Does a timing light really work?

[wiskers]

Well I know it will with a new timing chain, but what if it had 70k on the motor and it was a single roller chain? There should be slack in the chain, right? I've always bump the timing up a lil high to where it cranks hard when the motors warm and back it down a lil. They always run good this way. Few old timers taught me this. Didn't know if anybody has ever heard of this?
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