Expansion chamber performance?

Discussion in 'Performance Mods' started by JVROOM, Sep 28, 2012.


    JVROOM Member

    Hi, have a question. Working on the 66 cc slant head, put a expansion chamber on, doesn't run real good. Vibration, seems to be running rough and maybe missing a bit? Could I be leaning out? Think about putting the kit exhaust back on, do I need to rejet riching up a bit? Thanks for your comments.:confused:

  2. wheelbender6

    wheelbender6 Well-Known Member

    I'm sure rejetting would help. Many members have been thru that.
  3. MotorBicycleRacing

    MotorBicycleRacing Well-Known Member

    I got a question too.
    What expansion chamber?
    There are expansion chambers then there are EXPANSION CHAMBERS.

    JVROOM Member

    Expansion chamber question

    Hi, thanks for the replies. I guess rejeting means richen things up, regarding the expansion chamber, don't remember exactly where I got it but it's one of those small chrome ones, that bolts right onto the head, with a small outlet about 3/8 inch diameter exiting near the pedal crank:bowdown::bowdown:
  5. jaguar

    jaguar Well-Known Member

    God, what a horrible design. The first cone (the diffuser) needs to have a certain distance from the engine or it will send a reverse suction wave back to the engine too quickly and will suck out fuel mixture making the engine run weaker. When it is far enough away from the engine the wave returns later to counter the suction of the crankcase at top rpm as the piston rises from bottom dead center.
    If you are going to install an expansion chamber then take the time and effort to do it right so that it can perform the function for which it was designed.
  6. MotorBicycleRacing

    MotorBicycleRacing Well-Known Member

    Sounds like the infamous "banana" pipe.
    They are Chinese designed junk, jaguar gave you a more technical response. :bowdown:

    They aren't a real expansion chamber and have sort of baffle inside.
    The banana pipe makes a lot of noise which may give you the false impression
    that you are going faster.

    If you want a low pipe look at Sick Bike Parts pipe.
    Arrow Motorized Cycles makes a good up pipe.
  7. Ludwig II

    Ludwig II Member

    2 strokes are designed as harmonic devices. Anything that alters one aspect of the engine (in this case the exhaust) affects the rest of it, including the intake. Were there any modifications recommended by the pipe supplier?

    JVROOM Member

    Thank you for your help, I hope I won't always be a newbe. For 5 monhts I'm working on a 15-year-old Schwinn Phantom. Had one heck of the time getting past chain rub on the rear fat whitewall never thought I'd get there but made it.

    Ported the intake and exhaust, upgraded the carburetor, have a Jag CDI on the way, their in stock now. The bike is dripping with the chrome, and I'm down to fine tune. Focusing on the exhaust, and thinking about it reed kit.

    I've got five different chrome exhaust pipe/muffler systems laying on the floor. The expansion chamber question I guess is junk its the banana type. I have another one with about a 2 foot head pipe, and then the banana junk, which might fit, maybe but it looks dumb.

    I guess I've got three options, but the chrome version of the stock exhaust muffler back on, try for the banana junk with head pipe, or the banana junk and rejet. Yes it the design of these banana expansion chambers is really bad maybe I should just go back to the stock muffler design?

    What really surprised me was I had this bike motorized with an old five flags kit that user roller drive on the rearwheel. It was a snap to put it on and it worked great. But the roller unit finally wore out and are no longer available. I was able to set it up and ran it for a long time it was a piece of cake, instead of five months it took me about two days.

    Thanks for everyone's help sincerely, newbe burning out