1980 NC50 engine won't turnover

[nb4927]

There is a spark, i double checked. there is fuel getting to the carb... When I turn it over and release all you hear is the piston move up and down, its like there is no attempt at combustion..... the piston cycles up and down until all of the energy stored up in the crank is expended..... Any tips? where should I look? I have played with the choke and thats not helping... Any ideas?

Thanks