[pete]

If the bike has no mods...
ie... air cleaner / exhuast etc
then any jet change needed will be close to
what yamaha put in it...
if you have to go along way from standard jetting
there is other issues going on in the carb , intake
or exhaust systems , cam , ignition timing..
I would replace ALL brass in the carb to start with..

get a length of clear tube put it on the carb bowl drain
hold it up the side of the carb open the drain screw..
it will show where the fuel level is... should be at the
fuel bowl gasket...