Stretched timing chain?
Posted March 15, 2008 - 10:43 PM
Posted March 16, 2008 - 02:44 AM
You will need to remove the flywheel to replace the timing chain. A flywheel puller is required.
If you don't have a service manual for your bike you can download it here.
Here's good instructions for changing the timing chain written by Grayracer in the YZ400/426/450 forum.
Takes about an hour if you don't throw in any valve adjustments.
Start and run the bike for just long enough to clear the sump of oil. Remove the cam cover, ignition cover (no significant oil will be lost), flywheel (puller required, don't cheat), and stator plate (#27 Torx bit required). Remove the tensioner (see manual for unloading instructions) and the two bolts holding the bottom of the rear chain guide shoe. Rotate the bottom end of this part outward and remove it by pulling it down. (If it gets stubborn, you can just let it sit in there out of the way.)
Remove at least one cam (probably both will be required) and rotate the chain off the crank as you did the rear guide.
In reassembling, you have a couple of options:
a) place to the chain over the crank and bolt the rear guide back in place. Reinstall the stator and flywheel, and depend on the front and rear guides to keep the chain from looping off the bottom of the crank sprocket as they are designed to while you time and assemble the top. Probably safe, but you always find yourself wondering if the chain is really on right.
Remove the spark plug and use a screwdriver to find TDC, or eyeball it by lining up the flywheel key with the bore centerline and time and assemble the chain and cams with the whole run of chain visible. Foolproof, but be sure to double check the timing by pushing the flywheel and cover on by hand and using the real marks just before torquing everything all the way up. You don't want to have to take any more apart than necessary if you screw up.
I use option b myself.
Two mistakes commonly made are failing to torque the cam caps right, and timing the cams wrong because the cams weren't rolled back. Back the engine up to a few degrees before TDC, and then roll each cam back until the front run of chain is tight, and the slack is on the back side. Now, hold pressure against the rear guide shoe by pushing your finger through the tensioner hole and rotate the engine forward back to TDC. Then check the timing. Note that there's no point in worrying too much about the timing marks lining up perfectly. The marks simply are never going to be perfect. What you do is imagine the timing marks on in the same position on the next tooth, and see how that would look. If it makes it closer, move it. If not, it's OK.
Always carefully torque up the cam caps in three successively tighter steps. I use 75 in/lb instead of 86. 86 just seems too tight to me, and I believe Yamaha has reduced that value for the later models.
It's a pretty simple job. Remember to check your valve clearances before removing the cams in case any of them need correcting, as this will be the perfect time for that.
Posted March 16, 2008 - 06:28 AM
Posted March 16, 2008 - 10:09 AM
Posted March 16, 2008 - 10:54 AM
Posted March 16, 2008 - 12:52 PM
Posted March 16, 2008 - 06:09 PM
Posted March 16, 2008 - 07:48 PM