Re: Topping out at 20 mph...Too much fuel ?
Once the motor is warmed up, turn the choke fully on. If the motor doesn't die, you probably have an air leak.
Just thought I'd throw my $0.02 as a side note...these are not for kids. A kid that's good at riding a dirt bike, maybe, but not for most. Especially where there are cars. Most states won't let kids ride these anyway.