Fourstroking @ WOT... help.

Discussion in 'Motorized Bicycle General Discussion' started by huntnfish08, Jul 21, 2012.

  1. huntnfish08

    huntnfish08 New Member

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    My engine doesn't run right at WOT. It fourstrokes for about 4 seconds, runs good for 5 seconds then back to fourstroking. I'm not sure what is causing this so I don't know where to fix it.

    I took the bowl off the carb and cleaned out what little debris was in there.
    Tried the top two grooves in the needle to try leaning out the engine a bit.
    Different plug wire, as well as different plugs, and the stock plug...
    Still fourstroking.
    I don't know what to do, the engine wants to keep pulling, is pulling hard, then squat.

    Can someone offer a fix for this or head me in the right direction?
    Adam
     
  2. tooljunkie

    tooljunkie Member

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    first,engine type and size.
    if its a new motor,dont expect too much during break-in.
    during break in,dont run wide open for too long.
    let it warm up a little before driving it too hard.
    mine four strokes,much less after rejetting to smaller size.
    also by lowering the float slightly,it will run a tad leaner.
    i wouldnt fiddle with the jetting until you have a couple tanks of gas run through it.
     
  3. huntnfish08

    huntnfish08 New Member

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    Z2-80 BGF engine, "80cc"

    I've got over 100 miles on it before it ever started this fourstroking business. I could wind the engine all the way out, pedal faster to get more RPMs and then hold those R's. Now I get to about 28-29mph and it stumbles, picks the speed up and stumbles again. It's driving me nuts not knowing why.
     
  4. tooljunkie

    tooljunkie Member

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    just for giggles,try cleaning your air filter.
     
  5. huntnfish08

    huntnfish08 New Member

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    I tried running without it, no luck.
     
  6. Pilotgeek

    Pilotgeek New Member

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    Try running without your gas cap once. A few of them don't vent right from the factory (including mine). A smaller jet may be in order. You may want to do a plug chop to check jetting. I needed a smaller jet after break-in.

    Also, if you have a stock engine, are you sure you're doing 28-29mph? That's about as high as I would want to rev it with a 44t sprocket. When I first got my speedo, I set it for the stock 26" wheel size, but it was too fast by about 3mph according to gps. I had to measure the circumference and enter that into my speedometer.
     
  7. nightcruiser

    nightcruiser New Member

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    Is it definitely 4-stroking at WOT when it's slowing down? The reason I ask is last year I had a similar issue but the motor wasn't 4-stroking, it was vibrating and rattling intermittently and loosing power at top speed WOT. I rebuilt the bike over the winter, this year I have a lower mount on the muffler and that problem seems to have gone away. Not sure if it is the muffler mount that fixed the problem, cause I changed a LOT of stuff in the rebuild, but it is one of the changes that is most likely to have cured this ill...
    If it IS 4-stroking then you are probably running rich at WOT. Do a plug chop while running WOT and see what she looks like. (Warm bike, ride WOT, then hit the kill switch, pull the clutch in and release the throttle all at the same time. Pedal the bike home, let the motor cool and then pull and inspect the plug)
    If it does turn out to be 4-stroking and running rich you need to tune your carb a bit. You need to tell us what carb you are running before we can give you proper advice. To cure a rich condition at WOT you could try a float adjustment to lower the fuel level in the bowl (will generally lean out the mix) or run a smaller jet. Again, you'll get more accurate advice if you tell us what carb your running and what the plug looks like after a plug chop at WOT.

    PS If you're still running the kit plug you might try running a NGK BP6HS plug instead and see if she runs better....
     
    #7 nightcruiser, Jul 23, 2012
    Last edited: Jul 23, 2012
  8. nightcruiser

    nightcruiser New Member

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    Have you changed from a break-in fuel mix over to a standard fuel mix, did the 4-stroking come along with that change? This just came to mind cause you said it didnt 4-stroke for the first 100 miles...
     
  9. maurtis

    maurtis New Member

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    My RB80 did that at high RPM with the stock jet (about a 70) and SBP HD air filter. Going to a 68 jet caused me to only 4 stroke on WOT just after startup. After about 2 miles, no 4 stroking anywhere.
     

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