Hi my gas gas is running fine revs up well. But when going up hill and get to high end revs it's starts spluttering. Any ideas? 

Thanks 

Check the fuel vent from the filler cap,

May be that the carb has been fiddled with as its a fairly old 2001 bike, main jet may be too big. There are possibly many reasons but its a starting point. Maybe ensure that the air filter is clear, clean and not restricted, hoses not kinked or crushed.

Edited July 12, 20197 yr by section swept
Additional info

If your bike has the standard DellOrto PHBH 26, they were always set up running rich on the main jet, from memory this was 118 with a 30 pilot jet. By spluttering do you mean it is running rich? If so, then change the main jet to a 115 or 112 if not used on the road. I imagine that you have cleaned the air filter. Bye, Peter B.