Losing Top End RPMS

Well things have gone good for almost 40 hours runtime on my 80CC bke motor. I have an onboard tach and it used to turn 6100 RPM's if I wound it out. But I usually only ran it at 5000 for any length of time.
Now it starts up and runs good and strong. Still pulls good on hills and all the way to 5000, but now it wont turn any faster. Just goes flat and almost feels like its starving for fuel a little. There are no bubbles in the fuel line or filter.
What could be wrong? Could the motor just be wearing out?

Yeah, I got the old style simple slide carb on mine. Some say that the filter could be shaking and aerating the fuel. I didn't get a good look at the line from the filter to the carb to see if it there were air bubbles. That's how its running though, like it aint getting enough fuel past 5000 RPMs anymore. Maybe it's time for a filter. Which way does the black washer inside the filter supposed to be facing? Right now it is facing up

Ok I checked a bunch of things. The filter doesn't appear to be running out of fuel at high RPM's. I also pulled the carb drain screw and let it drain, then opened the shutoff and checked that it flowed real good. Then pulled the plug and found it black but not dripping or wet at all. Pulled the exhaust and checked that it flows really well with some compressed air. Not hardly any of a restriction at all. I already leaned the needle out one notch from factory setting. I really need to get a speed sprocket for this thing and quit running it wide open. I guess I need to lean it one more. But what is funny is that the thing was running before at 6000 either way. It has always ran a little rich at idle and up to mid throttle but then cleans out and runs good. It runs great between 3500 and 5000. Over 5000, you can barely feel it bucking a little then just wont go no faster. Maybe its ignition related?