By Mark Cantrell - Member # 6179 on http://www.thumpertalk.com/
Visit this thread on thumpertalk here http://www.thumpertalk.com/bike/ubb/ultimatebb.php?ubb=get_topic&f=10&t=004140
Firstly a note, this modification may only be needed on US WR's. For Example, the Australian WRF apparently has the same CDI map with or without the grey wire cut.
Some questions like "grey wire?" and "what's a CDI" or "what's a map" comes up over and over. You can search for grey wire for how to locate it, best way to cut it, etc. but this post is the very basics of what a CDI unit is. If you already know, stop reading at the period .
(since so many of us are really creaky, I use old car analogies that may help. Everyone else ignore the car talk.) (also, I put grey and gray and wire in the subject to help searchers.)
All of the following has to do with when the spark is sent to the plug. I don't know what the range is on this bike but on cars with distributors it was from about 10 degrees before top dead center to a few degrees after top dead center.
The faster an engine turns, the more advanced the spark. I think the reasoning is that it needs to start burning early to have lots of pressure for the early part of the power stroke. Anyway, it is advanced (fires early or earlier before top dead center) at higher RPMs.
To start and keep it from stalling at slow speeds when there is little momentum to keep it going (especially on one cylinder motors with light flywheels), it is retarded (fires earlier or later after top dead center) so firing won't roll it backwards stalling it.
On cars with distributors (and even on this bike, see Inspection and Adjustment chapter of your manual under Ignition timing check) we used a timing light, which is basically a high speed strobe light triggered by the spark plug wire. When the plug fires, it flashes and the effect is that you see the same place on the flywheel each time. Since it wiggles a little, you get the "I'm getting a light buzz" feeling if you watch it too long. On cars, the flywheel had calibration marks of about + or - 10 degrees from Top Dead Center (TDC). We would rotate the base of the carb to adjust it at idle. On your bike, the range of movement is the two sides of the H engraved in the flywheel visible under the (you guessed it) timing plug.
The car had two automatic adjustments that adjusted for running conditions: (1) it had centrifugal weights that advanced the spark as the engine sped up, and (2) it had a vacuum (advance or retard, been a long time, why am I blanking on this?) that essentially measured throttle position (the engine draws a higher vacuum when the throttle is closed and less vacuum when the throttle is open).
Your bike has a computer controlled (OK, really its just a ROM lookup table but close enough) ignition that has two inputs. It gets a signal from the stator (actually a second coil on the stator called the pickup coil) once per revolution. From this it calculates the engine RPM. It has a Throttle Position Sensor (TPS) attached to the carb that, believe it or not, reports the throttle position to the CDI.
From there its like a spreadsheet. The CDI look ups the RPM as a row and the throttle position as a column. The value in that cell is the degrees to retard or advance the spark. Engineers call this spreadsheet a map (like a map you would look things up in) so they won't be confused with administrative assistants. There is a description of the map the CDI (ignition unit that does all of this, mounted on right side of frame just under handlebars) uses in the Electrical section of the manual under the MAP-Controlled CDI unit. It even has a cool graph. Unforunately the graph doesn't represent the map actually delivered.
There are two different maps (spreadsheets) in the WR/YZ bikes. One is for the YZ and is selected if the grey wire isn't grounded. The other is for the WR and is selected if the grey wire is grounded. On a WR, many cut the grey wire to get the YZ map which is deemed better performance than the WR map. Some, like me, buy the Vortex CDI replacement unit that has two maps, one labeled traction and the other labeled power, with a little switch between the two on the handlebars. The WR map probably is to reduce backfiring on decelleration and to work with the restrictive baffle which requires the different cam timing. Just guessing there. Since WR people lust after YZ ignition maps, WR people cut grey wires (i.e., that mod is for WR people only). YZ people never add the grey wire.
The only other function of the CDI is as a rev limiter. This cuts out the spark when the bike revs above 13,500 RPM to prevent damage from reving too high.
New topic:
There are several 'timing's on this bike. The above is all about the ignition timing, the timing of the spark relative to the crankshaft position. There is also cam timing, the timing of when the cams actuate the valves relative to the crankshaft position, important for people converting from WR timing to YZ timing. There is accelerator pump timing, when the AP squirts relative to the throttle slide opening. Don't confuse them.
Another new topic:
The Sky Blue wire is another wire that goes to the CDI. It connects the neutral detect switch (electrical sensor just behind the shift lever) to the CDI. It changes the rev limiter to 7,500 RPM in neutral and may also reduce the voltage of the spark. I haven't touched it but Motoman393 is a believer and Sceptor, who is an electrical engineer, believes its possible. I don't know.