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Vespa mopeds (not to be confused with Vespa scooters)
use a rotary intake port instead of a piston port or reed
valve. Parts of the crankshaft cover/uncover a port in the
case to open/close the intake port and allow a fresh charge
into the crankcase. This system allows for better intake
and higher intake velocities. The only problem is
that you're limited to a 13mm intake/carb, due to the size
of the crank itself, and the geometry of the crankcase.
Now from what I understand, moped crankshafts are "built-
up" from several sections press-fit together. It should
then be possible to take apart two Vespa crankshafts and
fit the left sides of each crank together to make one
crankshaft that will work with two inlets. So instead of a
single port being covered/uncovered by a single crank
lobe, you'll have two intake ports for twice the flow. Of
course this will require you to modify the engine case with
another intake port. You'll also need a custom intake to
connect a single larger carburetor to the two inlets, since
two stock carbs won't fit in the available space.
At this point it might be easier to make a custom
crankcase from scratch, especially since it would allow you
to overcome some of the other shortcomings of the Vespa
engine design (only 3 head studs, off-center spark plug,
fixed spark timing, no boost port, etc.).
Vespa moped crankshaft
the top half shows where it is open, and the bottom where it closes against the crankcase port [discontinuuity, Jan 04 2013]
[discontinuuity, Jan 04 2013]
this is for a multi-cylinder engine, but you get the idea. [discontinuuity, Jan 04 2013]
more on Vespa rotary intakes
including more pictures of Vespa engine internals, and modifications of the stock engine [discontinuuity, Jan 04 2013]
RG500 Engine Cutaway.
Complex rotary valves. (x4) [Letsbuildafort, Jan 04 2013]
||I think you would have to make the crankcase from
scratch. Welding on that cheap metal (i have heard)
is next to impossible. Are two strokes still legal? In
Canada you can't even have one on a weed eater.
Can you bolt power onto a Vespa? Like a turbo or
compressor type? Maybe a little nitro?
||I've heard of people fixing broken moped
crankcases with JB Weld, so it's possible. The
trick would be in lining everything up so it works
||Two-strokes are definitely still legal, at least in the
US. For new on-road vehicles, I believe they are
limited to 50cc or below. There are various
regulations for two-stroke emissions from boats,
snowmobiles, off-road motorcycles, etc.
||Bolt-on performance for Vespa mopeds is mostly
limited to bigger displacement cylinders, or
expansion chamber exhaust pipes. My moped has
an expansion chamber, and reaches a dizzying
speed of 35 mph.
||At 35 mph your feet could be "bolt on" power adders.
||Simple answer: buy a bigger scooter. You want
complex rotary valves? Scope out the legendary
RG500! (with link).
There are several technical hurdles to achieving
more horsepower than simply increasing the
volume of your intake charge. If you go through
the trouble with JB-fabbing crankcases together
while maintaining your factory crank, cylinder,
intake tract or carb, just fit a bigger engine.
Choices that went into the production on the
motor for that poor little wasp were founded in
ease of production, projected managed power
output and cost. For a moped, it probably meets
those tenets pretty well; and to be frank - if its
worth doing, its worth OVERdoing.
|| "The Suzuki RG500 "Gamma" was a motorcycle built by Suzuki between 1985 and 1987 and inspired by the 1984 Suzuki RG500 "Gamma" Grand Prix motorcycle, capitalizing on Suzuki's seven consecutive constructors title wins in the 500 cc-class.
||The Gamma was powered by a two stroke, rotary valve, twin crank, square four engine displacing 498 cubic centimeters. The power output was 93.7 brake horsepower (69.9 kW) at 9,500 RPM."
||Wow, that sounds like fun. I owned a 1972 Kawasaki H2, three cylinder two stroke 750 cc, which was probably not as sophisticated, but it was an interesting machine to launch.
||Again from Wikipedia, "notoriously dangerous, being prone to wheelies and speed wobbles, the dangerous handling characteristics arising from its mediocre frame design and caused it to be nicknamed the "Widowmaker"".
||The "RG500 MK II" from 1978 was a real wonder of
half-cocked, stupid fast technology. I still mourn the
selling of my '77 RD400.
||I'm in the process of fixing up an 80cc scooter, so
really I have no need for a hot rod moped. But I want
to build one just the same. Souping up mopeds isn't
really logical, but it is fun.
||That Suzuki engine is pretty incredible, and I bet it
was something else to ride. Its disc-type rotary
valves are slightly more complex, but probably much
more efficient that the Vespa design.