Engine Intake Info

  • 'some late 85's, 86's and '87 SLE should be a flat reed engine (the flat reed engine came into play in late '85)
    • If the engine and swing arm move with the suspension, it is a flat reed
    • If the motor is rigidly attached to the frame it's piston port or pyramid reed
  • the 81 SLE was pyramid reed
  • Derbi RD's were pyramid reed
  • Derbi Diablos and Lagunas piston port
  • the Derbi DS50 scooter is flat reed

Technical Specs

Piston port engines

  • cylinder inducted
  • stroke: 43mm
  • bore: 38mm
  • displacement: 48.7cc
  • compression 8.5:1
  • direction of rotation: counter clockwise facing magneto

Download the repair manual for Derbi piston port motors here.

Pyramid reed engines

Pyramid reed engines share the same internals as piston port motors except they are case inducted.

  • Pyramid 3-reed valve, Crankcase mounted
  • stroke: 43mm
  • bore: 38mm
  • displacement: 48.6cc
  • compression 8.5:1
  • direction of rotation: counter clockwise facing magneto.

Flat reed engines

flat reed engines were completely redesigned from the earlier piston port/pyramid reed engines. They are not directly swappable to older bikes. Flat reed engines come stock with a 6 roller variator which accepts standard 16x3 variator weights. Flat reed motors also use cast aluminum rear pulleys instead of the stamped steel version on older engines.

  • Crankcase 2-reed valve induction with triple transfer port distribution
  • stroke: 40mm
  • bore: 39.87mm
  • displacement: 49cc
  • compression: 10:1
  • direction of rotation: clockwise facing magneto.
  • Max engine RPM: 6500 (stock configuration)
  • Flywheel and Clutch bell nuts are reverse thread

See a PDF file of the flat reed engine repair manual here.

Make a Water-Cooled Derbi

Please see this discussion from the forums.

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