gilera smt problem

[mradiii]

hi i have a gilera smt 50. the engines the same as the dirby as you probably no.
its has a 70cc kit on
k&n airfilter
race crank +bearings
arrow race exhaust.

it has standard carb on at the min till i can get a 21mm

but i can only get 40 out of it n it like pulls back n wont rev past like 6000 in any gear wt could i do? it like pops wel not pop its like it eaither cant get enough air or fuel but im not sure iv up jetted it and its slightly better on revs but i can still only get around 42mph what could the problem be apart from having a standard carb cud the needle need liftings? and also when its up to temp after bout 5 mins warming up then steady ride for a mile in the rain and wet roads it wont rev past like 5000 rpm what could this be ?

can some one please help because this bikes the only way i get round to school n work n back home