New member looking for tech help

Hi All i have a few auto scooters including aprilia 500 Atlantic (maxi scooter ) i have just brought a neco Abruzzi 125 this is my second one as I liked the first one so much, it is n the style of the classic vespa vbb.
Anyway the issue is I won’t go over 37 mph !! On the flat.!!
I have changed the rollers to mollossi and made a bit of difference bottom end, i have cleaned the carb and the tank new fuel etc, the spark is ok have new plug ready to go in.
Can anyone help with ideas ? My other neco does 52 mph and is standard also

Any help welcome
Cheers

 

Valve clearances and curb adjustment is my guess

 

Typical GY6 125cc engine.
Quite likely someone has changed the main jet to try and get some more out of it without success.
I've just had to re-jet a GY6 125 down to 90 main jet, (someone had put a 110 in there), moved the slide needle to the middle slot, new airbox and standard filter, it's happy now.

 

I'd agree with what's been said, also check the diaphram in the carb top !