JetSki motorcycle engine - Loud and Hot exhaust

Discussion in 'Propulsion' started by IronTom, Dec 1, 2019.

  1. IronTom
    Joined: Dec 2019
    Posts: 5
    Likes: 1, Points: 3
    Location: arizona

    IronTom Junior Member

    NO JET PUMP. Engine is Yamaha YZ125 water-cooled with drive shaft to 3blade metal prop. Enclosed in metal grille with trailing rudder. Classic inboard setup with engine's water pump drawing in lake water to the cooling jacket. Engine hot water exits thru 30' of copper tubing coils around the exhaust expansion pipe and back into the lake. The exhaust 'stinger' is routed up above the waterline, then down to the transom exhaust flapper.
    Note: NO water-injection into the exhaust. The copper tubing ends into the exhaust flapper.
    Problem --- Motocross race-bike NOISE, and lots of it. I replaced the plastic air-box with 7feet of 3inch flex hose leading up to the bow, which quieted the intake roar. I'm considering wrapping the coil-covered expansion chamber with fiberglass 'muffler tape' to dampen the Ring-Ding-Ding.
    We have seen small mopeds with taped exhaust and they are much quieter. However, it seems to serve no purpose on a big Harley, or a loud race bike. There must be another solution.
    An old shrimp-boat owner described his old diesel engine that had a muffler made from a ceramic-tile-sewer-pipe with tail pipe 'in the sky'. It laid below the engine in a sea water hot box, which also provided hot water used to control the shrimp/ice melt during un-loading. (Shrimps are frozen when caught).
    I'm old, but I'm not a sailor. Is this what they call a "water muffler" ?
    Could I build a water-cooled-ceramic-expansion-chamber muffler ?
    Your input is welcomed.
  2. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    We've made a small 2 stroke with a dry exhaust very quiet by using a simple straight through muffler after the expansion chamber stinger. If you have a dry exhaust you can easily add a straight through muffler and it will become very quiet. Our system had a classic expansion chamber and followed it up with a one inch ID straight through fiberglass packed muffler that was about a foot long. With our system the loudest noise was the hull slapping the water. We were at about 72 db at 15 feet away. If you just get the motorcycle exhaust and a following muffler and wrap the expansion chamber like you did you'll be fine. Going to a wet exhaust system is going to be a lot harder because then you can't use a glass packed muffler with water in the exhaust. You're better off staying dry and adding a straight through silencer after your expansion chamber. If you look exhaust systems for your motor cycle that comes with a proper muffler that is the way to go, you can mod it to fit and cool the outside of it the way you've done with your present system.
  3. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    Also get an airbox, the long tube helps, but an airbox will make the intake totally quiet. 2 strokes have a lot of intake noise and a go kart air box will totally kill the intake noise

  4. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    The long intake will also restrict the air flow significantly.

    You either need to go short, or go with a much greater diameter.

    A simple motorcycle muffler will do, find one with the correct diameter for the current stinger.
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