losing power at high rpm

Discussion in 'Motorized Bicycle General Discussion' started by Mike Hunt, Jan 7, 2010.

  1. Mike Hunt

    Mike Hunt New Member

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    stock engine, c-clip on the top notch, runs fine until i give it full throttle, after 5 seconds or so rpms start to decrease and if i hold it long enough it dies. doesnt sound like it 4stroking, just decreasing rpms until it dies. i used to be able to give it full throttle and it would keep going and going until i let go. what to do?
     
  2. bairdco

    bairdco a guy who makes cool bikes

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    check your fuel filter and everything else for debris, clean and re-gap your plug, de-carbonize the top of your piston and inside of your head, check your exhaust to make sure it's not clogged, try some new fuel...

    sounds like a weak spark or not enough fuel.
     
  3. Mike Hunt

    Mike Hunt New Member

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    ok so before i had the b7hs plug, swapped to b5hs, doesnt die at full throttle any more, but fourstrokes badly (the needle clip is already set to the leanest setting). i tried holding the throttle wide open and after a few seconds it got going again, then later on during the same trip it started fourstroking again at full throttle and wouldnt pull through no matter how long i held it. the only thing thats different from stock right now is the plug and the saucer style airfilter from sick bike parts. and a new spark cable from sbp. does the spark gap make a difference, because i think i set mine too high..
     
  4. bairdco

    bairdco a guy who makes cool bikes

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    too large of a gap could make it 4 stroke. i run mine at .025

    bad fuel or water in it could hurt it, too.

    you say you've changed the plug wire, is it on there tight? mine has a "snap" to it when it's on all the way. once, i didn't snap it on all the way, and it four stroked at high rpm.

    double check that all your wiring connections are tight.

    it might actually be too lean. it's easy enough to go down a notch to see if that helps.

    what kind of oil are you using, and what mixture?
     
  5. GearNut

    GearNut Active Member

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    The easiest way to de-carbonize your engine is to use SeaFoam additive in your fuel. Follow the directions on the bottle for how much to add, or do a search on it here on this site. It has been discussed before. Super simple and thorough cleaning!
     
  6. Mike Hunt

    Mike Hunt New Member

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    i have temporarily "solved" the problem by turning my fuel valve only part way open, thus reducing the flow of fuel. this took care of the fourstroking, but when i did a plug chop the thing was milky, virgin white. i'm kinda nervous about riding it like this for long, but if it was already too lean then how would cutting off fuel help it?
     
  7. GearNut

    GearNut Active Member

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    I can only surmise that it was not too lean. Limiting the gas flow at the fuel valve basically creates a situation where the fuel valve is now the main jet. If the carb cannot get the fuel flow enough to fill the float bowl and let the float and float valve/ needle do it's job then ultimately the fuel valve is used as the regulator instead of the jet.
    I hope that makes sense.
     
  8. BarelyAWake

    BarelyAWake New Member

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    All of this sounds so much like the fuel starvation issues I've had with my Schwinn, can ride all day at 3/4 throttle - but at WOT (and at speed) it would go great then do exactly the same as described above (recovering after a few moments after backing of throttle). I checked the entire fuel system for good flow (cap venting, petcock and filter, inlet on carb) - then bent the float tabs up as far as they'd go and still be able to shut off flow when the bowl was full (that carb is at a pretty steep angle so there's not much adj to the float I can do).

    That worked great and fixed the problem right up, I could ride all day at WOT without a problem till I switched carbs to the newer style - for the life of me I can't get the new one dialed in (same starvation problem) so I switched back.
     
    #8 BarelyAWake, Jan 17, 2010
    Last edited: Jan 17, 2010
  9. bairdco

    bairdco a guy who makes cool bikes

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    just wondering if you tried any of the things i suggested...
     
  10. Mike Hunt

    Mike Hunt New Member

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    hmm so i changed the needle clip to a richer setting and winds right out again at wot, no fourstroking, no need to fiddle with petcock. just came back from a cruise, gonna do a plug chop in a minute.
     
  11. GearNut

    GearNut Active Member

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    Cool beans!
     

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