It’s sort of futile hopping up a 2-horsepower bicycle engine because the rear sprocket-to-spoke connection is so weak. Spindly running gear has never stopped a hot-rodder before and bicycle motor aficionados are hot-rodders to the bone. While Mini Motor Madness is on hold awaiting parts I wandered into the wonderfully inexpensive world of high performance engine goodies.
The bog standard bicycle motor is not heavily finned. Overheating doesn’t seem to be a problem but with an aluminum bore and a recommended 16:1 oil/gas ratio it is clearly something to watch out for. Like all parts for these little motors there are a zillion sellers insuring fierce competition and low prices for the thrifty Rodder. The increase in the quantity of cylinder head metal alone would be helpful in keeping things square and dampening noise. The large, deep fins must do a great job of lowering cylinder temperatures. A cooler cylinder means you’ll be able to run a leaner mixture without melting internal parts. Less oil means more room for fuel going through those tiny orifices in your carb. It’s like changing jet size by playing with the fuel ratio. Any compression increases would probably be nullified by the low-grade gasoline cheap mini motor fans cannot help but use.
There is even more selection when it comes to exhaust systems. Two-piece, welded seam expansion chambers are the most numerous and various finishes are on offer to suit anyone’s budget. Expansion chambers are like free horsepower in that most times you won’t need to do much carburetor work to get the things in tune. Using a combination of black magic and sonic waves, expansion chambers most often move the powerband higher into the rev range and will allow the motor to spin up another few thousand RPM. Those extra RPM’s are a good thing for drafting Indian FTR750s on the long straightaways at The Sacramento Mile but a chambered motor may not get you over that last vertical shelf of a long, rocky hill climb. It’s the one where you don’t dare downshift for fear of losing momentum.
No matter how well designed, reed valves are an obstruction in the intake tract of a mini motor. They make up for this by stopping the constant re-carburation of the fuel mixture due to the reverse air pulses created by a sloppy piston port intake. Reeds also allow huge holes in the piston skirt to extend the degrees of rotation a crankcase can draw in the fuel/air mixture. Since nothing can blow back out the carburetor, an extra transfer port can be carved into the intake side of the cylinder wall. Combined with an expansion chamber, reed valves can more than make up for blocking the intake tract. Even without all the piston/transfer port butchery low speed running is improved by the cleaner intake signal.
With all the excitement focused on pumping more air through your mini motor you’ll find the stock carb wanting. Large carburetors are really cheap online so why not go all the way? For less than $200 you can convert your mild-mannered, reliable mini motor into an atomic bomb ready to explode between your legs at any moment. Sure it’s stupid. Sure you will burn more fuel and annoy all you neighbors, but when have those things mattered to a Rodder?