Engine Trouble Throttle problem

Discussion in '2-Stroke Engines' started by Callum2461, Jan 25, 2014.

  1. Callum2461

    Callum2461 New Member

    Hi, I am new on here and I have just purchased a 2 stroke, 70cc mountain bike with a 2 liter tank (brand new). It's a nice bike and it runs smooth, although, when I slowly twist the throttle to speed up, it does speed up, but when I get to around full throttle it starts backing off (slowing down) for some reason. If anyone could give me and idea of why I would very much appreciate it, thanks.

  2. crassius

    crassius Well-Known Member

    normally, you'll find it runs a bit rich during the first 2 or 3 hundred miles of the break-in period

    after that time, you'll be better able to tune it

    I avoid going full throttle until the engine has worn off the rough spots a bit.
  3. Callum2461

    Callum2461 New Member

    Ok no worries, thanks heaps for that :)
  4. HeadSmess

    HeadSmess Well-Known Member

    there is no break in period, the jet is brass, solid brass, how does that "break in" and change mixture at any point? it doesnt. ball bearings are the most highly toleranced parts that are mass manufactured on earth, what exactly "breaks in" on them? plain bearings dont make metal to metal contact EVER, so how do they "break in"? the piston rings get one chance to conform themselves to the cylinder wall, and thats in the first five to ten minutes. that is the ONLY "break in"... which takes high pressure, meaning...full throttle, full load!(once theyve idled for a minute or two and are nice and WARM) five minutes....not really that long is it?

    i avoid these "gentle break in" recommendations, and havent had any engine tell me afterwards that i did anything wrong...just the opposite, actually.

    if it revs nicely but splutters or "fourstrokes" at full throttle, and returns to a smooth sound up hills, its too rich. if it revs out nice but tends to die at full throttle...

    what you have is a slightly lean mixture, caused either by dirt in the fuel blocking the jet, some type of debris in there, it came with a(very unusual) small jet, or... you live at a ridiculous altitude!

    check for dirt, or drill the jet out another 0.05mm...
  5. crassius

    crassius Well-Known Member

    getting rings to seat properly will change mixture quite a bit as blowby will be taken up on next stroke

    bearings on these have been observed to have very rough races and balls at times with the bearing cage somewhat pinched too tightly in spots - all these things benefit from a more gentle approach to fitting themselves together