I'm j opening a new topic for drag bike mike's question so that finshammer's topic doesn't get too cluttered.

Mike,s question:

Zevenenergie, taking that little moped from an 8K redline to 13K redline must have been fun.  Do you think it was choked off a bit with that 8mm carb?  That's like the size of a pencil.  I've got carbs that big on my model plane engines.  To run the engine up that high, did you have to switch to thinner piston rings?

I increased the cc to 73 and used a piston with thinner rings, but that isn't really necessary. This little engine went from 4 hp to about 15 hp and ran 100+ km/h with a variator drive and the lifespan is not really great.

And yes, that 8mm is a significant obstacle, but the biggest power gain came from the scooter exhaust I had installed. A 2-stroke is very sensitive to the dimensions of the exhaust.

If you put a standard exhaust on a 50cc 21hp 2-stroke racer, it only has about 6 hp.

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I’m going to give some thought to using a reed valve on a four-stroke engine and modifying the camshaft so that the intake valve closes well after BDC, thereby achieving optimal intake timing and volumetric efficiency at any RPM. It would also completely prevent reversion.And perhaps it generates an interesting pulse effect in the intake duct.

How does the rotary valve fit in.  I am familiar with rotary valve engines that have the carburetors on the side.  The intake charge flows through the carb parallel to the crank axles.  The rotary valve is a thin disk with a section cut out to interrupt air flow and seal off the crankcase.  Kawasaki used rotary valves in the late 1960s.  Bridgestone also.  Those are the designs I am familiar with.

Your case has the intake flow perpendicular to the crank axles.  What does the rotary valve look like?  How does it seal off the crankcase?  You have any pictures?   9/15/26:  Does the crank itself serve as the rotary valve, if so, how does it form a seal with the case?

Looks like you did a great job on the intake manifold.  I assume you then welded it to your modified case.  Pretty cool.  I can see how that would make a huge difference.

Interesting idea regarding the reed valve on a 4-stroke.  Not sure how the airflow would be with the reeds and cage in the flow path.  Also, might be tricky to maintain inertia with the drastic change in cross section.  Certainly would be a challenging project.

You are spot on regarding the expansion chamber.  A ring-ding doesn't make much power without an expansion chamber to yank the fresh charge right through the carb, crankcase, and into the cylinder.  The folks that figured that out were genius.  I recently saw an article about some folks in Europe who are marketing an eight cylinder two-stroke motorcycle.  The expansion chamber setup is so complicated that the only way they can make it is with 3D printing.  All you can see is a maze of expansion chambers.  It must sound crazy.