This engine is one of two I got from a buddy NIP from an old military mig-27 foamie target plane kit. The backplate has the "91" stamp. The carb is horrible. Only useful at 3/4-WOT. Midrange and idle are junk. The crankshaft bushing race has worn some (even using all castor brew) but I flogged it with a bunch of props for lack of anything better to do.. It used to idle okay and just had a crappy midrange but I think the more-noticeable crank leakage might be causing the crappy idle. I'm not sure.

58°F, breezy, low humidity.
5% nitro 20% castor homebrew fuel
McCoy MC59 hot plug

MAS K-series props:
11x6 = 13,700 with muffler pressure
11x7 = 13,080 with muffler pressure
12x6 = 11,010 with muffler pressure, 11,070 without muffler pressure
12x8 = 10,900 without muffler pressure
APC props:
13x6 = 10,940 no muffler pressure
13x7 = 10,290 no muffler pressure

The 11" props must be just a smidge off-balance because they vibrated a bit. The rest of the props ran pretty smooth considering.

This engine would run a lot better with a better carb. I was going to change the mixture disk to a current-production piece but I'm not currently using the engine for anything so I'm not going to bother. Down the road if I find a plane that I want to fly extra slow, I'll clean the cobwebs and oil scum off and find a carb for them.