The engine
The engine is the same basic design as the CR250M, which is a pretty wild machine. The easiest way to get more high revving power is to bolt on the top-end from the CR. In doing this, you will lose some bottom end power, but once it comes into the power in the midrange, it pulls hard right up to a screaming top-end at about 8500RPM. My clutch engages at about 3800RPM and it will still madly spin the tires from a stop. CR/FL cylinder comparison
To make the swap,
you need the following parts from a ‘73-74 CR250M.
·
The
cylinder
·
The
head
·
The
intake manifold
·
The
head gasket
·
The
piston (the fl250 piston will work too but will yield lower cranking
compression.
·
Any
aftermarket fl250 exhaust system. The
stock fl250 muffler is too restrictive.
· Any 34-38mm carburetor. I recommend a Mikuni TM/TMX or Keihin PJ/PWK.
Unless
you are using an aftermarket ignition system, the
timing needs to be changed on the models with CDI ignition for better
performance at high RPM. You will lose some midrange pull but top-end
will be much better and your engine will live to thank you. Here is how to change the timing to 16 degrees
BTDC.
(NOTE: The timing does not need to be changed on the models with points
ignition)
With the stator and flywheel on, push the piston to BDC with a dowel or screwdriver and hold it there. Scratch a mark on the stator that lines up with the second firing mark. (There are three marks on the stock flywheel. One marked "T" for TDC and two more. The one you want to scratch a line beside now is the one closest to the "T".) Remove the stator and slot the holes about 8mm longer to the left when viewed from the top. (doing this lets you rotate the stator clockwise.) Put the stator back in and turn it roughly 6 degrees but don't tighten the screws, you need to be able to rotate the stator. Install the flywheel and key. Push the piston to BDC and hold it, turning the stator until the mark you just made lines up with the first firing mark (The one furthest from the "T" mark.) Now remove the flywheel and tighten the stator screws. Install the flywheel and key and
torque the flywheel nut. Once more, push the piston to BDC and check to be sure that the mark furthest from "T" still lines up with your scribed mark. Now you can run your engine at much higher rpm's with less chance of detonation and longer over-rev.
You can also do it per this paper but without the flywheel key, your ignition timing may spontaneously change.
The
DG porcupine head has much bigger cooling fins and a wider squishband.
It was designed for the fl250 but will work on the cr cylinder with two
additional counterbores.
I ran a DG performance race pipe that is well suited to the mid to high-revving CR top-end. It is much louder than stock but performance is greatly improved over the stock exhaust. If you need a new pipe, I would recommend the torque pipe from http://www.4x4tuff.com for the stock odyssey or CR cylinder or a rev pipe for a rev ported odyssey or cr.
My
odyssey is a ’78 model so I decided to replace the original points ignition
with the CDI system from a newer (‘81-84) model.
It will bolt right in but you will need to make a mounting plate for
the new coil and CDI box. An NGK race wire works well and easily swaps into the system.
