Engine Sputter at High RPM?

Discussion in '2 Stroke Bicycle Engines & Kits' started by cory151, Oct 28, 2010.

  1. cory151

    cory151 New Member

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    Hello, been a lurker on here for a bit, Im having a problem with a sputter at high RPM. The bike fires up fine and idles well, it:-|| just lost its top end somehow. A few days ago I installed a high flow air filter, NGK plug and an inline fuel filter (from Sick Bike Parts), took it for a testdrive and all was well. Road 4 miles to the post office today and it developed this problem. Any ideas?

    Thanks, Cory

    Bikes a 50cc Starfire nothing special
     
    #1 cory151, Oct 28, 2010
    Last edited: Oct 28, 2010
  2. Nougat

    Nougat New Member

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    Describe the sputter in more detail. If it's mild, with a barely noticeable drop in power, it's probably four stroking - which you should expect when it runs out of RPM at top speed.

    With a better air cleaner, it might be running lean, though (more air, same amount of fuel). Check your plug color and adjust the needle clip accordingly.

    May also be a clogging exhaust, unrelated to your changes, just coincidental. How long has this bike been running?
     
  3. Venice Motor Bikes

    Venice Motor Bikes Custom Builder / Dealer/Los Angeles

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    I concur... it's probably 4-stroking... (Just a hair too rich.)
    What plug did you put in it , & what did you set the gap to??
     
  4. FileStyle

    FileStyle New Member

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    I gap to .025-.027 How about the rest of you? Running a NGK B5HS
     
  5. cory151

    cory151 New Member

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    Im running the NGK B6HS I believe and it said its pre gapped. The loss of power is noticeable. Turning the needle valve on the carb doesnt seem to do much at all. Im at a loss, I could throw the old plug back in and see what happens. It wouldn't be leaking vacuum some place would it?
     
  6. Nougat

    Nougat New Member

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    Cory, "Turning the needle valve on the carb"? You sure you're not talking about the idle screw? And I bet you just need to snug the spark plug up, you're probably losing compression past a loose plug.
     
  7. cory151

    cory151 New Member

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    Thats actually exactly what it was doing, adjusting the idle and nothing else, I take it these arent really adjustable other than changing out a jet? Ill re-snug the plug and report back.

    Thanks again, to all contributors.
     
  8. corgi1

    corgi1 New Member

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    they may be pre gapped for a car which is usually 35 thou.on up to 80 or so...............
    The air filter causes less vaccum in the carb to pull gas up the jet,I havent desided that there is any venturi action working for these carbs........
    and the idle screw won't help the main jet as stated above....I would start w/gapping the plug at the correct gap xct2
     
  9. corgi1

    corgi1 New Member

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    they may be pre gapped for a car which is usually 35 thou.on up to 80 or so...............
    The air filter causes less vaccum in the carb to pull gas up the jet,I havent desided that there is any venturi action working for these carbs........
    and the idle screw won't help the main jet as stated above....I would start w/gapping the plug at the correct gap xct2
     
  10. corgi1

    corgi1 New Member

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    please remove the second post
     
  11. Nougat

    Nougat New Member

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    Cory, you can move the clip on the needle up, so the needle sits lower in the throttle slide. That will lean the mix between about 20% and 80% throttle. To deal with the bike being rich when the throttle is wide open, you'll need to use a smaller jet. Lots of people solder them shut and drill them to a different size; I prefer just replacing it with a different size jet.
     
  12. cory151

    cory151 New Member

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    Just wanted to thank everyone for the input, I finally found a solution to the problem of the hi speed sputter. It turns out the phillips head screw on the muffler had backed out and was rattling 1/2" in out of the housing. Who knew that the muffler could have caused a loss of power on the top RPM range.

    Another weird aspect of it was that my midrange power was greatly increased. Go figure, I may be in the market for a better muffler thats still somewhat compact.
     
  13. cory151

    cory151 New Member

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    [​IMG]

    Heres the plug
     
  14. Nougat

    Nougat New Member

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    That plug is a bit lean, should be browner.
     
  15. cory151

    cory151 New Member

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    Yeah that plug was from when the muffler was coming loose. That explains why it had so much more midrange and low end power, leaners meaner.
     

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