engine stop suddenly and is hard to start

I was doing a new friction drive bike until I started the engine(It was not monted yet and It was running great a hour before)It started but jammed And the crank is hard to pull(or imposible). It did the same thing last week but I was able to pull the crank and start it again. I dont know what it is. I had the good gas and oil ratio.
I dont want it to jammed while driving.

Thank you

Aaron