Sporadic top speed
 

While I have read around a little, I still new. So, please forgive me and
Point me to the right direction if I overlook something.

I have a 150cc, Roketa Super Hornet. Purchased new and have over 2000 miles on it now. It's had It's ups and downs, but, doesn't usually take me long to diagnose an issue and replace a part of needed. For the most, I've kept it stock. I'm fairly average weight and would get up to 60MPH with it at times, so, never felt the need to replace anything. Any parts that I had to replace in the past were still stock specifications. One things that's been bugging me for a couple months now is the top speed has been sporadic. Sometimes it wont do anymore than 40mph. I could stop at a traffic light, and, maybe get 50mph after that. Carburetor has been cleaned including jets and needle. Fuel filters replaced, no bad fuel, air filter is fine, no vacuum leaks, no exhaust leak, transmission has been inspected down to the gears. Engine inspected down to the piston rings. Valve clearance is fine. Spark plugs CDI and ignition coil has been replaced. I haven't noticed any other symptoms except sporadic top speed. Time of day _ night or temperature doesn't seem to be effecting it. One thing that I have replaced are the tires. The stock tires had a very poor compound. They slid a lot and I fell off a couple times. No problems with the new tires. I cant remember if the top speed was effected after I replaced the tires, but, I don't see why that would be sporadic.

Edit: Stand by. I may have figured out the problem, but, I need to continue to test it to be sure if the problem is fixed and not just having good luck. Because again, the top speed is sporadic. Sometimes It's fine, sometimes it isn't.