need help in getting more top end speed

Discussion in 'Motorized Bicycle General Discussion' started by gohst ryda, Jul 5, 2009.

  1. gohst ryda

    gohst ryda New Member

    Jun 17, 2009
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    hi i have a 80cc, yeah i know 69cc, motor , i have moded the exhust byb putting on an expansion chamber, runs very well , awesome power , goes up any hill no prob no peddling, but around 35kmph it seems to lack power and kind of wants to stall its self at around that speed. with the stock muffler i could get to faster speeds but less power.

    i was wondering if it might be a carb isue, do i need a bigger one??/ or do i need a larger jet hole to get more fuel in there???

    can anyone help.....
  2. Pablo

    Pablo Master Bike Builder & Forum Sponsor

    Dec 28, 2007
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    What does the plug look like after a hard run? (for that matter, what plug and wire?)

    What gears are you running?
  3. Nashville Kat

    Nashville Kat Active Member

    Apr 20, 2009
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    Usually the problem- reported all the time- seems to be too rich- to much gas to oil-

    Does the slightest chocking kill the motor?

    Close the petcock down by degrees- you may notice it runs better then-

    too rich

    most everyone here, myself included- usually thinks not getting enough gas at first

    close the petcock down all the time, or mix in more oil, probably the better solution- and motor will probably last longer-

    I drilled about five holes in the back of my air filter which helped a lot, more air to gas- leaningit down when it really gets flowing

    s smaller rear sprocket like the 36 I installed really helps, because the motor has always run better under load, and now cruises faster

    Narrower tires are the old bike racer trick- smoother tires roll much better than knobby too.

    keep things lightweight- especially rotating weight- wheels, crank, chain- we used to say when racing that an ounce off the wheels was worth two off the frame

    a bigger wheel is like putting on a slightly bigger gear- 27" or 700C is about four percentage up from 26" when you multiply it roughly by four to get 100 percent-

    so a motor turning the rear sprocket the same rPMs will move the bike 4 percent faster with the larger wheel- 25 mph would then be increased by one mph, to 26 mph,

    and greater then as the speed goes up- not as much difference as changing sprockets, but some- and the rolling resistence of a 1'1/4 inch tire is sooo much less than a 2.25" cruiser tire- if you don't mind the risk
  4. Bikeguy Joe

    Bikeguy Joe Godfather of Motorized Bicycles

    Jan 8, 2008
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    First determine if it's lean or rich. Most of the time with an expansion chamber, you will need to "upjet" or go to a richer jet.

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