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Reed Valves?

Discussion in 'Technical and Troubleshooting Torque' started by rodgerdodger, Sep 15, 2005.

  1. I've come references to reed valves a couple of times and was wondering how they're operated and why certain, but not all, (two stroke) bikes use them. How often do they need to be replaced/rebuilt/whatever happens with them? And whether the aftermarket valves offer any kind of advantage? Cheers.

  2. Aftermarket reed valves can have a different degree of stiffness depending on the material used - alloy, carbon fibre and I think Kevlar reed valves are available which will allow it to snap close quicker and therefore increase power.
  3. Older style two stokes used rotary valves.

    Older ones agains didn't have them at all.
  4. Yamaha strted using reed valves in the 70's mainly because they were cheap and reliable. Most 2 strokes before this had relied on rotary disk vavles (and some still do, hence the reference to some bikes not having reed valves)

    Reed valves offered the advantage of being able to induct mixture into the cylinder, rather than via the crankcase as rotary valves do. Rotary valve engines had their carbies on the side of the engine which, in racing terms, was not ideal.

    Rotary valves are actually more efficient than reed vavles but reed valve technology has been greatlty developed and works extremely well also.

    After-market valves can have advantages but only when used in combination with other legitimate tuning practices. They're not a "magic bullet" to extra horsepower all by themselves.
  5. Is this right?

    Yes rotary valves are on the side of the crankcase. Yes reed valves are on the cylinder.

    But it was my understanding that the mixture still had to be pumped be the crankcase, because 2-strokes has no suction stroke.

    Unless of course they are supercharged.

    sorry to nit-pick
  6. You're right ibast,

    Even though reed valve enginges have the carbies located on the cylinder, they still induct the fuel air charge into the crankcase before using the decending piston to pump it above the piston via the transfer port.

    Reed valves allow more aggressive cut-out on the lower rear of the piston (effectively the intake port)... kinda like extended duration of intake valve opening on 4 strokes.
  7. cool. I now know a bit more.