Back in April of this year, I replaced the cracked stock (iron 49cc) cylinder with an aftermarket aluminum 80cc cylinder, piston, pin, and head assembly. This added immense more power over the stock output; no longer did I need to feather the clutch when starting off - the stock engine died if I let the clutch out too quickly - and it gained torque and horsepower across the RPM spectrum. However, the new engine was limited by the geared-for-acceleration sprockets and teeny-tiny carburetor.
For your reference, here is how a high-performance two-stroke engine works:
Click the image to see the wiki article.
Here's what I did to my scoot this weekend:
- Replaced the sprockets. Stock is 12 teeth front and 47 teeth rear, resulting in a theoretical top speed of 62 mph at 12,000 RPM. My new sprockets are 15:43, with a theoretical top speed of 85 mph. Also, the rear sprocket is now aluminum, meaning shorter life but a lot less unsprung weight for quicker acceleration. I didn't replace the chain (it only has about 3000 miles, and the sprockets looked nearly new), but I did clean the grime out of it and thoroughly lube it.
- Replaced the stock carburetor. The stock carb has a tiny 12mm bore, and it also features a water-heated passageway to make it easier to start in cold weather while also reducing the intake air-mass. These features limit power in two ways, reducing flow and reducing the amount of fuel/air mix entering the engine. My new carb is a 24mm Mikuni unit. That's twice the diameter, resulting in something like four times the airflow. And sans heated intake, it moves a lot denser fuel/air charge.
- Clearly, the stock airbox with its tiny snorkel would have strangled this new carb, so I removed that and simply capped the Mikuni with a free-breathing, washable, pod-type filter.
- Next in line bringing fuel/air mix into the engine is the reed cage (that's the little rectangular flapper device between carb and engine in the graphic above). In a two-stroke engine, either a rotary valve (as in most Vespas) or a reed cage allows the intake charge into the engine at the right moment. My new cage uses bigger reeds that open further to let in much more intake charge. The reed petals are also made out of carbon fiber, which is far lighter than the stock metal petals, so they open and close much faster for better intake timing.
- I also did some other needed work, like replacing a cracked blinker lens and cleaning every part of the machine that is normally covered with body panels and the like. I must have spent several hours just cleaning and Armor-All-ing the plastic.
So, how did it work? Here are my top speeds so far:
Stock: In stock trim, this bike will do about 45 mph.
Stage One: Otherwise stock, the 80cc cylinder kit brought top speed to about 60 mph.
Stage Two: With new gearing, new carburetor, new reeds, and new air filter, I hit an indicated 76 mph at just under 11,000 RPM. Top safe RPM on this engine is 12,000 RPM, so the theoretical top speed of 85 mph is probably attainable with this new configuration.
Stage Three will include a new exhaust chamber (exhaust resonance makes a huge difference in two-stroke engine tuning). Stage Four: Back to 49cc cylinder kit (for the land-speed-record run) with nitrous injection! With the addition of a racing exhaust chamber and nitrous injection to the current intake and gearing setup, I hope to boost the little 49cc engine back up to its 80cc performance, meaning the 49cc stock-body/power-adder record should be mine!
However, I have to admit that the biggest barrier to higher speeds right now is me. I'm six feet tall and rather broad across the shoulders. With my helmet tucked behind the windscreen and resting on the gas tank, my elbows pressed against the middle of my thighs. Also, I weigh about 180-185 pounds, about as much as the scooter, itself. When it comes time for the real top-speed run in 49cc configuration, I think I'll ask my friend, Alex, to do the run for me. He is probably several inches and 40 pounds lighter, which means he'll get up to speed quicker and be able to reach a higher top speed. *sigh*
More news as I get it!