Of the two '78 Puch Sport Mark II's that I have that go into "four-cycle" mode at 23-25mph, thereby limiting the top speed, one of them I have had since '79 and it didn't always do that, it always use to get right up to 29-32mph, even with my 200+ pounds of dead weight. This is a problem that it kind of developed, actually associated with my having to screw around with cleaning the carb after it got gummed up while I was in Michigan for 10 months and it was in Florida. To be honest, it has never run right since then, oh by the way, another part of the problem, along with a float chamber full of varnish, was a rusted metering rod. I very, very carefully ploished the rod and I do not think that it is the problem at high speed because at WOT it is esentially completely withdrawn and all the fuel metering chores are the job of the main jet. (I left fuel in the ped on purpose, it had looked like I was getting a lot of rust inside the tank if I drained it and left it empty, so I leave it full and drain the fuel out and discard it each winter when I return, the stupid fuel shut-off valve leaked enough over time to let a drop or two into the float chamber which evaporated in the summer heat, and then another drop, and so on for 10 months, now I pull the hose off. The Puch fuel tank is integral with the frame down tube, so internal tank rust might eventially destroy the structural integrety of the frame, also bummer to clean out every time) I also have a '79 Newport II, same ped but for wheels and seat, it pulls 30mph anytime with me. So, I think there is something screwed up with the carb, although, that air cleaner suggestion is interesting, except I thought it was more of an induction noise silencer than filter. It isn't exhaust, been there, done that, got the T-shirt. Lots of noise but no faster! I am also going to screw around with the float system, my theory being that if the float needle/seat have deteriorated and can no longer seal off the flow of fuel when subjected to high speed engine vibrations, the level of fuel in the float chamber will be momentarily too high and result in an over rich mixture of fuel to air. And some day I would like someone to explain to me why a two-stroke cycle engine will fire only every other time (so called four cycling) when the mixture is too rich?