This is not CL, but I need advice from a Merco scholar.  I've got my 61 installed in an old restored trainer.  It has a Kavan carb, and I was able to make a muffler from my Tower Hobbies 46 fit beautifully.  I'm running Powermaster GMA 10/22(50-50 syn. and castor).  The plugs I'm using are OS A3's, and I'm running a Zinger 12-5 prop.  The engine starts easily, has a great idle, super transition from idle to wide open.  The trouble is that it won't hold a steady run at the high end.  It'll sag a bit, then pick back up, and although I didn't have a tach, it doesn't seem to be revving up as high as it should.  I think the engine is running hot, and there could be several causes. It may be over-propped, but I can't see where a 12-5 could be too much for a 61.  The engine may not like the back pressure from the muffler.  There may be a leak somewhere, but I can't find it.  BTW, I'm using a Hayes 12 0z. tank with muffler pressure.  I think my first option might be to run it w/o muffler pressure, but I don't see where muffler pressure would cause overheating.  The other thing I'm going to try is removing the baffle from the muffler.
There is another thing.  I've noticed that sometimes engines will run a lot better in the air than on the test stand, but I want it right on the ground before I put in in the air.  Am I on the right track?  Suggestions?

Thanks a bunch, George.  I took the muffler apart and found that, apparently during a moment of "brilliance", I had sealed the baffle in with red silicone.  Well, the silicone had made a mess and was partially restricting the outlet.  Now the muffler is clean and the baffle has been consigned to the "stuff" pile.  Hopefully, that was the problem.  We'll see.  I do RC, and I like it.  But CL is sure simpler, and much more exciting.

Glen

I ran the engine, today, on the test table.  Taking the baffle out of the muffler made all the difference in the world.  The high end is stable, the engine transitions from the idle(which is really smooth, with no loading up), through all ranges very smoothly, with no stumbling.  A concern; though, is that it is only turning about 9800 at the top.  I'll probably need a long take-off run, but I can handle that.  I did notice a tiny bit  of seepage from the head gasket; that is probably bringing the power down, some.  I'll probably call MECOA and see if they can supply some head bolts and gaskets.  All-in-all, a good engine.

Glen

Hi Glen,

I have several of the Merco engines and I never run them over 7800 rpm's. They do not like to run high rpm's. They will give you a lot of power at this rpm, if it not enough, you need to find a different engine. I use a 13/6 APC and 10/22 GMA fuel and the engine loves it. Treat her like a lady and she will purr for ya. Treat her wrong and she throw a fit, Gary

Interesting you state C/L is easier, we've had several R/C fliers join us and they decided it was to much work flying C/L, so they went back to R/C.