on my 1980 850 i rebuilt mine cleaned out !its a vacume valve with manuel shut off 9out of 10 times its the vac side thet stops working witch causes over flo of carbes when on kick stand!my advise is to drain tank take off tank take valves off tank be carfull there are filters inside tank thet fall off in tank when removing valves! you can get them out thoe thru the fill hole.valves are easy to take apart pay atention to the vac parts clean and flip vac gasket over grease outer edges with white grease and replace or put back together!0n valve handle do same thing!if that dont work yamaha dealers can get you parts !email me if you have more q or a´s.tom