Hi Mark,
Your problem is not caused by the main jet, it doesn’t run on the main jet below 3/4 throttle. The problem could well be air leaks in the transfer covers, carburettor gasket and crankcase doors. I don’t know how you checked for them, try squirting WD40 on the joints. If the revs rise while doing this it indicates the engine has sucked in WD40 via a leak. Also try removing the crankcase drain plugs in case there is an excess of oil in the crankcases then refit them.
Regards,
Bob Mather.
