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Exhaust Cam Timing

The exhaust cam timing on the WR250F and YZ250F are different (see Table 1). Although the exact reasons are unclear, the presumption is that the restricted intake and exhaust of the stock WRF run better with different timing. Also, the WRF timing creates a more linear powerband and less throttle response compared to the YZF which is engineered for MX/SX racing.

 

Table 1

 

 

 250F Cam Timing At-A-Glance

 

Setting cam timing with stock cams is relatively easily done by using the punch marks provided by Yamaha on the cam gears (see below). For stock timing, the punch marks at 9 O'clock on the exhaust cam and at 3 O'clock on the intake cam should align with the top of the head. For altering stock timing, use the table below. If stock timing is altered the exhaust cam punch mark will either be one tooth above or one tooth below the top of the head.

 

Table 2

Stock Cam / Desired Timing

Pins between 12 O'clock Marks @ TDC

WR cam / YZ timing

12 pins

WR cam / WR timing

13 pins

YZ cam / YZ timing

12 pins

YZ cam / WR timing

13 pins

 

 Timing from Scratch

 

(From TT member Mark Cantrell)

 

Procedure

  1. Take out tensioner.

  2. Remove both plugs from shifter side of crankcase, the center plug to turn, the 11:00 plug to locate the 'I' (not the H) in.

  3. Use a ratchet on end of crankshaft (through center plug) and zip tie ratchet to footpeg with the I lined up in the timing plug hole.

  4. With the intake cam (rear one) pointing straight back, align the punch mark on the backside of the intake cam sprocket with the top of the head. Since the slack in the timing chain is in the back, if it doesn't align exactly right at the top of the head, err slightly high. Zip tie chain to intake sprocket.

  1. Remove exhaust cam, count 13 (for WR timing) or 12 (for YZ timing) links (where sprocket teeth will go) from the punch mark on the top of the intake cam and mark that position on the chain with something noticeable (I use a Sharpie). Hold the exhaust cam close the to intake cam with the exhaust cams pointing horizontally to the front of the bike. With the exhaust cam in this position, there will be a punch mark at the top. Put the tooth above this punch mark in the chain link you marked that was 12 or 13 over. Zip-tie the chain to the exhaust cam.

  2. Reseat exhaust cam. If you are a WR and use WR timing or a YZ with YZ timing, the punch mark on the left side of the exhaust sprocket will align with the top of the head. If WR with YZ timing, it will be one link high. Go back and check that

    1. the I is still centered in the timing plug hole

    2. the intake cams point back

    3. the exhaust cams point frontward

    4. the rear punch mark on the intake is aligned or slight high with the top of the head

    5. the front punch mark of the exhaust sprocket is aligned with the top of the head (or 1 link high if WR with YZ timing).

  3. Bolt and torque exhaust cam, torque intake if you've had it off.

  4. Install chain tensioner (wind it up, hold the screwdriver with two hands, use the third hand to put hold tensioner in place, and fourth hand to bolt it up, then release screwdriver and put end bolt in it.

  5. Remove zip ties on intake sprocket, exhaust sprocket, and ratchet holding TDC. Remove ratchet and replace both plugs.

  6. Put on valve cover and two hoses, reinstall tank and seat and go riding.

 

 ThumperTalk Archives (updated 09/08/03)

 

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ThumperTalk Timeing

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ThumperTalk help with timing

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ThumperTalk need help with timing

Listed here are what I consider the most helpful threads. There are more threads on this topic in the ThumperTalk archives. To find them, search the YZ250F/WR250F forum. Suggested keywords: exhaust cam timing, timing, YZ timing, WR timing.

 

 

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Last Updated 09/19/2003