Lou55, I had a problem similar to yours with a 91fz....it would run fine full throttle, fine at idle, but if you cut back to maybe 1/4 throttle, as when setting up to land for example, it would go rich and not want to clean up again. I fiddled with the idle setting, I fiddled with the regulator, I fiddled with the high speed needle, all to no avail.
I was running my regular 15% sport fuel and somebody suggested trying 20% nitro, 20% oil. It didn't make any sense to me that this would solve the problem, but it did. You might try borrowing a tank of the stuff from somebody to see if it works for you.