i had this problem for a little bit on my 78, it turned out to be the kill switch on the handle bar wouldnt let it fully start, once it got going enough to vibrate a little it´d trigger the killswitch and it would keep it from running, this is just something small id start with, i didnt replace mine due to not being able to find the part and wanting to keep the bike as origional as i could mine still acts up from time to time but i just force it side to side then put it back in run and it cranks everytime, hope this helps!