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How to Adjust the Auto Clutch on a 50cc Bike

Updated February 21, 2017

The auto clutch on a 50cc bike is designed to engage at a specific rpm when you twist the throttle. If the clutch engages too soon, the engine bogs because it doesn't have sufficient torque to get the bike rolling. If the clutch engages late, the engine is going faster than the specified rpm and the clutch plates can burn or glaze. Mechanical experience and a special accessory tool and are needed to adjust the auto clutch on a 50cc bike.

Refer to the connection instructions for a battery-powered digital tachometer. Connect the unit to the appropriate ignition wires and ground on the bike as described. Start the bike and sit on the seat with both feet on the ground.

Hold the tachometer in your left hand. Slowly twist the throttle grip on the bike as you observe the reading on the tachometer. Note the rpm when the clutch engages. You increase the disc-spring tension if the rpm is below 7,200 when the clutch engages. Decrease the disc-spring tension if the rpm is over 7,500 when the clutch engages.

Turn off the engine. Disconnect the digital tachometer and set it aside.

Loosen and remove the 14mm pivot nut at the base of the foot brake pedal on the right side of the bike. Remove the pedal return spring and take the foot brake pedal off.

Loosen and remove the clutch cover bolts using a 6mm hex wrench. Carefully pry the clutch cover off with a screwdriver to expose the auto clutch. Set the cover and bolts aside.

Install a clutch drum tool onto the notches on the face of the clutch. Hold the handle of the tool with one hand to prevent the clutch from rotating. Loosen and remove the main bolt at the centre of the clutch with a 10mm hex wrench. Set the drum tool, main bolt and wrench aside.

Lift the disc-spring stack off the face of the clutch by hand to expose the spacers. Add a 5mm disc spacer under the stack to increase the disc-spring tension. Remove one of the existing 5mm preload shims to decrease the disc-spring tension.

Reposition the disc-spring stack atop the shims at the centre of the clutch. Thread the main bolt in by hand and tighten it with the 10mm wrench, using the clutch drum tool as a backup to prevent the clutch from rotating.

Reattach the clutch cover and secure it using the saved bolts that you tighten with the 6mm hex wrench. Reattach the foot brake pedal to the pivot and reconnect the return spring. Thread the saved pivot nut on at the base of the pedal and tighten it with the 14mm wrench.

Tip

Prolong the life of the clutch by changing the transmission fluid before any competitive riding or after four hours of riding.

Things You'll Need

  • Battery-powered digital tachometer
  • Metric wrenches
  • Metric hex wrenches
  • Screwdriver
  • Clutch drum tool
  • 5mm shim
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About the Author

William Machin began work in construction at the age of 15, while still in high school. In 35 years, he's gained expertise in all phases of residential construction, retrofit and remodeling. His hobbies include horses, motorcycles, road racing and sport fishing. He studied architecture at Taft Junior College.