Yes, it is an obvious exercize to swap parts between the two Puchs to aid in problem cause identification, however, I am reluctant to take a chance on screwing my sweet running Newport II to get my ratty Sport Mark II running at full speed again. But now that I've got a second bike with the same problem (I now have two slow Sport Mark II's and the faster Newport II) I am encouraged to get to the bottom of this. This summer, I picked up a pretty good looking carb from an eBay auction, and another off a junk J.C.Penney Pinto, so I have a bunch of stuff to work with without disabling the good ped. For now, the stuff (and I) are up here in Mich. and the mopeds are in Florida so I'll have to wait till I get down there to start experimenting.
Your hypothesis for why an overly rich fuel/air mixture causes a two-stroke cycle engine to fire every other cycle is an interesting one that I had not considered. My experience and knowledge in IC engine combustion theory, while fairly extensive, is very much limited to four-stroke cycle, spark ignited engines and I had been trying to think of how the residual trapped gases, for instance, could dilute the incoming charge to the extent necessary to prevent ignition. In fact, I wonder, if the mixture in the combustion chamber is still burning when the piston uncovers the transfer port between the cylinder and the crankcase, what prevents the flame front from burning back down the passage and igniting the mixture in the crankcase with disasterous results? Boy, I think that would get your attention. So, you mentioned a book on two-stroke cycle IC engine combustion theory, what do you recommend for me to read?