Engine Trouble Engine slow down and speeds up at high RPMs

Discussion in '2-Stroke Engines' started by Decidium, Apr 4, 2014.

  1. Decidium

    Decidium New Member

    Hello guys!

    I have a 60cc Cycle Pro engine on a mountain bike with a high compression slant head and a expansion chamber.
    Both upgrades are very recent and after about 50km of riding with them I was sometimes able to start hitting extremely high speeds, I am talking about a 20kph increase in speed from the original speed of 60kph... What I don't understand is that it needs something to help it get into that band, like going down a small hill or something. It just splutters for a bit when it is at the end of its power range and then it will accelerate like mad from 60 to 80 in less than 10 seconds... How can I smooth this out? I know that going that sort of speed is bad for the engine but I would like the power there if needed (like if a cop is pursuing me ;) )
    I have heard that boost bottles smooth it out, would it be useful to purchase one?
    Any help is appreciated!

  2. jaguar

    jaguar Well-Known Member

    normal expansion chambers rob Peter to pay Paul. That is, they reduce mid range power to boost top rpm power. That works fine if you have gears to shift to stay in the powerband but it is all wrong for a 1 speed bike. All you can do is reduce and stretch out the baffles return wave to have more mid range power and less top end power. Click on my signature link to read about making a baffle extension to accomplish this.
  3. Decidium

    Decidium New Member

    I managed to smooth it out a bit by leaning the fuel mixture out a bit. Works great now!
  4. 074KU

    074KU Member


    Sounds like your ready to go to Bonneville and have more skill (or luck) than anyone on here..
  5. Decidium

    Decidium New Member

    Yeah I have no idea... It used to go about 30-40kph with everything stock. Then I bought an expansion chamber, a high compression head, a big bore inlet manifold, some fully synthetic racing oil and I modified my air filter to allow more air to enter the engine. I'm also 16 and weigh 60kgs and live in New Zealand, so that might be a big part of it as well. I can get it up to 70kph now that I leaned it out but it has better acceleration and the power band seems smoother for some reason.
  6. butre

    butre Well-Known Member

    I could easily see 80 kph, my bike does around 45 mph with similar mods and more weight
  7. 074KU

    074KU Member

    I dunno what top speeds you skinnys get, 50mph isn't a speed I have witnessed or seen any credible video of however.. I will be the first to admit rolling down hills (gravity only) at 70+kph Not really a smart thing to do, probably quite "safe." If you are going to go that fast get some decent brakes some motorcycle gear and make sure your common sense is always switched on.. Bicycles aren't really meant to go that fast.
  8. velzie

    velzie Member

    What kind of x-chamber/head?
  9. Decidium

    Decidium New Member