Inlet-side power valves on two-strokes.

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Well-Known Member
Dec 26, 2021
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Kawerau, New Zealand
This is an aside from my thread on my two-stroke mods.

1) I saw on "2Stroke Stuffing" where an inlet-side power valve was used.
There do not seem to be any articles on this coming up in a google search.

Is this a new thing?

This raises the possibility of converting a reed valve engine to work more like a disc valve engine.
Crank-case and inlet resonance, combined with pipe draw, allow the reed valve to be totally bypassed.
No production engines seem to use this system.
It seems to work best when the engine is fully on the pipe.

The exhaust power valve is common.

How do exhaust power valves work? Is this an electronically controlled actuator or servo?

2) I saw an article on Zenith gocart gearbox (shifter) engines which claim 40 hp at 14500 rpm on a reed valve, and a 30mm carb.

This seems rather a high power for this rpm.

at 54.5mm stroke, the engine should turn more rpms than this. Maybe a longer rod would be a good idea, to limit piston side-thrust.
Formula one engines go right up to 2:1 rod ratios for high rpm use.

I am eying up a Yamaha YZ 125 engine as a potential starter parts donor.
I will research expected output and rpms, and tuning, a bit more.
Back in the 80's I owned a yz 125 with a ypvs or yamaha power valve system.
It was mechanically driven with linkage by rpm as I remember.
Never seen one on the intake side, only read valves. The reason I had to look at this thread.

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