Engine Troubleshooting

Discussion in 'Motorized Bicycle Trouble Shooting' started by LukeG, Sep 28, 2015.

  1. LukeG

    LukeG New Member

    Sep 17, 2014
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    Hi everyone,

    I've been having some strange engine issues and would appreciate any insight from some more experienced engine people.

    For the last 2-3 months, my bike has gradually gotten slower - lost pickup on the low end and lower top speed on the high end. I haven't changed any major parts. I've replaced gaskets, re-tightened the head, and adjusted chain tension, all with no effect.

    The weird thing is that every once in a while I get a few seconds of incredibly improved performance, like how my engine used to run. This usually occurs right before I run out of gas - I have 10-20 seconds where the pickup is easily doubled, then the engine dies. I assume this means that I need to do something with my air/fuel ratio...any ideas?

    I have a 66cc/80cc Flying Horse with a "high performance carburetor": http://www.bikeberry.com/66-80cc-high-performance-carburetor.html

    Thanks in advance!

  2. Slogger

    Slogger Member

    Sep 8, 2014
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    Not knowing how many hours are on the engine, I can say that mine required leaner and leaner jetting to run cleanly as it broke in.
    The engine is essentially a pump. New, its rings are not seated and its efficiency is poor.
    As it breaks in it seems to run better with less gas.
    It may be time to try some rejetting.
    There are lots of threads about how to do it, no need to rewrite all that info.

    First, check your spark plug and re-torque the head nuts, check the intake for leaks then have at it. I've also read that the mufflers can get clogged, although mine never did that.
    Good luck!
    #2 Slogger, Sep 28, 2015
    Last edited: Sep 28, 2015
  3. Juantrejo879

    Juantrejo879 New Member

    Feb 7, 2016
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    I have a mountain bike with an 80cc motor. When I ride it everything is fine for about 30 seconds. After that the bike goes and slows down over and over again. And even if I let go of the gas it keeps going and when I hold the clutch down to break, you can hear the motor working really hard. I need to get it fixed and would like to know what is the closest location to Houston Texas 77083
  4. Greg58

    Greg58 Well-Known Member

    May 1, 2011
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    Welcome Juan, you have found a place where there's a lot of people that know motorbikes. If you can tell us a little more about your bike we'll try to help, how old is it:? How many miles does it have on it? What fuel to oil ratio are you using etc. With more info you'll get more replies.
  5. crassius

    crassius Well-Known Member

    Sep 30, 2012
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    sounds like an air leak at intake or slide or cable sticking
  6. curtisfox

    curtisfox Well-Known Member

    Dec 29, 2008
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    I would say he is running to rich and when it starts to run out of gas it gets leaner, check the plug color. ....................Curt
  7. maniac57

    maniac57 Old, Fat, and still faster than you

    Oct 8, 2011
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    The slow loss of power sounds like wear and compression loss.
    The burst of power as you run out of fuel indicates too rich jetting in your carb.
    It gains power as it leans out. But beware, TOO lean jetting melts engine parts and breaks things in spectacular ways.
    Most chinadoll kits are FAR too rich stock. Try a 69 main and go from there...

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