And don't 'fix it' by adjusting the fuel flow to the carburetor with the petcock.
The petcock should be wide open and the fuel flow is controled by the float and the float needle valve. If shutting off the fuel allows the engine to run then you have an over rich condition. First take Venice's advice and check that the main jet is still in place. It if isn't, screw it back in. If it is in place and tight check the plastic float. It should 'float' in gasoline. If there is any gas inside it you'll need to replace it. The float controls a needle valve which maintains the proper level of fuel in the carburetor bowl.
Any engine will seemingly 'run better' if you temporarily starve it for fuel. You're causing a lean condition that makes the engine run a higher RPM than it normally should but continuing to run it like that is asking for premature engine failure. One reason for not controling fuel flow with the petcock.
Let us know what you find.
Tom