03 cam mod problem

5 replies to this topic
  • Rick_Graham

Posted March 16, 2005 - 06:34 PM


OK, here's the deal. I was very carefull to follow all the instructions and as far as the install went everything was just fine. I double and triple checked timing, valve clearances, chain tensioner, cam lobe positions and torque values. I tried to fire it up after reassembly and it just wouldn't light off. I pulled the plug and it was wet so I installed a new plug, no dice! So apart it came to double check timing and everything was spot on. So I then started rotating the crank slowly to verify how all parts were interacting and watched the timing chain jump several teeth :) . Obviously a new cam chain tensoner is on order tomorrow. Now my timing mark/cam positions are way off and I need to make sure to get it back to TDC on the compression stroke. Can I just put the crank on the timing mark and reset the cams or would I potentially be 180 out? I'm not thinking clearly at the moment and need a hand :) . Thanks

  • Harry Paratestes

Posted March 16, 2005 - 08:00 PM


If the timing mark is lined up and the piston is at the top then set the cams where they need to be and you're good to go. Just roll it over and make sure everything does what it should when it should.

  • 02YZ426

Posted March 16, 2005 - 08:03 PM


Just make sure that the crank is in time, line up your cams, slip the chain back on and you should be fine. Just make sure your timing marks still line up after you install the tensioner. Rotate the motor over by hand a few times and re-check your timing. If all is good, then it should fire up for you no problem.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • WheelsUp

Posted March 16, 2005 - 10:53 PM


It is not possible to have the crank 180 degrees out if the piston is at TDC.

The crank turns 720 degrees for every 360 of the cams.
If it's a TDC, it's at TDC. The piston and crank don't care if it's the end of the compression stroke or the end of the exhaust stroke. It's the cams which determine what stroke it is on.

It would be possible to have a CAM 180 degrees out, but I'd be willing to bet that it would be pretty difficult to do. The lobes would be compressing the valves... it would be obvious that you had screwed up. If BOTH cams were 180 degrees out (equally difficult to do), then you'd be back to "normal", except that you'd have installed the cams at the end of the exhaust stroke (all valves open) instead of at the end of the compression stroke (all valves closed)

Looking at the brake side of the engine, when the YZ cams are in properly and the piston is at TDC, the intake cam will be at about 10:00 and the exhaust cam will be at about 2:00. For WR timing, the exhaust will be closer to 3:00.

  • Rick_Graham

Posted March 17, 2005 - 05:25 AM


Thanks for confirming what I thought. I've had a real blast with this bike and want it going as soon as possible. The wait for the correct valve shims (which took a month)and now the tensioner is really making me jones for a ride!

  • grayracer513

Posted March 17, 2005 - 12:24 PM


The crank turns 720 degrees for every 360 of the cams.
If it's a TDC, it's at TDC. The piston and crank don't care if it's the end of the compression stroke or the end of the exhaust stroke. It's the cams which determine what stroke it is on.

That's right. The term, "180 out" comes from car people, and other applications where the component that controls ignition timing, the distributor in the case of a car, is installed independently. In the typical automobile, the crank is brought up to TDC, and the distributor is indexed to point to the #1 cylinder. The problem starts because, as was said, the camshaft can be in either of two positions when the crank is at TDC, and it is the cam that determines which stroke the engine is on. And, in fact, it's actually 360 degrees out of time, but would be correct if the distributor had been rotated 180.

In most CDI single cylinder four strokes, the flywheel triggers the ignition at every TDC, even though that creates a "waste" spark at the end of the exhaust/beginning of the intake stroke. So there really is no "which TDC" once the cams are in; it's always the right one.

Your original cams have 3 timing marks; one marked E, one marked I, and one in the middle. The one in the center should be pointing straight up parallel to the centerline of the cylinder, and on the intake, the one marked I should be fairly close to lining up with the edge of the cam cover gasket surface on the head. The '03 exhaust cam sprocket has only two marks at about 90 degrees to one another. When it's in right, one will parallel the cylinder centerline, and the other will line up with the edge of the head. Just for fun, in case you're worried about it, deliberately move one off the cams one tooth out of their correct position and see how it looks. DO NOT rotate the engine while you have it like this. Not much doubt, is there?

Be very, very careful when torquing the cam caps. This cannot be over emphasized. Screwing up the torque down procedure is the leading reason that cams bind up and seize, and may have been the cause of the skipping problem you had. You should be able to move the cams the amount that the chain slack will allow pretty easily by hand.

Related Content


Megabomb Fitment by 288yz450

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 1 reply

James Stewart back on a YZ450F by YamaLink

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  47 replies

YZ450F 03 Sparks driving me crazy by SirAttard

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 5 replies

Chubby dad, looking at bikes , First trip to the orange/black forum ! by Slow_ride

Dirt Bike   Make / Model Specific   KTM   250-530 EXC/MXC/SXC/XC-W/XCR-W (4-Strokes)
  • Hot  28 replies

Michigan Motocross Tires by 288yz450

Dirt Bike   Dirt Bike Regional Discussion   North
  • 1 reply

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.