My OS 33GT is acting up. If I turn the prop over by hand, the compression stroke sounds fine and the compression is good. On the exhaust stroke, I hear a distinct metallic clink. As one might expect, it sounds even worse running or with the starter applied.

Before I start disassembly (which I'm not all that comfortable with), does anyone have any ideas what might be wrong?

Sounds like a bad crank or rod bearing. Pull the cylinder off and you will quickly see if this is the problem. In many cases the top rod bearing is the cause which is an easy fix if it hasn’t started to come apart. Let us know what you find.

Thx Truckracer. Tomorrow's project.

I've seen where a bad thrust bearing will allow the connecting rod to contact the case.....when people applied the starter. See if you can move the prop shaft back and forth. Might be able to verify by pulling back cover, and watching for contact.

Just to close the loop, I disassembled the OS 33GT until I hit a road block. Couldn't get the crank pin stop screw out. I know its a left hand screw but I didn't have a wide blade screw driver and even heat would not budge it. In any case, I didn't have ring pliers either. Before going to the HW store I noticed that the clanking noise was gone when I rotated the shaft with the head, backplate, and muffler off. So I put it back together one piece at a time, testing for the noise at each step. Eventually it was all back together, sounded fine hitting with the starter with the ignition off and then ran fine with the ignition on (apart from needing some fine-tuning).

During disassembly I did notice a washer on the table that did not belong to any part of the engine. I think it fell out of the muffler and was what was causing the clank and the horrendous noise with the engine running. How it got there, I have no idea. At least I got the carbon build up cleaned off the piston while it was apart.