I feel your pain and your frustration. I used to have lots of troubles with Motobecanes until I finally figured out these beasts. Now I've got six of them that all run extremely well, including a red '78 Moby that I ride everywhere without a lick of trouble.
Basically, you're confronted with two choices....hang in there and keep plugging away until you finally conquer your Moby's demons, or consider it a "learning experience", sell it, and buy yourself a more reliable Puch, Tomos or any Italian moped with a Minarelli V1 or Franco-Morini motor. If you're spending more time working on your moped as opposed to riding it, then I would go with my latter advice.