I have my 25cc Homelite Conversion in a CMP Hellcat. Engine is on EI with carb from a G26. Engine runs great on the ground, but as soon as it gets in the air, it 4-strokes. In turns the engine will peak out and sound right, but then in straight line flight, it four strokes again. I think it is a tuning issue, but wanted some more opinions. Is it too rich? Starved of fuel in turns causing it to peak out? Any ideas.

First of all the G26 carb is too big for a Homelite 25 in a airplane situation

The G23 carb WA167-1 11.11mm would be about the maximum size a Homelite 25 can handle on a airplane

Some of this depends on if you are running a 16 X 10 or a 18 X 8 prop and if the engine as been modified as to compression ratio, combustion chamber shape, or port timing

Another factor and one that you can apply to what you have, is that you need a velocity stack or venturi extension on your carb. You can use the one for the Zenoah or other available ones even sold by Tower Hobbies. This will help stabilize the incoming air going into the carb.

The next thing is the little vent hole on the cover of the regulator on the carb needs to be positioned down stream of the airflow or have a brass tube soldered into the hole and a piece of Tygon ran from the vent hole to a neutral pressure zone inside the fuselage

These are efforts to make the airflow around your carb be consistent

Yep, as usual W8YE has nailed it. Here is a good tuning document, including a picture of remote venting of the carb as mentioned.

With careful tuning and a free breathing muffler you might get away with the carb you have. If it runs good and doesn't stumble when you open the throttle rapidly, you're good to go....

I am betting running a remote vent to the regulator cover will cure the in flight richening problem.

AV8TOR

This sounds exactly right! I'll have to give this a try. Thanks.