Increase single phase AC motor speed

Hey guys I have made a ball throwing machine. For which I have used 2 exhaust fan motors - Single phase AC motor , output 11w, max current .8A, class E, RTG-cont AOM, Capacitor 4MFD, min speed -1200RPM.

I have assembled all machine. After assembly i found that motors speed is less hence ball speed is also less. Now I want to increase both motors speed externally without disasembling the machine. 

please guide me with cheaper and effective option.

seal the ball in more tightly?

11 W may not be enough for the intended purpose.