Oops, I Fragged My Motor!

Discussion in 'Motorized Bicycle General Discussion' started by Huffathump, Mar 31, 2009.

  1. Huffathump

    Huffathump New Member

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    so, yesterday my motor started running mysteriously well. Too well. And definitely too fast- I havent mounted a speedo to this particular bike yet, but I was going faster than I have ever gone on a MB.
    .trk
    I also noticed an occasional "ping" from the clutch area, but this motor (BGF) has always done that.
    When I got to my destination I noticed that the choke didnt kill the motor instantly, only after a little sputter. Uh-oh. I know what that means. But I figured what the heck- I have run these motors so lean that they would stall. How bad could a little air leak hurt?

    On the way home I found out. On the last mile the "ping" became "PING!!!" was joined by all manner of dreadful noises and a couple of times it seemed like the clutch or SOMETHING gave out, allowing the motor to rev freely. I was sure I was going to end up walking home... but the poor little thing got me and my groceries home safely. Just as I had dropped the kickstand and was getting ready to shut off the motor, one more "PING!" stops the motor for me. Very Uh-oh.

    So I think maybe all is not lost- I strip down the motor and find the dread results (cue ominous music) First the cylinder- It looked OK at first, but it has lost ALL compression. I can turn the crank with my hand- with the spark plug in it just makes a slight "huffff" on the compression stroke.

    Then there is the bottom end. The "ping" appears to have been the woodruff key from the small gear on the end of the crank shaft. It appears to have somehow escaped its confines and bounced around in the clutch housing and got caught between the gears numerous times. The small gear now spins freely of the crankshaft sometimes, and spins the shaft other times.

    Bottom line, this motor is fodder for the parts bin, which kind of sucks since it it less than a month old. But mainly I am curious how this all happened at once, and on such a young motor. I dont think I really had it going all that fast- I just dont normally break about 30 mph on my bikes.

    Also, how on earth did the thing make it home at all, with the woodruff key out of place and no compression to speak of?

    Any and all theories would be welcome.

    RIP BGF 66cc

    I am switching the old PK "80cc limited" over from my mountain bike to keep the cruiser on the road while I wait for a new motor.
     
  2. dvddtz

    dvddtz New Member

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    My BGF motor only lasted 2 mths. but my rod bearing fell appart, and pieces got wedged between cylinder and piston. No telling why, some are good some are bad no matter where you get them from.:-||
     
  3. Finfan

    Finfan New Member

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    It's that legendary Chinese quality control once again!!! :-||
     
  4. captainrichhill

    captainrichhill New Member

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    Lean mixture is the WORST thing for these little 2 stroke motors. You can get away with too rich without too much trouble. Maybe a loss of power a fouled plug or two. They run hotter then 4 stroke anyway and when you starve it of fuel/oil you're killing it. It doesn't take long..bf.
     
  5. Huffathump

    Huffathump New Member

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    ^ yes indeed. (see my post in the troubleshooting section) After replacing the motor I discovered root of the problem- the carb was leaking air. Not just a little either. It started small (choke on and it sputtered before stop) but by the time the motor fragged the leak was so bad that I could run the (new) engine with the choke fully on. It would even START with the choke on! A quick squirt of starter fluid stalls it out instantly- proving the theory.

    i suppose i should rebuild the old carb just for the experience... but at this point I would be tempted to use a sledgehammer to do the rebuild, so call it more fodder for the parts bin.

    meanwhile, the bike is back on the road and the weather is nice- time to go catch some bugs in my teeth!
     
  6. ZnsaneRyder

    ZnsaneRyder New Member

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    Save ALL PARTS. When you have your 2nd engine, and you end up needing something, you will be thankful.
     
  7. Bikeguy Joe

    Bikeguy Joe Godfather of Motorized Bicycles

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    Well, you learned an expensive lesson. Lean engines run hot and then they start to self destruct.
     

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