Performance Guides

Rejetting the carburetor is a required step after any performance modification on a Chinese 50cc scooter. A lean engine is a damaged engine. This guide covers reading the spark plug, choosing the right main jet size for your specific mods, and dialing in the mixture...

A performance exhaust is one of the most effective and affordable upgrades for a Chinese 50cc scooter. The stock exhaust is heavily restrictive — a free-flowing performance exhaust releases that restriction and noticeably improves power. This guide covers the full...

Upgrading the clutch springs on a Chinese 50cc scooter delays engagement to higher RPM, keeping the engine in its power band longer for improved acceleration. This guide covers the complete clutch spring swap on a QMB139 engine. Tools Needed Clutch puller toolSocket...

Dr. Pulley sliding weights are one of the best bang-for-buck upgrades for a Chinese 50cc scooter. This guide explains the difference between sliders and round rollers for the QMB139 engine, how to choose the right 16x13 mm gram weight, and how to install them. Tools...

Roller weights are one of the best and cheapest CVT performance upgrades for a Chinese 50cc scooter. The QMB139 uses 16x13 mm roller weights — different from the GY6 150cc. This guide explains how to choose the right weight for your riding style and how to install...

A performance variator upgrades the CVT ratio spread on a QMB139 50cc scooter for better acceleration and higher top speed. The swap takes about an hour and uses basic tools. This guide covers confirming compatibility, removal, installation, and tuning. Tools Needed...

Installing a 70cc big bore kit on a QMB139 50cc engine is one of the most popular Chinese scooter performance builds — and easier than it sounds. The QMB139's vertical cylinder is accessible from the top of the scooter, and most builds are complete in under 3 hours...

The most effective performance upgrade for a Chinese 50cc scooter is a big bore kit. By replacing the cylinder and piston with a larger bore, you increase displacement from 50cc to 70cc or 80cc — producing significantly more power. This guide covers how to choose the...

A performance exhaust is one of the most impactful bolt-on upgrades for a GY6 scooter — reducing backpressure, improving exhaust scavenging, and freeing up power across the RPM range. This guide walks through the full installation from panel removal to rejetting....

Any time you increase airflow through a GY6 engine — new exhaust, big bore kit, performance air filter — you must rejet the carburetor to add more fuel. Running lean is the fastest way to destroy a piston. This guide covers reading the spark plug and dialing in the...