Adding a second bottom gasket for clinder and...

Discussion in 'High Performance Bicycle Engine Tips and Motorized' started by moosinatrix, Dec 10, 2009.

  1. moosinatrix

    moosinatrix New Member

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    hey guys!!! it has been a while. I was just pondering how i could impprove my bike with just some stock mods when i thought up an evilly and maniacally brilliant mod, just wanted to see what the bike motor gurus had to say about it.



    I was looking at my axhaust port the other day when i notice that my postin at BDC stil stick up about 1/8 of an inch above the bottom of my exhaust port. well i was wondering if it would improve performance to add a gasket to the bottom of the jug, and have the head milled. Good idea??? If i did have the head milled whats the most you can take off without having clearance problems between piston and sprak plug, and not blowing you engine to peices with too much compression..shft.
     
  2. camlifter

    camlifter New Member

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    you'll need to build a spacer, will take more than a gasket to raise it high enough to notice. i rasied the jug on one of my jet 80s .125" this put the piston even with the exhaust, just don't go so much that you expose the top of the piston on the intake side. then mill off the same amount from the top of the jug. gave a lot of low and midrange power. been running it all summer.
     
  3. Venice Motor Bikes

    Venice Motor Bikes Custom Builder / Dealer/Los Angeles

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    That changes all the port timing! But will it change it enough to make a difference (performance wise) one way or the other?
     
  4. moosinatrix

    moosinatrix New Member

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    whats the diamter od the piston on the 80cc so i can get a ring compressor also whats the bigget jug kit i can find for that bottom end??
     
  5. camlifter

    camlifter New Member

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    it moves the power band down to a lower rpm, gives great low end and midrange performance. will let you run a smaller rear sprocket and still pull it.
    you don't need a ring compressor for these, just use your fingers, real ez to do.
    i don't know of anyone making a big bore kit for the 66cc motor.
    there was a thread where someone mixed and matched cranks and piston to make a stroker, you could raise the ports even more with that idea. these engines lose a lot on the higher rpms, so the more you could raise the ports the better it would run.
     
  6. Venice Motor Bikes

    Venice Motor Bikes Custom Builder / Dealer/Los Angeles

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    Just like camlifter said. you don't need a ring compressor; but there's a tiny pin in the ring groove that holds the ring gap in place... if you aren't careful to remember it's there, you will break the rings!!!
     
  7. Riding Rich

    Riding Rich New Member

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    Ahh i would check out my porting thread.
    Actually the port being restricted at BDC dosent matter much.
    Look at the exhaust diagrams for exhaust port timing.
    The transfer is completed out the exhaust by the end of the Blow Down duration.

    The distance from the top of the exhaust port to the top of the Transfer port.
    Only 17 degrees of crankshaft rotation.
    This occours at the top portion of the exhaust port and the piston restriction prolly helps keep the fresh fuel mix in the cylinder.

    I ran my Grubee with 2 base gaskets ( Not For Long ). It Didn't work too well.

    Port timing is a tricky balance of things.
    Allmost completed for the 68cc.

    Raising the exhaust port top slightly may help . ( about .030 inch max )
    The intake really needs lowered a little.
    The transfers i wouldn't touch.

    Very soon i will have figured the optimal Low end and Optimal High End port adjustments.

    Don't worry as i get my hands on them i'll be doing the other motors.
    -------------------------------------------------------------------------------------
    Hopefully Taco sends me his PK-Skyhawk hybred.
    That will be a very interesting motor.
    -------------------------------------------------------------------------------------

    NOW AS FAR AS THAT HEAD ( YES YES YES )

    The higher you get the compression the better.
    Up to 9:1 corrected compression ratio.
    About a 50% increase in compression.

    #1) Take the head and jug off of the motor.
    #2) put good emery cloth on a pc of glass.
    #3) Sand the head about .020 / .030 inch off.
    " The little circular ring will be gone "
    #4) Sand the jug on the top surface.
    take about .015 / .020 inch off of it.
    #5) Use a cookie sheet or pop can or copper roof flashing to cut an ultra thin head gasket out. About .005 / .010 in is what i use.
    SBP ALSO SELLS A THIN HEAD GASKET.

    #6) Put the jug and head back on but don't bolt it down.
    crank the motor over by hand wile holding the head down tightly.
    You will feel if the piston hits the head.

    Even better put clay on the piston and check that way.

    Bump up the compression and match all the port gaskets.
    You'll be surprised how much torque you gain.

    Move a port the wrong way and youll find out that .030 inch is enough to make the motor run real bad.
     

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