Dave I changed my fuel and made my own, 20 nitro, 20% cool power, 60 %methanol. All problems seemed fixed. The engine (120 sc) started after 5 flicks and runs perfectly.

Should I use caster oil in this mix of fuel and what %?

I had to adjust the idle and now it sits on 2400 with not much variation. The motor is going into a zen 90.

What prop do you reccommend. I am concerned about residual thrust on landing. 15 x 8? 14 x 10?

Thanks again

One more question Dave

This engine has a tuned pipe. It looks like a 60 size pipe. Is there much to be gained by using a tuned pipe on this 4stroke?

Or should I use an existing 120 muffler. The pipe set up is quite awkward on the zen90. The regular muffler would be an easier installation.

Thanks again

First of all, I would just go ahead and run the synthetic oil, that is what we recommend. Put a muffler on it and you'll be happier, the 4 stroke pipes are a pain and really only make the engine think it's running open faced. You might lose a couple of hundred but less hassle makes it worthwhile. This thing will probably blow the wings off your Zen anyway.
Both the 15x8 and 14x10 are way too small. This engine will turn a 15x12 or 16x10 without a problem. Shoot for about 8800 to 9000 for best results.

I also have a ZEN 90 with the YS 120 in it. Hows your CG? Did you have to add much weight to the tail ?