I have an 87 250x and i am currently jetting it, but I have a question. On the place where the main jet screws on there is a little cup type piece and i was wondering if it should be on with the open part facing up or down. If anyone knows I would appreciate your support.

 

I assume you have a Mikuni and if so the part on my Warrior faces down

 

the open part faces down, with the nitch faceing the breether vent on the bowl. The plastic keeps fuel from splashing around and causeing bogging when landing off a jump or in heavy bumps.