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 timing 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.).-- discontinuuity, Jan 04 2013 Vespa moped crankshaft http://uphillbattle...ds/old/olympia2.jpgthe top half shows where it is open, and the bottom where it closes against the crankcase port [discontinuuity, Jan 04 2013] intake port http://i260.photobu...140275/09f9adb2.jpg [discontinuuity, Jan 04 2013] built-up crankshaft http://2.bp.blogspo...aft+Exploded+01.jpgthis is for a multi-cylinder engine, but you get the idea. [discontinuuity, Jan 04 2013] more on Vespa rotary intakes http://outofcontrol...per-stock-ciao.htmlincluding more pictures of Vespa engine internals, and modifications of the stock engine [discontinuuity, Jan 04 2013] RG500 Engine Cutaway. http://www.southbay...tachmentid=98443&d=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?-- Brian the Painter, Jan 04 2013 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 properly.
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.-- discontinuuity, Jan 04 2013 At 35 mph your feet could be "bolt on" power adders.-- Brian the Painter, Jan 04 2013 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.-- Letsbuildafort, Jan 04 2013 From Wikipedia:
"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"".-- normzone, Jan 04 2013 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.-- Letsbuildafort, Jan 04 2013 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.-- discontinuuity, Jan 05 2013 random, halfbakery