KevinKRC:

I had the exact same thing happen to me with my Tower .61. I didn't have nearly 100 hours on it though. I was flying around just fine, and then suddenly I realized the throttle was stuck at half throttle. I ran the plane out of fuel, and when I got it down, I realized the carburetor was frozen up. I took the carburetor off and drenched it with after-run oil. This allowed me to move it, so I put it back together, and haven't had a problem since. I've probably got 200 flights on the engine since then, but that was a weird experience. I never did figure out why it happened, as I didn't see any groove on it, like you did. By the way, the engine was not new when this happened either. It was on it's second plane, in fact.

The main issue I have with the Tower .61 is that I couldn't find a muffler extension for it anywhere. I had to stick that huge muffler out of the top of a Giles 202 because I couldn't find a muffler extension to make it clear the fuselage in the bottom. I've never seen a header for this engine either, but maybe I'm just looking in the wrong places.

-bsc