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I have a 06 grandvista 250. it is having a carb issue,the vacuume piston in the barb is not moving. the bike runs and runs well,it just is not getting enough fule/air..and top speed is like 40 mph. i have taken the carb out and cleaned it and inspected the vacuume piston diphram,and there is no visable tears in the membrane.. i have adjusted the piolt jet to about 2 1/2 turns out..pretty standard adjustment. so as my trouble shooting conticues.i have removed the vacuume piston from the bike and it runs like a champ. has tons of pep and top/bottom end and i can actully cruise with traffic..so just picking youre brain's any ideas ill take.. only thing i thought was a bad piston..vacuume is great..but not sure.. and if anyone has a carb digram and a vacuume diagram it would be awesome.. thanks  -Joshua