How to Increase the Power of the 48cc Engine of Motorized Bicycles
version en español

A " motorized bicycle" it is a bicycle with a 48cc two stroke engine installed. Along with the engine comes one fuel tank, ignition system, chain, and sprocket. Like a motorcycle the control for the carburetor is at the right hand, and the control for the clutch is at the left hand of the handlebars. The fuel to use is "Extra" and the oil to put inside the tank (2 ounces per tank) is "Valvoline Super 2T" available in Coral or "Super M" synthetic (almost smokeless) from the motorcycle clothing shop in Millenium Mall. Without changes to the carburetor the kilometers you can travel per each tank are 60. With changes they are 80. The maximum speed is 26mph. The engine does not have multiple gears. It is "one speed". The sprockets are sized for a good compromise between torque and speed. It is a good idea to put lights in front and back to avoid accidents at night by drivers of cars that cannot see bicycles well. In order to start the motor your need to pull toward you the clutch lever, pedal the bicycle, and then release the lever to engage the motor. There is a choke lever on the carburetor that needs to be repositioned when the climate is cold so that the engine starts and runs easily until it is hot. Then you can return the handle to the normal position. The whole kit comes from China and is Ok but has much room for improvement. Replacement and improvement parts are available on-line from the States at pistonbikes.com gasbike.net sickbikeparts.com and bicycle-engines.com. Here's some things worthwhile buying: LED headlight $15 front and back lights $11 horn $9 speedometer $12 round key cable lock $17  B5HS spark plug $2.30

Without modifications this engine only has 2 horsepower and is not strong enough to easily haul heavy people or climb the inclined streets which are common here in Cuenca. This page lists the necessary modifications to make it where you can climb most streets up to around 10 degrees inclination. 

Reduce the hole diameter in the carburetors main jet to .6mm to maintain the ideal ratio of air to gasoline (15 to 1) in this high atmosphere where the air is less dense. (look at this chart). Less gasoline in places of high altitude surprisingly gives more engine power (and around 80km per tank which is 1/2 gallon).


Install an extension (10-14cm) between the carburetor and the motor which makes use of vacuum/pressure wave resonance to push 15% more air/gas mixture into the engine for 15% more power when your speed is less than 15mph. This is very important if you want to be able to climb inclined streets.

Cut 1.5mm from the bottom of the intake skirt of the piston to permit the air/gas mixture to enter the engine 4 degrees earlier in each engine cycle. The period of intake port opening will then be 121 degrees as this chart shows is the best for both low and high rpm's. Or if you want the best climbing power and don't care about losing 1-2mph in top speed then cut the skirt 2mm for an intake period of 125 degrees.

The engine compression of air/gas is less in places of high altitude and needs to be corrected by reducing the compressed combustion area. This can be done by not using the head gasket. But to prevent any compression leaks both mating surfaces need to be leveled by moving them in a circular pattern on sandpaper on glass. Also you can increase the compression even more by having the head mating surface milled down by .5mm.

Use a rotary tool to change the ports in the cylinder to be equal to the measurements in the drawing below. This will increase power throughout the whole rpm range.


Replace the junk stock ignition system for 10% more power. Available from BioElectric/CCnow for $60. Click here to read more.

References: "The Effect of Crankcase Volume and the Inlet System on the Delivery Ratio of Two-Stroke Cycle Engine" by Fujio Nagao of the University of Kyoto

Motorized Bicycles are available from the business "Auto Comercio" on Avenida de las Americas (east of Super Maxi) in Cuenca. Call Esteban Mogrovejo at 080-718-421. They charge $250 to install the engine kit on your bicycle, or $400 for a complete motorized bicycle.