There seems to be a ghost or a gremlin in my engine. I have an older K&B .61 that I got second hand a while back. It's a strong engine when it wants to be. I've noticed something though. Sometimes it will have awesome compression, then other times it doesn't seem to have any. I had to replace the bearings a couple months ago, and I noticed that the sleeve is in almost perfect condition. I removed the piston and cleaned out the slot the ring sets in. Everyting internally looks fine. I had it out this afternoon and I noticed that it had good compression through the first 3 flights. When I went to start it for the 4th flight, it had very weak compression. It started and ran, but kind of sluggish. About 1/2 way through the flight, it just took off and started revving up like it's supposed to. When I landed, it had good compression again. I would think if it had a cracked crankcase, gouged up sleeve or a weak ring that it would have poor compression all the time. I haven't taken the head off since the rebuild, but I wouldn't think the sleeve would be turning. Anyone else have any ideas?

All those K&B .61's have ghosts in them....they come that way from the factory. They
used to have a large surplus of ghosts, and that was the only way they could get rid
of the....ship 'em out.

Seriously though, I'm been running those engines for over 35 years....and I think it's
the chrome liner that does it, and it depends on how the engine shuts down. If the
engine were to run out of fuel, or lean out and die for some reason....it would be a hot
and dry shut down....the compression would seem low.

On the other hand, if you were running the engine on the ground and richened the
mixture way up before you shut it down....you would have a cool/wet shut down, that
would result in the feel of more compression because of the amount of oil left on the
cylinder wall.

Also you have to take into account how hot the engine is. The hotter it is, the more the
cylinder and liner will have expanded...cause a slightly larger ring gap, that will result
in a feel of lower compression.

So it's the amount of oil on the cylinder wall and the ring gap that effect the compression.

FBD.

It sounds like you took all the precautions in your rebuild but it sounds like the ring is sticking

Edit: Dave and I wrote responses at the same time. I would like to emphasize that I agree completely with what Dave says about the residual oil in the engine, the engine temperature, and how long it has been setting around.

It shouldn't be getting too hot. I'm still running it on the rich side trying to use up the rest of a gallon of 10 / 20 (break in fuel). I'm going to see if I can find a museum somewhere that will part with a new ring and cylinder. I really don't want to junk it, when it's running right, the thing screams. But then again I've got it on a P51 PTS (ARF version) and it's only pulling 6 pounds. I had hopes of putting it on my Stinger when I get it built.

....is there castor in the fuel (I hope) ?

You can still get all the parts for the engine. You can also get rings, carbs, muffler, and bearings from other sources

I just rebuilt my K&B .61 with a new ring, piston and conrod from Mecoa. Before the rebuild mine would do the same thing. After some runs the compression was great, after other runs it would almost windmill. That's why I just went ahead and rebuilt it.

David

I've been experiencing that "compression thing" for years. I've fried a couple engines, but
never worn one out. They can act funny like that.....

FBD.

I won't run without it.
Normally, when I have to redo a break in, I'll deglaze the cylinder, and run some Byron's 10% with 16% oil, and add 4 caps of straight castor to the gallon.. I wasn't looking when I picked up this last gallon, it already had 20% castor / synth blend. I added the customary 4 capfulls of castor anyways. It leaves a pretty smoke trail. I'll order another sleeve and ring from MECOA. If it still does it, I'll just go with it. Too bad I can't do it like a car - fill the crankcase with 90 weight and put it on the auction block. LOL

....I'm the same way....I will not run an engine without castor....no matter what. The thin
finned K&B .61 ringed engines thrive on castor....plain and simple. In the beginning of last
season I flew my Ultra Stick 40 (K&B .61) unknowingly on some stale (last years) fuel. I set
the engine correctly, but it wasn't leaving the proper smoke trail. Just as I cut the throttle to
bring her in, the engine died. I got her in and checked....almost no compression. [:@]

I took the head off and scratched the heck out of the cylinder wall with some good 120 emery.
I ran it for about an hour on the bench, fresh 15% fuel 18% oil (1/3 castor with extra castor added)
....kinda' rich....and the engine healed itself.

....ya' gotta love the castor, eh ?

FBD.