yn50 slow up hill fast downhill and jet size for air filter ??

when going up hills my mbk ovetto (yamaha neos yn50)
just seems to die the revs will just drop and it will slow down and do 10mph :L
but if i get speed somtimes it will go up the hill for 40mph for a bit then suddenly drop
i have had a new viper exhaust put on since my old one had a hole in wich i thought was the problem but it still does the same on the hill. i have also put an iridium spark plug in but still does same on the hills
but its briiliant on flat and down hil i have had 60mph down hill and nearlly 50 on flat at 1 point
but i just dont understand why its so slow up the only thing i can think of is its because of my gasket on the exhaust it isnt very good and dosent connect the exhaust on brilantly can anyone help me with my hill problem???
also ive got a standered filter but have a racing filter to put on but goes like 10mph when its on how many jet sizes should i go up ???
would changing my rollers help aswell if so lighter or heavyer??
please post back asap.
lewis.