Motorized Bike Performance Upgrades

Discussion in 'High Performance Bicycle Engine Tips and Motorized' started by CARider, May 20, 2013.

  1. CARider

    CARider New Member

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    So I have a BGF China girl motorized bike on a fixie, and I'm looking for upgrades I could use with it to make it perform better. It is a good bike, but I want it to run good n' fast for as long as possible. In this time frame, I'd like to get good performance too. I'm aware of the red "speed carb" that is very cheap on ebay, but I'm wondering what that will do to help.
    Thanks, and sorry for me being a noob here!
    -Zane Smith
     
  2. biknut

    biknut Active Member

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    A Speed carb will give you a good idle, but not much extra performance. The best bang for the buck would be a SBP expansion chamber, a well chosen rear sprocket, and jetting/tuning. You should be able to get any "80" to run 35-40 mph with just these mods.
     
  3. maniac57

    maniac57 Old, Fat, and still faster than you

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    I agree. A tuned pipe and proper tuning is the first upgrade to look into. Once you have it running right, you can gear it to your needs.
    In my opinion, the stock carb is fine with a stock cylinder. Once you get beyond that point, you should have a pretty decent idea what to do next
     
  4. CARider

    CARider New Member

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    Thanks for the info! That was very helpful. Now I have another question: has anyone here ever had problems with squeaking noise coming from the clutch/engine sprocket area? It seems to go away (I think) when I ride it, but when I'm pedaling with the clutch in or just moving it, I hear the sound.
    Thanks again for all your help!
     
  5. Tyler6357

    Tyler6357 Member

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    Grease your clutch.
     
  6. CARider

    CARider New Member

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    Alright. Thanks again!
     
  7. 2door

    2door Moderator
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    As for your "performance mods".....Do ONE thing at a time. Don't add a bunch of stuff all at once. The reason is if there's a problem you won't know where to look for it. Also making your changes one at a time will give you a basis of comparison to work with.
    Good luck.

    Tom
     
  8. biknut

    biknut Active Member

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    I think you're trying to say, grease the transmission shaft? Actually greasing the clutch wouldn't be a good thing lol.

    On the clutch side it is a good idea to grease the pinion gear.
     
  9. dodge dude94

    dodge dude94 New Member

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    If you want to yank the engine and have the tools to do this....deck and polish the head, port the exhaust port, and port your intake manifold.


    *threadjack* *argh!*

    Will a kind of tall cone filter fit in a Cranbrook frame without need an offset intake?

    *threadjack over*
     
  10. CARider

    CARider New Member

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    Aw man Tyler! I have grease all over my clutch handle now. Dang it. My bike is ruined.



    No, thanks guys, I opened it up and greased the transmision shaft, and it fixed the problem right away. Thanks!

    Thanks dodgedude, I've got it now.

    I'm going to try to get some pictures of my bike up here. It's the same bike as the "McDonalds" colored fixie from Walmart, but it is this Target version that is white and green. I've noticed each store that carries bikes around here has their own paint job and brand name for the exact name bike.
     
  11. 48ccbiker

    48ccbiker New Member

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    You get what you pay for. Buy a cheap carb and get cheap performance. Buy a Mikuni, jet it right, and live happy forever after! A 2 stroke just cant perform right without the proper size carb and/or without it jetted really good.
    Use sandpaper on glass to mill off 1mm from the head to increase cylinder compression.
    Make your intake "extended" for a good low rpm boost of power.
    Boost engine power by boosting spark power.
     
  12. stephen1

    stephen1 New Member

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    so a tuned pipe and sprocket and jet for the carb will get you down the road at 35 to 40 mph thats what i wanted to know
     
  13. dodge dude94

    dodge dude94 New Member

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    IMO sprocket -> tuned pipe -> jetting -> open air cleaner -> more jetting.

    It's not the parts that gets you there, it's the tuning and combination of parts.

    Ya put on the sprocket, see what that does, then buy a tuned pipe, see what that makes your engine do (likely run lean), then re-jet the carb and buy an open filter.
     
  14. 48ccbiker

    48ccbiker New Member

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    After doing those basics you may have to buy a rotary tool and use a cutting disc to raise the exhaust port some to get the extra speed you desire. Maybe 1mm at a time to see how much increase you get. Also lower the intake port the same amount. Both have limited port durations that limit speed no matter what else you do to the bike.
     
  15. mew905

    mew905 New Member

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    It's a tricky tradeoff though, as far as I understand, you get more high end power by increasing the timing, but lose out on the low end (which is likely why you said to increase intake length). Jenning's book says increasing exhaust port area (in any direction, up, down, side to side) will boost power, some high power motors use up to 70% bore width (and some even more but they used bridged ports), but a 62% bore exhaust port should be safe to do (about 30mm wide). you can get more power still by raising it, but there's that timing tradeoff, the morinis pull 9hp at 11.5k RPM IIRC ([torque x RPM]/5252). Using that equation you can see that more power can be had by simply increasing the max RPM but of course there's a limit to that thanks to physics (and size limitations, your ports can only get so tall haha) being that our motors dont exactly have strong materials, we need to boost power within our RPM band, which is relatively short (which is good for beginners because we can focus our efforts). Some claim 11,000 RPM, but I dont think that'd be safe, I'd agree 9000 RPM absolute maximum for safety. So essentially, choose your target max RPM, gear appropriately, balance the crank for your RPM, build a torque pipe, widen the ports (and raise them to match your target RPM). raise compression, raise case compression to ~1.5:1, correct the transfer port flow, put on an intake header, reeds if you choose, and a proper sized carb. Perfection isn't easy, but it sure is worth it.
     

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