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2 Stroke Tuning

Discussion in 'Maintenance and Servicing' started by Nicholai_Chev, Dec 27, 2015.

  1. Hi guys, need a little guidance on tuning a 70cc 2 stroke scooter.

    I've had to change to premix (32:1) after the factory oil tank mixer started leaking.
    After doing so the engine wouldn't rev beyond idle on the size 80 Main jet.
    -->On a size 90 jet their is plenty of power however the engine and exhaust feel very hot
    -->On a size 100 jet the engine hesitates just of idle to full power however is fine once the rpm builds up.

    Before I blow myself up, should I put a 95 jet in or focus on the needle clip?

  2. #2 mike8863, Dec 28, 2015
    Last edited: Dec 28, 2015
    When you have gone from oil injected to premix you have increased the viscosity of the fuel rendering your jetting too lean ( add oil to fuel, 'thickens' the fuel meaning less fuel gets through the jets ).
    'Basically' there are 3 fuel metering systems in a carb. Idle, needle AND seat and main jet....and they work TOGETHER to provide the correct amount of fuel.
    ( tip . the 'pilot air screw' works in reverse...the more you open the screw the more air you introduce into the idle circuit and the less fuel is drawn from the idle jet. )

    From your decription the 100 main jet is nearer to the mark, but sorting the idle and midrange is an issue...learn to read sparkplugs.

    • Agree Agree x 1
  3. You definitely need a larger main jet. It's more to do with the oil, which doesn't go towards creating your air/fuel mixture, taking up volume which would previously have been occupied by fuel which did, than being a viscosity thing.
  4. Thanks for the help

    As the engine was running better (presumably leaner) on the 90, would it be best to aim towards 105 or leave the jet alone and adjust the needle?
    Will be getting a few new sparkplugs today to test.
  5. General rule is to get the main jet right first and then work down the range of throttle settings. That's because, at full throttle, the fuelling is almost entirely dependent on the main jet and isn't affected by anything else So it's relatively easy to get right and then remains fixed while you fiddle with everything else.

    I'm not sure if you'll be able to do meaningful plug chops on an auto scooter as you can't do the correct cut the ignition and simultaneously pull the clutch procedure.
    • Winner Winner x 1
  6. While not adding to the original topic, your comment re plug chop has reminded me of a question I have about two-stroke auto scooters.

    On a normal two-stroke bike, if you are thrashing the tits off of it and the engine "nips up" you VERY QUICKLY pull in the clutch lever.

    So what, if anything, can you do if a CVT scooter's two-stroke motor "nips up"?

    Will it lock up the back wheel, or just drift to a gentle stop?
  7. My experience of fairly primitive CVT systems is that they're fairly "soft". Whilst I haven't had it happen, I suspect that, at least on dry bitumen, the belt would slip before the tyre let go.