my 01ds650 bombardier is running to rich i suppose,the spark plugs are black and after one good run it wont start. if i put a new plug it will start up again a run until the plugs get dark and wet. how do i fix this? do i change the jets? if yes what size?

p.s. i believe its this problem because i had the same engine modded and now that i put a stock engine its doing this. i have a exhaust and K&N filter thats about it now.

any help would be appreciated. thanks.