Rick,

I dont fly helicopters......but clearly there is a problem either with the engine or the mechanical system of the helicopter. I understand that cooling in a helicopter can be a real problem (the fan is small & the engine shoe-horned into a very tight & congested space).

If the engine is slowing down at full throttle, but after 5 mins it begins to slow down, certainly sounds like an overheating situation.

As a starting point, I suggest you bench run the engine using the same fuel & watch what happens. If it will run out the tank without any problems, then the engine is not the issue.

Is there tightness in the helicopter bearings? Binding of shafts?

As I mix my own fuel, I am not familar with the fuel you are using. What is the oil content? Regardless, I would suggest AT LEAST 20% oil.

If all the above are ok, then perhaps the cooling needs to be increased. Extra ducting for both the in & out air may be the answer.

Finally, is the engine large enough for the helicopter? A more powerful engine would not need to be run at wide open throttle & do the job more easily.

Bone