50cc scooter wont go over 20mph most of the time?

Hi there, ive recently installed a leo vince tt exhaust to my yamaha neos 50 2004. I added the rollers and the springs recieved with the exhaust as reccomended. Now my problem is that my neos will be great after you start it up, and pull the throttle first time, the acceleration is what i imagine it should be all the time and i will quickly get to about 45mph, however that quickly dissapears. After that initial take off if i slow down at all then my bike will not get over 20 mph anywhere, i was coming from work today, and on a flat road my bike wouldnt get over 20mph, which annoyed me, and even more so the traffic stuck behind me. I had to go down a dual carriageway where it then shot up to about 50mph. I then tried to go home and to get home i have an awfully steep hill to get up and half way up the revs dissapeared and it came to a quick stop but did not die. I think it might either be that the contrast spring cannot keep up with the rollers and clutch springs as its still a stock one, or my bike is over-heating VERY quickly, or that i may need lighter rollers or too up-jet my carb... Im new to the whole tuning scooters thing so i might seem like im talking bull**t to you more experienced riders. But i could really use some help on this guys.
Cheers in advance :)