loss of speed

OK so I took my 80cc\66cc motor off of my Schwinn chopper and put it on a modified bmx bike. The chopper had a top speed of 26mph but my bmx bike only goes 21. Same motor I had to extend the intake about 4 inches and the gas tank isn't as steep as it was on the chopper. Could the intake for the carb be the problem? If so how can I fix it?

assuming the tire sizes for the two bikes are the same then yes it is the extended intake because the length has a resonance effect which makes longer favor low rpm power whereas you want more emphasis on top rpm power. Try a shorter intake or take some metal off the top of the exhaust port so the engine will rev higher. (2mm or so for starters)

Yes same tire size. I'll give the exhaust port a try.

youre being misled....

get a smaller sprocket on the bmx, because NO. the schwinn stingray (i assume?) while having a 20 inch rim, has a much higher profile TYRE. by about 4" or so. due to pi (3.14 or so) that increase the circumference by about 12".

i just went and held them side by side just to make sure, and my schwinn tyre is flat, to boot. its a LOT bigger on the overall diameter.


so the bmx tyre covers less ground per revolution... therefore has to spin faster to travel at the same speed.


i found a 32-36t about ideal on 20", though i do run a 28t myself... and why can i get away with that with only a 48cc engine? hmmmms... im told the 66 is far superior

are you finding it gets up hills better though? im sure it will be...