Hi, I have a 2005 Linhai 260

After cleaning the carburetor the bike now has a hangin rev when in neutral, acceleration is down approx 40%, top speed is down at least 20km/h.

It feels like it's running on 3 1/4 throttle.

 

Problem is, did you introduce the fault by getting something wrong when cleaning the carb out, or was that the fault that caused you to strip the carb in the first place? Fit a new plug anyway and if that doesn't help, strip the carb again and check you cleaned it properly and put it together right. Get an exploded parts picture if you can, so you can be sure everything is there and put back in the right order.