is less fuel difference in mixture required to effect a greater change in heat produced.

Therefore your backfires are created by a lean mixture or the glowplug is too hot for the mixture you are running.

Backfires most often occur when the engine is trying to accelerate but can happen in level flight too if too much heat builds up in the engine.

Enjoy,

Jim