How to Increase the Power of the 48cc Engine of Motorized Bicycles

A " motorized bicycle" it is a bicycle with a gasoline powered 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" or "Super M" synthetic oil by Maxima. 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 inclined streets. This page lists the necessary modifications to make it where you can climb most streets up to around 10 degrees inclination. 

Make sure the hole diameter in the carburetors main jet is within .02mm of .6mm (#60) to maintain the ideal ratio of air to gasoline (15 to 1) at or near seal level. (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).This photo shows the center jet hole filled with solder and a hole drilled into it for the correct sizing.

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.

The cylinder compression in this engine is low at 100 pounds per square inch. 20% more compression will give 10% more speed/power. This can be acheived by reducing the head surface by 3mm. It can easily be machined off or you can use course sandpaper on glass and reduce the head surface manually. Standard cylinder compression is 100psi and will increase to 120psi after this modification. But it is best to replace the weak bearing in the piston if you do this modification.

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.

There is also an ignition system for this engine (CDI with the ignition coil) which is available for $75. 2.5% more engine power is derived for each 33% increase in spark energy according to research, "Devices to Improve the performance of a conventional two-stroke spark ignition engine". To read more about how to increase power 10% with this system click here.

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