I got a Pro .61 a little over a year ago to put in my VMar Spit. (Yeah, I know, VMar=crap.)
The plane has been sitting on the bench for quite some time now and I figured I'd better get that engine running!
I got the engine used-ran once-here on RCU.
I mounted the engine on my test stand and removed the cone shaped baffle from the muffler. Put on an 11x6 APC and filled my test stand tank with Wildcat 10% Premium Xtra, (18% oil, 80/20 castor/syn mix). There was an old used Fox plug in the engine so I just left it in there for break-in.

First 10 oz. tank: held 13,980 RPM consistently. Peaked at 14,100 RPM. Idle: ~2300 RPM. Mid-range was rich. Low-end was... I guessed rich.
Second & third tanks: Same as first but got a slightly higher peak. Around 14,150 RPM.
During the fourth and fifth tanks I started richening the low-end. Wrong. Mid-range got worse. Started leaning the low-end and got it to idle around 1800 RPM. Mid-range still slightly rich but MUCH better. Peak was now holding nicely at 14,040. One more click leaner and it would hover around 14,100 RPM. Yet another click leaner and it would vary between 14,040 and 14,200 RPM.

The stock muffler was without an O-ring where the rear cone meets the chamber so it weeped a good bit of oil.
The front bearing-as Dave Gierke mentioned in the .61 Shootout-leaked a small amount of fuel/oil during operation but it didn't seem to affect anything. I still have that old Fox plug in it. Tons of compression and all seems well. When sunlight permits I'll probably lean the low-end a little more to try and smooth it out even more.

All-in-all I'm rather impressed and slightly deaf now. Hearing protection is ABSOLUTELY required when you remove baffles!

It'll be interesting to see if my homemade Pitts muffler can hold up to this kind of punishment.