FMF Slip On says "the silencer you purchased will not fit on OEM"

It won't take but a few minutes to take of the carb and float bowl to look at the main jet, running richer mix like 25-1 will help a little but you really need to plug chop to get it right
 
never assume the previous owner did anything..... but wrong, confirm for yourself !
that main should/may need bumped up to a 240/250 for the oil injection delete, to account for the new mixed in oil, which slightly reduces the amount of fuel.
then plug chop to confirm it's correct.
Amen to that ! Mine just had the line cut flush on the carb from the oil pump, and was not plugged !