Engine won't rev all the way while riding

Discussion in '2-Stroke Engines' started by Soflowrider, May 27, 2016.

  1. Soflowrider

    Soflowrider New Member

    hello new to the forum. Just installed my 66cc motor on my bike and started riding. I installed a tach so I could keep an eye on my RPMs while riding, I have since noticed that while the motor is engaged and at WOT I am only getting 2700rpms but when I have the clutch engaged the motor revs up to 5000 RPMs. I do have some upgrades to my motor, high compression head, expansion chamber, 30 tooth sprocket and dellorto carb with a #65 jet. The engine had this problem before I added the chamber, sprocket and the carb. My engine kit came with a high compression head which I swapped with the stock one before ever starting the engine. My top speed is only 33. Mph I figure I should be moving with this parts. Also the engine does excelerte past 2700 RPMs when going down hill but then settles back down to 2700. Any one have any ideas on what it may be? (One pic is of current condition other is of the high compression head)
     

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  2. crassius

    crassius Well-Known Member

    unless your weight is less than about 50 lbs, I'd guess your sprocket is too small - try a 41t or 44t to get a baseline of how the motor pulls
     
  3. Soflowrider

    Soflowrider New Member

    Ok I was going to try the stock one this weekend and see if it had any difference. Thanks for the quick reply
     
  4. butre

    butre Well-Known Member

    2700 rpm with a 30 tooth would be something like 16 miles per hour. how sure are you that your tach is reading right?
     
  5. Soflowrider

    Soflowrider New Member


    Not to sure followed the instructions to get it set for single piston 2 stroke. But have nothing to base it off of. I have similar tach on a 4-stroke long tail mud motor and it's dead accurate. But it doesn't sound like its reached its full potential at 2700.
     
  6. HeadSmess

    HeadSmess Well-Known Member

    i dont trust them cheap nasty tachs... reading anywhere from 5k to 16k when everything sounds right.

    try to locate it as far away from the CDI as possible and use a resistor type plug, cuts down on EMI which makes the digital internals go ballistic.

    do you really need one? they dont help you go faster!
     
  7. Soflowrider

    Soflowrider New Member


    No I don't really need it, but it still doesn't change the fact that my engine is not making full power under a load.
     
  8. Timbone

    Timbone Active Member

    Not sure exactly your high compression head plays in this, but your 30T sprocket makes a BIG A gear. And your 65 jet is gonna be leaning things out at WOT. Doubtful you have enough power to turn that cog. Greedy for speed...
     
  9. Soflowrider

    Soflowrider New Member

    Just a little greedy! What jet would you recommend using? I was going to put back the 44t sprocket and see if any speeds increased. And I read somewhere that the metal gaskets that come with the kit are a one time deal wasn't sure if maybe was losing compression from an air leak of so sort.
     
  10. Timbone

    Timbone Active Member

    65 works for me. I don't think that is the problem. Shorten that gear already. 39, 40, 41 if you need speed.
     
  11. Soflowrider

    Soflowrider New Member

    Ok il give that a try.
     
  12. HeadSmess

    HeadSmess Well-Known Member

    36t is good for a 26" wheel, no real need to get smaller. bigger is better for hilly areas.

    jetting is up to altitude, fuel, engine tune... do some research, plug chops, and good old "this revs up without fourstroking and makes more power than..." tests.

    start small, get bigger until it starts fourstroking and then go back one.
    use ngk b(r)5hs or similar, get a decent reading on the plug.

    dont run more oil than 25:1. ie, between 30:1, 50:1, ok,

    20:1, 16:1...waste of oil, dirty, and less power.

    and some engines are just junk and never want to get up and boogie.
     
  13. butre

    butre Well-Known Member

    what jet works for you isn't what will work for other people. different modifications and different density altitude will require different jets
     
  14. Timbone

    Timbone Active Member

    Stock jet on a HT 2 stroke is a 70. Never had one that wouldn't 4 stroke at WOT with that jet. 65 is a good place to start. All this is just suggestion. YMMV, of course.
     
  15. Steve Best

    Steve Best Active Member

    Your engine is not reving up enough to make max power.
    All the advice given here looks good.

    I'm running a 70 jet, 44t stock gear and doing 35-38mph dependent on conditions.
    My speed is pretty much rpm limited at this point and is turning much higher rpm than OP.
    I did lower the float level slightly, as Timbone notes, NT carb is rich from the factory.
    Cooler weather and higher altitude it may still be the jet for you.

    You tach sounds wonky. Only 2700 rpm at 33mph does not make sense even with a 30T sprocket.
    Wow. 30T sprocket. Acceleration and low speed must be terrible! 33mph is very respectable.
    I agree with other posters to try a taller sprocket. You will probably like it better and see more speed.

    Steve
     
  16. butre

    butre Well-Known Member

    at my altitude they tend to run lean at 65. somewhere between a 66 and a 68 is required.
     
  17. Soflowrider

    Soflowrider New Member

    Just swapped back
    I'm at sea level still working on jetting, currently running 65 main jet my friend has luck with the 60 so I'm going to try that next and see. changed my sprocket back the 44t and it gets up and goes now 35mph @ WOT still trying to squeeze a little more out of it.
     
    Steve Best and butre like this.
  18. butre

    butre Well-Known Member

    a 60 isn't likely to do good for you at sea level. do a plug chop, don't just rely on what your friends tell you works.

    find out what the typical density altitude in your area is and I can give you a good baseline.
     
  19. Soflowrider

    Soflowrider New Member


    What is found the density altitude is somewhere between 2250'-2110'
     
  20. butre

    butre Well-Known Member

    yeah, start at a 63 and go richer from there.

    do actual plug chops.

    around here our da can be as low as -1500 or even -2000 ft during the winter and as high as 2500 during the summer.

    that can easily be a 10% difference in power
     
    Last edited: May 29, 2016
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