What needs to be modified to upgrade a 125cc motorcycle to 150cc?
4 Answers
Replace the cylinder, piston, piston rings, etc. The numbers 125 and 150 refer to the engine cylinder displacement. According to the "Motor Vehicle Registration Regulations," replacing the engine requires applying to the local vehicle management department. Replacing the piston and cylinder head: Between 125cc and 150cc, the piston, piston rings, cylinder head volume, and valve sizes are different. Besides some slight differences in the carburetor, other parts are completely the same. Replacement must be done by professionals. Replacing the cylinder kit and cylinder head: If only the cylinder kit is replaced without increasing the exhaust speed of the valves, it can cause the engine to overheat, and the power increase will not be significant. Modifying the displacement: You can replace the cylinder with a 150cc one and then replace the clutch plates with 150cc ones. This will significantly increase the engine's power.
To upgrade a 125cc motorcycle to 150cc, the core engine components must be modified. First, a larger cylinder block and piston set need to be installed, as the increased displacement requires more combustion chamber space. The crankshaft must also be replaced because the stroke changes, and the stock one can't handle the additional torque. The carburetor or fuel injection system needs recalibration for the air-fuel mixture; otherwise, insufficient fuel delivery could damage the engine. It's also advisable to reinforce the transmission system—clutch plates and gear ratios should match the new power output. Don’t forget to readjust the air-fuel ratio after the upgrade, or overheating and cylinder scoring may occur. This modification demands high technical expertise, so it's best handled by a professional mechanic. Post-upgrade, engine stability must be thoroughly tested, as a nearly 20% displacement increase is no small matter.
I just modified this, mainly focusing on the engine trio: the cylinder block, pistons, and crankshaft are must-replace items. The original bore is around 62.5mm, and it needs to be enlarged to 63.5-64mm to achieve 150cc. Forged pistons are recommended for better durability, as the stock cast iron ones can't handle it. The clutch springs need to be stiffened, otherwise, there will be severe slipping during acceleration. Remember to modify the diameter of the mid-section of the exhaust, as the stock exhaust will limit power output. A key reminder is to switch to full synthetic oil, as the increased displacement raises cooling demands. After the modification, it's essential to dyno tune it. A friend of mine didn't tune it and ended up with a seized engine, and the repair cost more than the modification itself.
Motorcycle displacement modification is essentially an engine rebuild. The core involves replacing the cylinder sleeve with a larger bore and matching piston rings, and swapping the crankshaft for a model with a longer stroke. The fuel injection system requires remapping, usually involving an ECU flash. It's advisable to reinforce the transmission gears, otherwise third gear is prone to gear grinding. The cooling system should ideally be upgraded with an oil cooler - I once had my temperature gauge max out during a long ride. After modification, it's recommended to go one size wider on tires to prevent fishtailing during cornering. The entire process takes about three days, with costs depending on parts quality - basic modifications can be done for around 2,000 yuan.
