Just need a new motor?

Discussion in 'Motorized Bicycle General Discussion' started by gunslingerburrito, Nov 22, 2012.

  1. gunslingerburrito

    gunslingerburrito New Member

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    The bike was running fairly well until a few months ago.

    It seemed to get gradually harder to get up to speed, and it almost seemed like it was bogging.

    I leaned it out, and that didn't do much. Which was odd, since this normally had a good effect on the performance.

    I tried a different oil.
    No change.

    I tried a new carb out of curiosity.
    I forgot to plug up the air leaks, and it idled at scary-high rpm's for about 10 seconds until I could stop and kill the motor.


    Then, it stopped cranking completely.

    It would almost crank when I would pedal in gear, but instantly died when I'd pull in the clutch.

    I tried a new magneto, thinking it wasn't getting a spark.
    No change.

    Ditto with a new CDI.

    So, today I pulled the head and the cylinder.
    I'll reply with pics after thanksgiving, but here's what I saw.

    First, there was a ton of carbon build up on the top of the piston and the firing cup in the head.

    The cylinder was mirror smooth. No scratches on either the piston or the cylinder walls.

    The piston had a brown burned looking patch on one side (guessing that's not good).

    The cylinder also had a lot of side to side play from the bottom of the connecting rod up.

    So, my questions are:

    A) What does that sound like to you?
    B) Can it be fixed?

    And most importantly:
    C) would it be cheaper/easier to just buy a new engine?


    This is a 58cc Grubee Starfire Gen-II A.
    It's a year old, and has a lot of miles on it.

    Thanks guys, and happy thanksgiving!
     
  2. bigbutterbean

    bigbutterbean New Member

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    When you say it had a lot of play from the connecting rod up, do you mean the conrod itself moves back and forth? If so, this sounds like worn out crank bearings to me. I had an engine that was about a year old with a lot of miles, and I was using the wrong kind of oil (Pennzoil marine 2 stroke), and my rings went, but when I tried rebuilding the engine, I wasted money on two top end rebuild kits because I did not yet know that the crank bearings were shot. What I now think happened was that the bearings went first. The reason I think this is because when the bearings go bad, it allows the conrod to wiggle back and forth on the crankshaft, which wont happen with good bearings. So I think when the conrod wiggled back and forth, it caused the piston and rings to come into contact with the cylinder at odd/bad angles, which caused the rings to get chewed up. I would recommend a new engine, and that would be fine.
     
  3. bigbutterbean

    bigbutterbean New Member

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    The crank bearings CAN be fixed, and it would be cheaper than buying a new engine, but it will require you to split the crankcase, which means you will need to know or understand how to do it (there is a tutorial on it somewhere here on the forum) and also know how to correctly install the new bearings and put everything back together properly (not sure if there is a tutorial on that. Installing a new engine is a little more expensive (I buy replacement engines on ebay, where they are usually the least expensive), but will require far less work. So I suppose its up to you which way you want to go.
     

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