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