My 2 stroke is 4 stroking..

Discussion in 'Motorized Bicycle General Discussion' started by WaterDog, Oct 30, 2010.

  1. WaterDog

    WaterDog New Member

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    Ok, i 'assume' my bike is 4 stroking above 75% of throttle. Great pick up across the power band until i hit 65-75% of throttle. The sound changes like its missing every other stroke and the acceleration stops. Heres the deal..gas flow is fine, high performance air filter breathes great, no intake leak from the brake cleaner test, needle set on second notch from the top, spark plus is nice chocolate brown with no oil or white burns, exhaust is modified slightly and flows fine, 100 total miles on engine and using 40:1 synthetic oil.. now what in the world could cause my problem. Its still fun to ride and never fails to idle, start or run. Just wish i could figure out the high end problem to get over 23-25 miles an hour. Thx for reading this guys, any help is appreciated.
     
  2. FreeWheeler

    FreeWheeler New Member

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    Try a smaller jet? Easy to do with a hand drill bit set, solder, lighter.
     
  3. Nougat

    Nougat New Member

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    Or buy a couple smaller sizes from Sick Bike Parts. But yes, downjetting is what you need to do.
     
  4. WaterDog

    WaterDog New Member

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    So the concensus is that i am running rich?? Even though the plug is perfect?
     
  5. Nougat

    Nougat New Member

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    You could try a wider plug gap. That should give you a stronger spark, which would ignite the fuel a little better. Or a hotter plug might accomplish the same thing.

    I'm making all this up, but it sounds good, doesn't it?
     
  6. timboellner

    timboellner Member

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    Yes it's too rich.
    You can solder and re-drill the jet if you have the right bits.
    Keep in mind that 3/100 of an inch difference in drill size can mean the world of difference how your engine runs.
    Check this link and look just past the middle of the page to the Drill Bit Conversion Table.
    Drill bit sizes - Wikipedia, the free encyclopedia

    You'll have to experiment a little to get it just right but believe me it's going to make you smile when you get it dialed in.

    Get yourself these 2 drill bits at a hobby store. I think I paid about $2 each.

    #70 gauge drill bit (0.028 inches 0.711mm..... Usually close to the stock size)
    #71 gauge drill bit. (0.026 inches 0.660mm. smaller than stock
    Main jets are measured in millimeters #70 = .7mm
    #68= .68mm etc.

    Sick Bikes sell the jets but if you are handy you can save a little time and money doing it yourself.
    #68 or #66 work best for me

    Good Luck,
    TIM
     
  7. WaterDog

    WaterDog New Member

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  8. Nougat

    Nougat New Member

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    You're right, that's not rich, that's lean. And there's some crud on there. I'd replace that plug, or at least clean it real good. You might have an air leak?
     
  9. WaterDog

    WaterDog New Member

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    Yeah, i think im going to have to look for an air leak again. Its sealed up pretty tight. But since the problem only happens at high throttle, i suppose even a small leak could be the culprit.
     
  10. msrfan

    msrfan Active Member

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    I discovered on teardown of one of my BGF motors that the seals are junk. They were flaking and looked used. I went to a bearing house and picked up some high quality seals and cured my problem. Most metric seals have the sizes molded right into them, like 14.8 27 5, and 14.8 27 7. My crank shaft measured 15mm, so the seals I ordered were TCM #15X27X5TC, and TCM #15X27X7TC. Metric seals use the bore, o.d., and thickness as the part number.
     
  11. Nashville Kat

    Nashville Kat Active Member

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    I think you're too light on the oil mix- the plug should be darker- I don't know what the scales are- Does synthetic oil work in these things? Better more oil than less- at least the motor wont blow.

    I got some results when I had this problem by closing down the petcock a bit- which seemed to indicate flooding. But then I added a little more oil and that seemed to help.

    The sure fire best bet to stop four stroking is a billet intake from creative engineering or pirate cycles, if they have them in stock. I bought both a NTS 66 carb and a billet intake. I put the billet on first to see the effect. I've never got around to putting the 66 carb on the bike runs so well with just the billet.

    I'll probably do it soon though, just as much for the extra shut off valve- and I'm putting a three piece alloy crank in.
     
    #11 Nashville Kat, Oct 31, 2010
    Last edited: Oct 31, 2010
  12. Slowrollers

    Slowrollers New Member

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  13. timboellner

    timboellner Member

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    Sorry I didn't look at that plug.
    I totally agree now it's way lean.
    I also agree with the air leak idea.
     
  14. WaterDog

    WaterDog New Member

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    Good suggestion. I did notice that the stock pipe that the carb connects to has 3 or 4 slits in the end that the carb clamp is supposed to seal. Mine is far from precision and there was a leak there before. I just wrapped silicon tape around these slits and slid the carb back on. I thought i sealed it up but i should check again. Can you use permatec gasket sealer in the intake gasket? I used it on my head gasket and it sealed up a leak that was causing some compression loss.
     
  15. Nougat

    Nougat New Member

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    I used Permatex 97b, since it's rated as being "gasoline resistant." But when I ordered a new jet from SBP, I decided to get a pack of their carb o-rings (made from gasoline resistant Viton). The Permatex I'd put on before was all gummy and not really stuck to anything anymore so much as just kind of "slimed" on.

    The o-ring works better than the Permatex did even when it was new.
     
  16. Nougat

    Nougat New Member

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    For the record:

    After downjetting to 68, mine was still 4 stroking a bit much. I just tested my earlier plug gap theory. I'd tapped to about 024 before, so last night I pulled the plug. The condition and color was perfect. Put it back in after yapping to 032, and just now got back from a ride. No more four stroking at all (unless I back off the throttle, of course).

    Science!
     
  17. FreeWheeler

    FreeWheeler New Member

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    Are you going to say your plug was perfect chocolate color, but then post a pic of a crispy lean plug?

    More seriously, I went the opposite direction on my plug and validates what your saying. I had no 4 stroking with the gap I used and then I closed it a bit and it breaks in and out unless a load. I felt the plug was a bit lean before and betting that improved. Have to pull it to see the color, but the gap affected 2 vs. 4 stroking.
     
  18. Nougat

    Nougat New Member

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    Freewheeler - that pic wasn't my plug. Mine was exactly milk chocolate on the electrode. Half the insulator was the same, the other half was lighter tan. I have to think that was from where the intake hits the plug. I don't know for sure, but I think the tan side was facing the intake. I tried to index the plug when I put it back in, but getting it tight into an aluminum head doesn't really allow for that.
     
  19. FreeWheeler

    FreeWheeler New Member

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    I was kidding about how this thread started with a perfect chocolate plug and some annoying 4 stroking, followed by well meaning suggestions based on those assumptions and then the truth. That's all. I dont doubt it when you say youre plug is good. :)

    Sarcasm!
     

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