Hi. Thank you all for your suggestions. Unfortunately most of what you have suggested already is checked. The spark plug is of “R-type”. The ignition cable and plug cap is also shielded. I suspected there was be a bad connection somewhere in the plug cap, causing extra sparks, but I have checked this without finding any faults.

I have tested two different receivers. Both of high quality. On was ppm the other pcm. Same result []
The antenna is directed away from the servo leads.

I feel I have tested everything but filtering the servo leads. Hence my question of filters. I should add that the airplane is not of the conventional type. It’s a canard wing where the engine Is placed in the rear. I have 4 servos in the wing and the receiver is placed in the fuselage. The cables to the wing mounted servos are long and passes about 12-15cm from the motor. Due to this unusual configuration I think filtering the servo cables is important.

So back to my original question. What type of filters are there that I could use? Home made or commercial. I know the power box from duralite is one possible solution but it’s a bit expensive, and it only filters the receiver from getting interference though servo leads. As I have heard, the servos them self might need filtering.

thanks
/David