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Old 03-10-2002, 12:25 PM
Arthur Dalton Arthur Dalton is offline
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Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,768
If the throttle is closed, you are pulling a vacuum.
You want the cylinder at atmosphiric pressure before the compression stroke. If it is not, you will get a low reading.
Opening the throttle insures the piston is full of air Before the compression stroke.
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