My mate has re-installed a used Irvine 39 in a Cougar. They used to work OK together. The prop has been changed from a 12x4 to 11x5. It used to pull an 11x5 too. The plug is brand new OS 8.

Now, what happens is that there is a big mixture change when the plane is moved from level to nose up and the main needle is very sensitive around the running point, around 4 clicks will go from way too lean to way too rich. The mixture won't hold between runs, I guess due to the sensitiveness.

Are there any things that could stand out that could cause this problem?

Cheers,

Mel.

isn't that alot of prop for that poor lil motor to be pulling? maybe overheated the engine. dunno i haven't run an Irvine but the ones at our club run the same sized props as our 40's
on secone thought i guess the 11x5 would be ok but the 12x4 might be tryin to move too much air

also did you change fuel types? nitro content? oil type? elevation? tank position? are you getting proper tank pressure from the muffler?
well thats all i can think of right now


john

Hallo!
To much nitro (more than 10% ), tank mounted with the centerline not in line with the fuel-inlet orifice in the carb when the airplane sits horizontal and too large fueltank.

Jan k