I have a Homelite 30cc on an UltraStick that I’ve flown several times and it has been running great with a MA 16x8. This past weekend though, when I arrived at the flying field, it was raining. After about 10 minutes, the rain stopped and the sun popped out (typical weather in South Carolina) making the air very thick, muggy, and humid. When I cranked the Homelite, I could not get it to run as well as it had been. It would not rev up to the full RPM like it was supposed to. One of the guys at the field said it was the extreme humidity that was causing this. We tried adjusting the needles and it helped a little, but it still would not run correctly. Was it indeed the extra moisture in the air that was affecting it, or is there likely some other problem? This is my first plane with a gas engine and I’m still learning about them. I’m hoping to work on it this weekend. Thanks.

High humidity can affect engines by raising the "density altitude" and causing an engine to lose a little power and run a bit rich.

Having said that however, other than possibly a very small adjustment to the carb mixture needle(s), it is nothing that would seriously affect the performance of a model airplane engine.

Check and/or replace your spark plug, then check the fuel system. Filter in carb clogged? Fuel old? Regulator diaphragm in carb old/stiff? Etc.

Nobody has ever been able to see or catch one, but sometimes little gremlins go to work on planes between outings. They can cause all sorts of problems....

Good luck,
AV8TOR