1) replace all the gas.
2) add some Sea Foam to the new gas.
Still having problems
3) check fuel lines
4) replace carb (easier than trying to clean it most times)
5) check auto choke (if it has one)
Still having problems
6) check spark plugs / replace
7) check coil (a bad coil can also cause your problem.. As the motor warms up the coil malfunctions)
Still having problems
8) check engine compression
(you can find videos for everything above on YouTube)