My #2 plane had been hanging in my shop for about six months. A week or so ago I took it out and flew it -- and had a crappy engine run!
Whatever changed in the plane was just a result of hanging in the shop -- I haven't done anything. Where before I would launch at 9750 RPM and get a decent run on 6% nitro, now I can't get more than 9400 RPM, and that's with about 15% nitro.
It has good compression, it makes no funny noises, I changed the glow plug, checked that the head bolts were tight, and made sure that a mud dauber hadn't built a nest in the muffler.
It doesn't seem to be a fuel flow problem -- I can easily open up the needle valve to get a 4-stroke. It just doesn't seem to want to run fast any more.
To whatever degree it matters, it was given to me as a well-broken in engine, and reported as having been modified by Tom Dixon.
So I'm kinda stumped about what to try next. Suggestions are welcome.