The yellow mixturescrew on the carburetor can be adjusted for different altitudes. In general turn the screw in for lower altitudes or more top end power and out for more bottom end. If desired the silver screw adjusts idle speed. There is another idle adjust under the black rubber cap where the throttle enters the carb. In general it should be all the way in.
Mosquito has a low top speed.
Make sure the tires are properly inflated to 36-40 psi. Check the large and small chains for smooth operation. If it has become worn installing a new one is inexpensive. If the problem seems to stem from carburetion check the screws holding the intake manifold to the engine and the gray gasket. It should be tight against the cylinder.