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Old 02-10-2010, 08:19 PM
Goobz_MX718's Avatar
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Default TDC of compression stroke or exhaust stroke

How do you find out if the piston is at TDC of the compression stroke or the exhause stroke. The cams jumped teeth on the timing chain so the cams are not a reliable resource to figuring this out. No matter which stroke i'm on, neither of the two cams marks line up where they are supposed to. I'm replacing the chain and tensioner on my 2006 YZ450F. Believe it or not, the valves were not harmed during this mishap. Head and cyclinder was removed and put back together.
The cams are removed and the engine has been cycled over to find TDC. Having a hard time understanding what triggers the spark plug to fire. Thought i could figure this out by looking at the stator, but it's part of the crank so it spins 1:1 with the piston. I guess it's the CDI. How can i figure this out.

Please help
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Old 02-11-2010, 09:17 AM
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Originally Posted by Goobz_MX718 View Post
How do you find out if the piston is at TDC of the compression stroke or the exhause stroke. The cams jumped teeth on the timing chain so the cams are not a reliable resource to figuring this out. No matter which stroke i'm on, neither of the two cams marks line up where they are supposed to. I'm replacing the chain and tensioner on my 2006 YZ450F. Believe it or not, the valves were not harmed during this mishap. Head and cyclinder was removed and put back together.
The cams are removed and the engine has been cycled over to find TDC. Having a hard time understanding what triggers the spark plug to fire. Thought i could figure this out by looking at the stator, but it's part of the crank so it spins 1:1 with the piston. I guess it's the CDI. How can i figure this out.

Please help
After giving this some thought maybe i have a better question. Does the spark plug fire on every stroke and the only thing constituting it being a compression stroke is the cams. In this case, it wouldn't matter if i was TDC on the compression stroke or exhause stroke as long as i was at TDC. Someone please correct me?
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Old 02-11-2010, 10:10 PM
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Originally Posted by Goobz_MX718 View Post
After giving this some thought maybe i have a better question. Does the spark plug fire on every stroke and the only thing constituting it being a compression stroke is the cams. In this case, it wouldn't matter if i was TDC on the compression stroke or exhause stroke as long as i was at TDC. Someone please correct me?
That is correct! You can line up the timing marks on the mag or unscrew the sparkplug and put a long screw driver down the hole...as you turn the motor over you can see top dead center and time the cams to that!
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