High Compression Engine: I Don't Like It.

Discussion in 'Motorized Bicycle General Discussion' started by rohmell, Aug 31, 2011.

  1. rohmell

    rohmell Active Member

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    My HT engine blew its piston pin needle bearing, and one of the needles managed to get in between the piston and cylinder. The engine still started and ran, but power was way down.

    I had a spare piston and cylinder, and before installing it, I gave the cylinder a nice hone and put it all together.

    At first, it started easy, just like the old one, a couple feet down the driveway and off it went.

    As the the cylinder, rings and piston broke in and mated to one another, compression went up and starting got harder and harder, more resistance when dropping the clutch to start.

    Now I have to pedal like crazy and get up plenty of speed or else the rear tire will lock up and skid due to the compression resistance.

    I actually preferred it when the engine had low compression and I could start it within a few feet,with hardly any speed, now it takes high speed and slamming my butt down onto the seat when I drop the clutch to make sure I have good traction on the rear wheel in order to start.
     
  2. 2door

    2door Moderator
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    LOL, remides me of the old drag racing days before dragsters had starters. The tow/push truck would get the car going down the strip then the driver would dump the clutch. Every now and then you'd see a particualrly high compression engine lock the rear tires and slide along probably making nice flat spots on the slicks.

    Rohmell, looks like you need to experiment with a compression release valve. :)

    Tom
     
  3. mapbike

    mapbike Active Member

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    You could just add an extra head gasket and that will reduce compression a bit.

    Just a thought....Shrug

    Peace, map
     
  4. Pilotgeek

    Pilotgeek New Member

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    I wouldn't complain about it. Sure, it's a bit more work to start it, but it then does all the work of moving for you after that. Seems like a minor problem.
     
  5. rohmell

    rohmell Active Member

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    Took the top-end apart, put the upper piston ring on the bottom, and the lower piston ring up top, re-honed the cylinder at a lesser angle, put everything back together and YAY, a low-compression engine again. That is, until all of the parts start to mate to one another again...

    A few feet of scooting down the driveway, and it's an instant, easy start.
    Power seems to be the same to me, whether high compression or low.
     
  6. killercanuck

    killercanuck New Member

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    Like mapbike said, you could add another head gasket if it gets too hard to start again. You must be running a small rear sprocket.

    gl.
     
  7. rohmell

    rohmell Active Member

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    Nope, using the standard 44T sprocket. Everything is stock. The frame is a small one with 24" wheels so maybe it's too lightweight for its own good.
     
  8. happycheapskate

    happycheapskate New Member

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    Actually, using a small rear sprocket makes starting EASIER by far!

    When my bike had the stock 44 t it was a PITA to cold start! After changing to the 36, it is a breeze, even if it doesn't fire right away or I need to choke it some, I can keep pedaling, or usually just pedal and coast and it will start.

    With a 44t cog, the wheel turns the motor more rotations per turn of the tire, than with a smaller cog. Perhaps changing to a 40 or 36 will help you with your starting problem. Also, making sure everything is properly lubricated (chain, engine bevel gears, and wheel hubs) will help.



     
  9. killercanuck

    killercanuck New Member

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    ouch, 24" tires wouldn't help either.

    As Happy pointed out, I had my thinking backwards. Smaller would make it easier to start too. Sry for mixup.

    gl.
     

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