Fired up my 29 101 after a bottom end up rebuild ... the motor seems to be running sweet as a nut with a nice idle ... I rebuilt it with oil control rings set with a big gap (30 thou) ... after 5 mins of running at mid range it's spitting lots of oil out of the breather .. ( early style left hand case breather) I'm assuming that the oil control rings are doing too good a job ... I've been told to simply gently file some flats on the oil control rings to allow some oil to go past them ... any advice from folk running these rings ???

Yes as Tim said, don't file the rings! If you're using a 3 piece oil ring you can reduce the ring pressure by cutting off a small portion (1, not 2 coils) of the spring. But remember oil should not be in the combustion chamber, it only makes trouble with soot and coking.

Check oil level in the crankcase frequently and reduce the oil pump delivery with 1/4 turn at the time. With little consumption the pump ends up delivering very little. Therefor it is important you have a check on what roads you are entering, a fast long stretch might require you opening up the pump a 1/4 turn. (Before, not after!) A bit too much oil is in any case better than too little oil!

I allow an overflow of 2-4 seconds when opening the level screw, up to max a deciliter is ok in my opinion but I'm getting anxious when no oil or just drips coming out...

I meant to say that I've always assumed that 101's were very oil smoky ... this engine is not smoking at all ... but there is plenty of oil in the cases for sure .. I would prefer to avoid fouled plugs and I've had mixed advice on exactly how the total loss system disposes of surplus crankcase oil .. I can see now that it's a careful balancing act for the first few runs until an optimum is reached ... I am feeling more relaxed about my bottom end rebuild .. and the crank balance seems pretty smooth .... no nasty vibes ....... yet .....