The flipper is weak or barely moves.

Weak flipper: dirty end-of-stroke switch

Cause: The end-of-stroke switch is not making good contact.

Checks

• Clip a lead across the coil middle and end lugs (bridging the end-of-stroke switch) and tap the button briefly; a strong flipper confirms the switch.

Fix

• Clean, adjust or replace the end-of-stroke switch.

Only tap the button with the jumper fitted; holding it overheats the coil.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C
• WPC Alpha-Numeric
• WPC Dot Matrix

Related tips

• Weak flipper: rectifier or worn parts
• Weak flippers from pitted EOS switches
• Contact files only on flipper switches
• Weak, chattering or buzzing flipper: end-of-stroke switch
• Pre-Fliptronics: weak flipper
• Flipper fuse blows after a rebuild: EOS switch barely opens
• Every System 11 flipper has an EOS switch and coil diodes
• One or both flippers weak

Source: Pinball Machines: How They Work & Troubleshooting, Norbert Snicer