carburetion problems

Discussion in 'Introduce Yourself' started by Bgnlb, Aug 18, 2014.

  1. Bgnlb

    Bgnlb Member

    I have a 66cc with a expansion pipe, it won't rev out unless I shut off petcock. I had the same problem on my other bike.l put the stock pipe on and dropped the needle all the way down and problem went away. Why won't run with the expansion pipe?
     

  2. Anhevius

    Anhevius New Member

    The best thing I can figure is that you're running without enough back pressure on the motor. It's not something that's truly commonly known, but on a carbureted engine you need some restriction to the exhaust to provide a certain amount of back pressure. Or you could be getting weak spark blowout.

    To test that, you'll need two people and a stand to pick the rear wheel, or disconnect the drive chain.

    With the engine COLD, start it. Once it's warmed enough to respond to throttle pulls easily (maybe a minute or so), put your hand over the exhaust. (CHECK TO MAKE SURE IT'S NOT TOO HOT FIRST. I take no responsibility for burns). With moderate pressure, try to keep your hand over the exhaust, but don't try too hard. Have the other person work the motor through the RPM range once or twice. Then try another pull without the exhaust covered. You'll know if it is due to a massively weaker pull with the exhaust unblocked.

    If that isn't the issue, then look at spark next. If you're using the spark plug that came with it, pull it, throw it out, and get an NGK iridium plug. The plugs that come with any kit motor are absolute trash. Try it with the new plug. If that doesn't fix it, get a multimeter and check your voltages and resistances from the magneto and the CDI.

    You can also do a spark check. Pull the plug, and reconnect the boot. Ground the plug by holding the metal area of it against the engine block (hold it by the rubber boot). In a dark area, turn the pedals with the clutch engaged. Watch the plug. You should get a bright spark with an audible snapping noise each revolution. If it's dim, then double-check your wiring and everything from the magneto to the coil.

    Now, if it is the expansion chamber, there are a few ways to go about fixing it. My prefered method is introducing a sharp bend in the exhaust pipe as soon as possible as it comes out from the motor. Or, if you've got decent welding skills, you can make your own baffle pipe. Just get a length of exhaust pipe with a flanged end that will go over the end of your exhaust. Using a jigsaw or bandsaw (NOT a torch), split it in half. Tack weld in a few plates, about every two inches. Each plate should obstruct half of the pipe, and it doesn't need to be a perfect fit. You're just looking for creating resistance for the air flow. Weld the pipe back together, and connect it to the end of your exhaust. You can use exhaust clamps, JB Weld, or weld it on if you like. I prefer a good solid weld, but that's just me.
     
  3. jaguar

    jaguar Well-Known Member

    expansion pipes create more dynamic compression at high rpm which requires different jetting. It also stuffs in more fuel/air mixture that was trying to escape out the exhaust pipe which probably makes it richer.
     
  4. Bgnlb

    Bgnlb Member

    Thanks for the advice, these dam things sure finicky. For the Price I guess its expected my other bike runs fine now but I went thru the same thing it ran good after break in and with the exp pipe.then all of the sudden it wouldn't rev out I switched out the carb and put on the stock pipe and problem solved.
     
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