Carl Cranke 250 Porting

by Alan Buehner

Originally printed in the 2000 issue #8 of Still....Keeping Track

In January, I contacted Greg Johnson of West Virginia who faxed me a copy of a Penton West Dealer Service Bulletin (no. 7) issued by Carl Cranke who was the Service Manager at the time. In my conversation with Carl at Sears Point in April, I asked him if his porting is just for enduro riding. His response was that it was good for any kind of riding. In talking to people who have ridden one of Carl's bikes, their response was that the power was available anywhere on the throttle and that it was controllable and predictable. The following is his porting specs:

  1. Set the deck height (distance from top of cylinder to top of the top ring) at 1.0 mm or .040 inches.
  2. Install a degree wheel and find top dead center
  3. Rotate engine clockwise 86 degrees and scribe a line above the exhaust port.
  4. Rotate engine 114 degrees A.T.D.C. and scribe a line above second set of transfer ports.
  5. Rotate engine 116 degrees A.T.D.C. and scribe a line above the front transfer and rear boost ports.
  6. Rotate engine 80 degrees A.T.D.C. and scribe a line on the piston along the bottom of the inlet port.
  7. Raise and widen all ports to the scribe lines according to the diagram supplied. (NOTE: be sure to radius exhaust port to top edge).
  8. Match and blend transfer port passages at bottom of cylinder to match crank cases.
  9. Match inlet manifold to inlet port. (Use gasket as a guide).
  10. Use an expansion chamber with the dimensions supplied.
  11. For tight race tracks, use an internal rotor ignition (timing 2.8mm B.T.D.C.)
  12. Carburation will remain basically the same, but will vary according to local track conditions.
  13. Port timing in degrees of total duration:
Standard CylinderModified Cylinder
Inlet Port160 degree160 degree
Second Transfers124-125 degree132 degree
First Transfer and
Boost Port
121-122 degree128 degree
Exhaust Port184-185 degree188-190 degree



NOTE: Head pipe length taken from cylinder flange -
measured through center line of pipe.