Hi all, thanks for the reply's, we have found the problem. The clue was in point 11 on my first post list; the crank seals were replaced. The friend who did it is a very accomplished engineer on two stroke engines but has no experience of Rotax, I didn't help by not passing on the rotax rebuild manual to help him put it back together.
It was a combination of incorrect balance shaft transfer gear timing and the gear spacer O ring failing.
The transfer gears were timed wrong by lining all the marks up - which is WRONG. Line the crank gear mark up with the arrow on the crankcase and line the balance shaft gear mark so it is pointing to 12 O clock!
Secondly he had reused the old spacer O ring which goes behind the gear on the crankshaft to act as a spacer to ensure the gears are in line and fully mesh. The old ring was hard and had split through and come out; it was in the bottom of the transfer gear case and had looked like it had gone through the transfer gears a few times!
I timed the gears correctly and put a new O ring it (there was one in the full gasket set). Set it as this hoping this would do the trick?
Out at Fulbeck IKR at the weekend and first practice session out with all the same set up as last time out and immediately revved up to 14250!!
SORTED !
I still have a mid range popping / missing / hesitation to sort. This seems to improve or rev cleanly through if I am more progressive with the throttle or as it happening if I back off the throttle and then gradually ease it down it recovers and revs through, hence think it may be carb related. I didn't have a chance to start swapping bits to find it.
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