....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.