i have a '78 puch mkII and it was doing the same thing. i had switched the carb from a 12mm to a 17mm and even though i had a new air filter and such on, the main jet was too big. we put the 12mm carb jet in the 17mm carb and it helped. then my friend used a drill index to make the jet a little bigger. after every size increase we put the carb back on and tried it. now it can go full throttle without sputtering or dying out.