I recently picked up an urban express. Despite tons of tweaking, it only hits 20mph on flat and it will four-stroke at WOT. I've tried it with a pretty restrictive air filter and with no air filter at all, and it doesn't seem to make much difference as far as when it starts four stroking or how it four-strokes. It does that thing where it will four-stroke for about half a second, and then sound normal for a second, and then four stroke for another half second, etc. It also sounds normal when it's going up an incline (i.e. the four stroking disappears), unless then incline is too steep. I've tried everything to get rid of the four stroking, or even change it (like, make it worse or better) but I can't even get it to change. It makes me think I'm missing something... I figure if it was a normal thing like a dirty filter or a carb issue I should be able to get it to happen earlier or later or make it more consistent or something by going through a range of filters and taking my carb settings to the extreme. These bikes have set timing, and either way the timing is really close (not dead on, which is weird, but close and I don't think its "lose 10mph of top speed and four stroke exactly the same way every time regardless of filter or carb settings" off). I just replaced the belt with no change in performance, cleaned the whole exhaust, new plug, cleaned the air filter just in case, checked for airleaks, didn't replace all the gaskets but they looked really, really fine, has good compression but I replaced the rings anyways. I'm stumped! Any help or advice would be greatly appreciated! Thanks in advance!