I think I have this engine broken in. It ran very good on the test stand, with clear spent caster oil. When I fly the plane, a King Kobra, it leans out, and the oil turns dark to black. I tried richening up the mixture, as to get a stumble on full throttle. I still get the black. I looked at the cylinder walls, and they look fine. Could this engine still need more break in running time?

I have a Tower 75 engine that ran the same way on the test stand and once on the plane it would begin to lean out. After much inspection I realized that the low speed needle would suck in once the engine started. feel the looseness of your low speed needle and see if that is the problem hope this helps.

Ramelen1, I had a problem with black oil while breaking in my
Tower 75 engine, and found the problem to be a loose end cap
on the muffler. It was loose and vibrating and rotating around
causing the aluminum to wear, turning the oil very black. After using some lock-tite on all the threads and tightening
everything up real tight, the problem was solved. I suspect that
is all that is wrong in your situation with your Tower 61. Good
luck.

My muffler end cap does loosen up, and spin. I don't see how this is making the oil black?

The black is aluminum vibrating/rubbing against aluminum like in a loose end cap situation. I guess fine particles find their way into the exhaust, so the story goes. Seems to hold water based on my experiences.

With the loose end cap moving around against the other
aluminum housing, it is polishing the two parts where they
contact each other. If you have ever polished aluminum you
know what happens, the polishing cloth gets black really
quick. So is the oil, as it flows over this area that is being
polished by movement of the aluminum parts polishing each
other.

I'm not good at explaining things, but believe me, this condition
will definitely turn the spent oil black.

I guess you guys are right. The oil was kind of black, with a gray tint to it. It also wasn't clear. It was more like a paint. It was light the first couple of flights, then it turned dark. I thought I trashed my engine. I have tightened the muffler as tight as I dare. I have broken that long screw before on other engines. The nut and screw are both tight when checking. Thanks for the help.

If the muffler's moving about, then aluminum particles will get into the exhaust and turn it black.

You can easily check by removing the muffler and holding a piece of white card stock behind the engine to collect the exhaust residue directly from the engine. You'll be able to easily see if you're still getting metal particles.

About the muffler. Don't get the long bolt too tight. You want it just tight enough to keep the muffler parts from moving. If the bolt's too tight, it will break when the muffler expands from the heat of running.