HI,

Sounds like maybe the compression ratio has change by skim,
not sure how much it would alter by 30thou,
are you saying it is pinking, could do with seeing what the co% is on load
to check the fuelling, maybe weak mixture, do you have the correct grade of plug's
fitted( heatrange) when it struggles as your on a carb, try putting the choke on
slightly to see if it helps, ie richens the mixture when you get the flat spot.
sounds like weak mixture,

DAVE R