Oil Blockoff Question

yes you can just remove the gear and plug the holes make sure they are plugged good though and air tight especially the one going to the carb. As for the jetting. you dont need to change jetting just because you premix its just preffered to premix over the injection. you only need to jet when the amount of air or fuel entering the engine and leaving the engine changes
 
The oil thins the gas. Thicker going through the jet. I have a fmf pipe, K&N filter, and Six 1 1/2 inch holes in the cover. Did have a 250 jet in it. Now i rebuilt the top and bottom end and need to break in. But wanted to use premix for break in. So is 270 big enough?
 
The oil thins the gas. Thicker going through the jet. I have a fmf pipe, K&N filter, and Six 1 1/2 inch holes in the cover. Did have a 250 jet in it. Now i rebuilt the top and bottom end and need to break in. But wanted to use premix for break in. So is 270 big enough?

again, premixing has nothing to do with your jet size. your jet size it determined by the mods you have done to allow more/less airflow into the motor. a 270 would be about right for a stock blaster with reeds, pipe and airbox lid on mabey a touch lean.
 
i think it will definitely be lean with those mods you listed since the air box has holes in it. i would try a 290 or 300 for starters to make sure you are rich and work your way back with some plug chops to the magic number.