OK,I have an older OS Max 90 2 stroke nib that I have installed in my Great Planes Little Toni.It starts right up and purrs at idle,but after it runs for a while the idle speed increases so much that I can not come in slow enough to land. I have to wait until I run out of fuel and land it dead stick. So I tether her up and start it again with the same results,idles fine for a few minutes but gradually speeds up.Here is what has me puzzled,when I trim the throttle down to close the barrel all the way to kill the engine , it still runs. I was thinking I had a vacuum leak so I have tightened all the screws and checked the O-ring around the carb to no avail. Any I deas?

Thanks Rich

It's difficult to say what the exact problem may be. If the engine tends to accelerate as it warms up, then something's making it go lean. It's possible that vibration is agitating air into the fuel that's in the tank, which then leans the mixture. It's also possible that there's some kind of air leak that opens-up after the engine's been run a bit. Rubber o-rings may be bad due to their age. The engine is quite old, and the rubber seals in the carburetor may not be doing their job. You may also want to replace the front and rear cover gaskets. You can easily make some out of baker's parchment paper. We'd not have any gaskets for the engine in the parts stock...it's been out of production for too long.