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Any standard airhead from the period needs some upgrades, unless you are collecting pristine stock collector bikes. I got a '74 R90S which is hardly recognizable from stock. In my mind every modification has been an improvement.
It's possible a previous owner switched cams on the '76. In fact, it's possible a lot of swaps have been done which wouldn't visible to the eye.
Just a few general comments, remember, an R90S is no big deal from the standard 900 except for few trim items, a 3 to 1 rear gear, 9.5 compression ratio, couple of gauges, that's about it. Trannies were weak on the '74s, but you can always update that item. However, if you treat the tranny easy, always clutch it no power shifting, it'll goo along time.
I've owned mine since '86, it's my favorite bike, and I've owned a few. Just working on it this afternoon, for fun. Good luck, enjoy. Cheers. Thank you BMW.
a 75 R90 came with a 3.09:1 ratio. The 76 model had a 3.20 ratio. Might want to check that on your bikes.
Going smaller on the gap should retard the timing. I'm not sure I'd go much below 0.010". While it shouldn't be necessary, some take off the points backing plate and extend the holes for more rotation. What I would do first is try a brand new set of points if you haven't done that yet.