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yz400 stumper...
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[QUOTE="Tony Williams, post: 61846, member: 17759"] >>> on a four stroke, what would engine temperature or throttle opening have to do with compression? <<<<< Will, It would have nothing to do with mechanical compression ratio, but certainly both of these factors can play a role in a compression test. Depending on the type of test, you will need the throttle open. This is typically for the "tire pressure gauge" type test, where you screw the test in the spark plug hole, open the throttle, disconnect the ignition, and kick. Typically, a higher reading will be read with a warm piston, particularly a worn engine and/or one with a forged piston. The other compression type test is a differential test, whereby you send compressed air through the spark plug hole, and read the difference between the known pressure going in and the pressure the piston is able to maintain at the restricted flow rate of the tester. No kicking required, and you can more easily judge if the valves are leaking (air will be escaping past out the exhaust pipe or air cleaner), if a cylinder is cracked (air bubbling through the coolant), or the piston rings are leaking (air leaking out the crankcase vent tube). In this test, also, a warm engine will indicate a better sealing piston rings. In the piston airplane world, unfortunately, replacing pistons and cylinders long before the engine needs an overhaul is common. And many times, a cylinder that doesn't want to pass cold, will pass warm. Tony ------------------ Tony Williams YZ400F ('98) #131B YZ80 ('00) #92 [/QUOTE]
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yz400 stumper...
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