I have a '78 Puch Maxi Sport that had the same problem. (The Maxi and Pinto are the same 'peds, just sold under different names). After many frustrations of replacing spark plug, cap, fuel line..I dismantled the engine and cleaned it all out...then it worked. From what I could figure...there was so much carbon build-up in the cylinder head, that the gas just couldnt flow properly. I would reccomend dismantling the engine and cleaning it out good. Make sure you clean the carb as well. If thats too much work, or if you dont have the tools, try cleaning out the carb and cylindar head and see if that works.
p.s. After i cleaned my engine I found the idle to be waaay off, so you may have to readjust the idle afterwards. Email me if you need more advice