This brings back alot of memories, mostly motocross. Speaking of racing aroma, I was Bel-Ray man myself, but when I started, there were still guys mixing Castrol R, which was fabulous!
I would check the jets, the float valve, (very prone to sticking after prolonged inactivity), and anything else that could be subject to gumming up.
A 140 main vs a 250 stock main is a very lean switch: Check the colour of your spark plugs....