did you replace with a new carb. also make sure that the choke is adjusted properly and check out the fuel pump . got to be getting to much fuel through the carb. the valves could also cause this problem.
I had the very same problem. The piston was broke on the exhaust side. It would load the cylinder w/fuel but wouldnt start due to no compression.check your compression. see if it will run with exhaust unhooked.mine would but barely.when you unhook the exhaust there is no back pressure on the piston.So it may start.hope this helps. good luck.