Benelli K2 100cc Acceleration & Top Speed Issues

[Snek]

Hi, I'm new to this forum and I hope that someone could help me out with an issue I'm having.

I have a Benelli K2 100 (2 Stroke), It's all standard apart from a Endy Performance exhaust and 5G rollers (Rollers are near new). The problem I have found is that from a stand still it will have great acceleration up until about 40mph, then it drops off and at 50mph it sounds like it's going to blow up! I have achieved a top speed of 55mph on a good day, but have read posts saying that the top speed should be between 60-70mph (In a stock form). I am also a light rider, weighing 9 stone (126 Pounds). Does the 60-70mph seem a bit to ambitious?

The other issue is that if I take the bike down to 10mph or so then try to accelerate full throttle (Or any position), the acceleration is not as good as it is from a stand still. It's like the RPM's rise slowly and are not as high. Also, from a stand still it pulls a little slow up to about 5 mph then shoots off, the RPM's then drop a little at 20. The bike has 5100 miles on it, from new. Air-Filter is clean, and plug is near new. Drive belt is fine and looks OK.

Any advice? Or is this completely normal?

Thanks.