i think my "80cc" has issues. top speed only 28mph with shifter kit...

Discussion in 'Motorized Bicycle General Discussion' started by jerseykat1, Oct 12, 2008.

  1. old motorbike

    old motorbike New Member

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    Mine does the same even with shifter. It went 29 mph after about 1/4 mile. It's going to take some tuning to make it go faster.
     
  2. jerseykat1

    jerseykat1 New Member

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    i think my motor is running good now, it is only limited by its ability to breath, i believe after i get a tuned pipe and port the intake and exhause i will be able to hit 35mph or more. I did however manage to hit 30 mph.

    The problem is that the engine cant rev any higher because the port size is just not big enough (this is my theory), and i believe this is the problem because it runs better and revs higher without any air filter, so if the ports were bigger it would rev higher and thus allow a higher top speed.

    I am not sure how far in i should go when porting the heads, should i port it all the way to the cylinder? will it run bad if i do that? could i port it without taking it apart and just use compressed air to blow out the debris or i guess i could just remove the cylinder head to get the debris out. anyone have any before and after pics of a ported China girl?
     
  3. Bikeguy Joe

    Bikeguy Joe Godfather of Motorized Bicycles

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    Hold on there killer!

    Before you port anything, I would suggest a little basic understanding of what you are doing or you may end up with an expensive experiment....:crash:

    You would be porting the cylinder, not the head (2 stroke).
    Yes, you'll have to take the cylinder off and dremel all the way from the beginning to the end inside the cylinder (all the way through). Do you know how much or where to remove material?

    I would suggest that you try matching the exhaust gasket and exhaust mounting flange to the exhaust port first. Once you do that, you will see an improvement and you don't have to remove the cylinder to do it.

    Also, get some miles on the engine....like 2-300 before you worry about how fast it will or won't go.
     
    #43 Bikeguy Joe, Oct 17, 2008
    Last edited: Oct 17, 2008
  4. Ghost0

    Ghost0 New Member

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    Excellent advice Joe. Start small and work your way up. So far my engine is completely stock. I have done nothing to the engine. I have matched my intake and exhaust headers. Added a tuned pipe and air filter. I have also rejetted the carb. I can now do 35 easy and can hit 40 if there is no wind. Once you get to around 30 resistance, mostly wind resistance, plays such a huge roll and increases exponentially with every mile an hour faster you go.

    Have you tried different gears for top speed. For instance I have a 6 speed cassette and my bike will actually get a higher top speed in 5th gear. 6th is just too tall for the engine to pull. I will do some proper porting soon. Plan on trimming the piston skirt to unblock the intake as well as lower and widen it slightly. I will also raise the exhaust port slightly. But like Joe said a little at a time.
     
  5. Goat Herder

    Goat Herder Gutter Rider

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    Your still sitten on a new motor too? Give it chance to break in some of these ch take 500 miles to loosen up!
     
  6. jerseykat1

    jerseykat1 New Member

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    thanks for the advice guys. i want planning on doing any porting or anything until atleast 2-300 miles. i was just trying to get advice on how to port these engines as i am not to familiar with modifying 2 stroke motors. my intake port is oval in shape and larger on the motor side but round at the manifold, so should i make a custom manifold that's the same way, because from what i can tell the intake port is oval all the way into the cylinder, and a little larger, but as you know the stock intake manfold is round and not enough material there for me to grind away to a perfect or almost perfect match.

    The exhaust port looks like i can easily match up, i need to take another look at that and make sure.

    But i think the first mod i do will just be a tuned pipe, seems like that gives the best bang for the buck.
     
  7. Ghost0

    Ghost0 New Member

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    Look at my post in this thread
    http://motorbicycling.com/f3/success-story-4065.html
    You can see that you can easily accomplish manifold matching. And yes you can do both intake and exhaust side. Your goal should be to at least make a smooth transition from oval to round.
     
    #47 Ghost0, Oct 17, 2008
    Last edited: Oct 17, 2008
  8. jerseykat1

    jerseykat1 New Member

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    will give it a go.
     
  9. Goat Herder

    Goat Herder Gutter Rider

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    (^)you can match ports now just saying be nice to the ch, while breaking things in.
     
  10. jerseykat1

    jerseykat1 New Member

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    ok i port matched the exhaust side, and test drove it, but its always more difficult to tell if there is any gain at night (it does feel stronger but it is colder tonight which could be helping).

    will give the intake side a porting tomorrow, think i will bore out the intake manifold a bit as well.
     
  11. Goat Herder

    Goat Herder Gutter Rider

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    Have fun I know ya will(^)
     

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