Larry,

Don't know if this would help any; but, on single ignition engines timing can drastically affect RPM. If everything else is equal (thottle the same) advanced timing will result in a higher RPM......retarded timing will result in a lower RPM...retarded timing taken to the extreme (short of killing the engine) can also result in a big RPM reduction with the charge burning partially on the way out of the engine which would possibly raise the EGT.

When you mentioned 50 and 500.....the 50 almost seems like less of a drop than a person would normally get on Rotax. If considering timing....is there any chance that the side which drops off 500 and raises the EGT is running on a severely retarded timing setting?

I don't have a clue how timing is controlled on a Rotax. Don't recall if you have the soft start (which I believe retards the timing to prevent kickback) and I don't have any idea if there is any possibility of the soft start affecting the running mode.

Again, not saying that I know anything about what the problem is; but, thought I wanted to share this possibility in case it might play into the issue.

Wish you luck in solving this.

Dave S