Motorcycle Repair: 350 bigbear, diaphragm, big bear

Question
I have a friends Yamaha big bear 350 it starts up fine and idles fine but when hit the throttle it just bogs down.
Also noticed that the needle slide doesn't move up just stays there. Checked the diaphragm and has no tears in it, what can cause the needle not to move up?

Answer
Richard. the spring on the top side of the slide keeps it down. There are vaccum orfices under and go around to the top side by a passage on the rim of the casting. The velocity of the air going through the venturi create a low pressure point or a vaccum that is routed to the top of the diaphragm allowing the slide to move up overpowering the spring.