Carby Running lean on WOT, Need Help.

Discussion in '2-Stroke Engines' started by DaveLikesToBike, Apr 6, 2011.

  1. DaveLikesToBike

    DaveLikesToBike New Member

    I did a plug chop after warm up at WOT for about 2.5 miles with a NGK B7HS plug. The reading was lean, it was an ashy white. It runs smooth all the way up to 32-33 MPH. From everything I've found about problems with rich/lean most people have trouble with it being too rich at WOT and four stroking. I have the opposite which I find weird. I've read you rejet it to a bigger size to let more gas in but I don't know really what size is in there and what one to go up to. People say the stock size is either .70mm or .79mm in the 66/80cc engines.

    So you can get a better idea, here's what I'm working with:
    I have a 66/80cc engine, unknown manufacture because I bought it used. It's broken in and I run synthetic oil at 32:1. I use a .035mm plug gap. It has a NTTC carb on it which I believe is an NT. I've sealed around the clamp where it was letting air in. My stock intake filter fell off and was ran over by a car so I have a Sick Bike Parts high flow air filter on. I also added straight piping to the end of the muffler to get the exhaust fumes away from me while I'm sitting idle at stop lights, I kept the end cap on the muffler. I live at sea level in Florida.

    One off topic question I had was: can a BMX 1/2 x 1/8 chain be used as the motor drive chain. I have a KMC Z chain that fits but I was wondering if they are strong enough to not break at high RPMS which would probably end up with me crashing?

    If you can help me out with my problem or direct me towards a thread that answers my questions I would really appreciate it. Thanks for your help.:euro:

  2. HeadSmess

    HeadSmess Well-Known Member

    bmx chain seems to work well, but does have a lot more "flop" in it, so can vibrate hella bad! maybe with some plastic "guides" either side it would be fine...

    get your micro drills from ebay... a .30 thru to 1.25 set is like $20, $30 ? get a chuck at the same time. if you go too big, theres always solder...(and fine wire in the jet as i read here somewhere...)
  3. DaveLikesToBike

    DaveLikesToBike New Member

    I order a .72 and a .74 jet from Sick Bike Parts, but if they're still not big enough then I guess drilling is the best way. I hope the new jets just work out and fix the problem. But knowing these engines nothing is ever that easy.

    I tried a bmx chain, the 410 1/2 x 1/8 one and it was a little bit too small for the engine sprocket but fit on the back sprocket fine. I guess I'll just have to find a 415. I'm only changing it because the chain on there is stretched in some spots and it doesn't stay the same tension all the way around.

    I wish these engines weren't such a pain, but I guess what's the fun in that right?:shout:
  4. motorpsycho

    motorpsycho Active Member

    your chain is probably not stretched in spots, your rear sprocket is probably not centered on the hub. if a chain stretches, it stretches all the way around, not just in selected spots. if the rear sprocket is slightly off center, your chain will go "tight, loose, tight, loose..." as it makes it's way around.
  5. DaveLikesToBike

    DaveLikesToBike New Member

    I'll check it, it looked straight. I'll take chain off and spin the wheel with something stationary next to the sprocket to see if it wobbles. That'd be nice if that just needs some tightening and that problem would be fixed. Thanks for the advice.
  6. HeadSmess

    HeadSmess Well-Known Member

    sorry psycho...chains can and do wear unevenly... the golden rule when tensioning is to do it at the TIGHTEST point... all chains do it. manufacturing tolerances mainly.

    but yeah yeah, having the outside of the sprocket true and concentric sorta helps things along :) well ok. really helps things along...

    use something like a peg clipped to the chain stay for a reference, have the bolts semi tight, and (preferably with a piece of old chain or timber in between!) smack the sprocket into line with a hammer... then tighten up the bolts...

    i used to spend a while evenly doing up the bolts, checking for wobble...then got sick of worrying, just tightened em up, and they seem to pull up pretty good.

    having it eccentric will wear the chain too quickly, having it wobble will throw the chain every other time you hit a bump...

    back on bmx chain...if you use it, use the high strength one... and get the LONG one!!!!! like, 102 links or more....grrrrr!