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Collection, Preservation and Display of Old Lawn Mowers

What is the correct method of installing reed valves?

The reed valves, just a piece of shaped shim steel, has finally arrived for the two stroke engine on my American made edging machine. A slight problem now faces me, a short distance from the mounting holes I now find a kink in the metal, I imagine this is to help keep the crankcase closed upon compression. Mounting it one way up would therefore keep it slightly open, until compression pressure closed it, fitted the other way around, it would be tightly closed until the suction from the ascending piston opened it?

I'm not really sure I've got that 100% correct and I invite others' comments please.


hillsider Mon, 01/02/2021

Personally I would fit the reed valves so that they are held in the closed position.
As you point out they open to suck fuel into the crankcase as the piston rises and creates a vacuum in the crankcase but any delay in them closing will affect the transfer of the mixture to the combustion chamber so having that bit of tension on the reed will improve the way they work.


wristpin Mon, 01/02/2021

The reeds must allow mixture to be drawn in on the upward stroke of the piston but must close to "trap" it there so that the downward stroke forces it into the cylinder via the transfer port(s).

Carb side


Crankcase side

Crankcase side 

DJD Mon, 01/02/2021

Thank you both for your quick replies, after sending the post I vaguely remembered reading an old bike article about the sound of the reeds 'snapping shut' a bit bigger than mine I imagine!

I have to make the covering parts which limit the amount they open I imagine, plus also stopping them bending right back, once you hit the loud pedal!

Cheers again for your help.