have not been lurking here for a while, lots going on. We traded thoughts on hard starting MD7A's a while back. You probably already know, if the compression is not up to snuff, you will not get the HP no matter what size prop. I adjusted my valves a year ago, and don't think I get the same speed as before, so I think they might be a little sloppy. I might have a real mechanic redo them, but I think the rings are probably the problem.
Unfortunately our old beasts are just getting tired!