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Verifying that piston is TDC on compression...

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After many unsuccessful attempts to get my bike running I have come up with a few questions? Does the spark plug only fire on the compresson stroke? The reason I ask is this. When setting my cam timing I am getting putting the piston at TDC using the second mark on the flywheel/magneto and veryifying TDC through the spark plug hole. Then I am setting the cam, lobes facing the rear, mark on the gear lining up with the mark on the cam holder. So my thought is that does the bike somehow know to only fire on the compression stroke. It seems to me that if the firing mark is the first mark on the flywheel that the plug will fire at the top of the compression stroke and at the top of the exhaust stroke. Does the ICM regulate this so that the plug sparks only on the compression stroke? If it fires only on compression and not exhaust could I possibly have it set to fire on exhuast?

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Are you sure you valves are in spec still after putting the cam chain tensioner back on? I know a lot of people including myself who have rebuilt there 250's and did not back off the cam chain tensioner when re-installing it. This would result in your valve clearances going to zero or way out of wack and the bike not wanting to run.

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Your piston doesn't know if it's on the compression or exhaust stroke until you connect the cam chain with the cam lobes facing towards the rear of the bike. Yes, it fires every time the piston is at the top, even if it's on the odd stroke. It's not too difficult to get the cam chain off one tooth, and then you'll be lucky to start it. Using the sight hole on the right side of the bike is usually more error-proof than using the flywheel mark, but I suppose that both ways are effective. It's a pain in the hindquarters, but moving the cam chain one tooth either way usually finds the solution.

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