Rob says that the SR42 (800 engine with the older 750 style airbox) should run just fine with 125 main jets. Even with a more open exhaust.
I dropped my needles back down the the middle position but, being a bit conservative (and not wanting to make too many changes at once) have kept the mains at 130. This is using the standard SR41/750 airbox but with a GSXR 1000 silencer.
Thanks for all your advice (and everyone else' too)
Yesterday I started the bike with tank off using a remote fuel source. Standing over the 800 engine without any body work on, I noticed at once that this engine was MUCH quieter in the top end than my old 750 engine. Overall the engine seems much tighter (it was at 37,000 k. when purchased).
The bike started right up once I got the decomp problem solved.
I noticed that the decomp. shaft that goes into the cylinder head is different for the manual (SR41/42) system than the shaft for the "automatic" (SR43) system.
I should have take photos but was in the middle of my S. Holmes diagnostic state tempered with abject fear that I'd done something mental and ruined a weeks work....so no photos.
Anyway....both shafts have a flat spot ground into the shaft at about the 9 o'clock position when looking straight at the engine from the right side.
The SR41 shaft has an additional flat spot that the SR43 shaft does not possess.
This, plus the fact that the 43 shaft did not protrude far enough to play nice with the manual cable from the 41 led me to believe that I could just swap all the pieces.
Everything fit together and the bike started on the next try.
The tank went back on yesterday evening.
The seat, bodywork and crashbars will be mounted today and I should have a ride sometime this afternoon.
I'll see if I find any differences between the 750 and 800 engine. But to be fair, I think my 750 engine is a bit tired.