TT had some issues with their heli engines in the past but R&D got them worked out.
The new engines are great. I've got a .50 and a .70 that have been fantastic.
The older ABC .36, .46 and .61 were basically airplane versions made to work in a heli.
The carbs were made too rich in the midrange --too much taper in the low needle.
A new or reground needle would help but being ABC means the correct mix is critical.
IMO, heli engines should be ringed, especially the bigger ones.
TT's new line-up (.39, .50, .70 & .90) are ringed motors with cast heads and good carbs.

Kirk