Slight modification to the above advice in bullet

3) If the engine loses RPM at all it's too lean. It doesn't have to die, just sag. You should hear a slight RPM increase when holding the plane nose-up, or maybe no change at all.

Btw, what kind of engine? older 4-stroke were known for kicking backwards and throwing props. I've also had a few brand new .40 size two strokes with prop throwing problems when they were flooded on start, they would sometimes kick.

You can also lower the compression of the engine or reduce the nitro a little, that sometimes helps.

Beleive it or not, some "long" glowplugs are longer than others. OS plugs are particuarlly short. I've now had 2 students with funky running engines that were fixed by going to a shorter plug. If I recall, one was an OS 46FX the other was a Super Tiger .36. One was, in fact, kicking backwards a lot. When flip starting it always took off backwards, so I started it running forwards by flipping it backwards. But going to a shorter plug fixed the problem.