I don't have a huge experience with the heli engines. However what you are describing is a slightly lean condition on the low end.

Also think glow plug too. The engine is new and its breaking in...at this point in their life...small little bits of metal are grinding away inside the engine and it breaks in. This are microscopic pieces that are being clearanced as the rings, bearings, and other parts seat themselves.

If these little bits get on the glow plug coil nit will burn a hot spot in it and burn the platting off the plug coil. The glow plug can still glow....but it won't be as effective...You need to change the plug when this happens. Its a common thing with all glow engines that are breaking in.

This problem is very noticeable in the 4 stroke engines as they use the type #F plug of the YS plug. This is a hot plug...The #8 that you would use in the heli engine is a colder plug and would be less susceptible to this problem but it could be the easy solution.

After about 15-20 flights the engine will be broken in and this will be much less of an issue and the glow plug will last much longer periods of time.

Troy Newman
Team YS