My son has the above bike and it has started to accelerate very slowly when cold however it eventually tops out at an indicated 42mph when warm.

Its starts and idles OK.
I have replaced the plug and air filter with no change.

Looking at the carb I noticed that the idle jet is marked 35S, which I believe should be a 0.35mm bore. I have left the jet soaking overnight in cleaner and blown it out with compressed air and sprayed cleaner thro' the jet.

I can run a 0.22mm dia wire thro' the jet but can't with a 0.31 dia wire.
Am I onto something here?

Any thoughts would be appreciated.