A TB sync is where they make sure all of the throttle plates open at the same amount so that of idle transition is smooth. It only affect the off idle transition. Symptoms would be a very rough idle, maybe even sounds like it is not hitting on all of the cylinders.
What they do is hook up a vacuum line kit with either a digital or mercury system and measure what each one of the TB are pulling as far as vacuum. Then the balance them so they all pull the same amount of vacuum.
I think the specs call for it to be done with when the valves are run.
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