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yfz has no compression... leakdown test was good!?!?!?!?

This is a discussion on yfz has no compression... leakdown test was good!?!?!?!? within the Yamaha YFZ450/450R (2004-Present) forums, part of the Performance /Technical category; ok... i was riding.. started loosing power... check compression 90lbs. after talking to the dealer they assured me it was probably just the rings even ...


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Old 10-12-2004, 04:57 PM   #1
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Default yfz has no compression... leakdown test was good!?!?!?!?

ok... i was riding.. started loosing power... check compression 90lbs. after talking to the dealer they assured me it was probably just the rings even though the old ones looked fine. put it back together... 90lbs compression.

head/valves are fine, rings are fine... i put it together correct, timimg is good...
did a leakdown test an hour ago, held all the pressure we gave it... looked great... did another compression test at the dealer and they got between 65 - 80 on 2 different testers.

what is wrong? decompression seems to be working good...
dealer is lost... they gonna call yamaha tomorrow but we all know how smart they are...

help me!!!
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Old 10-12-2004, 05:27 PM   #2
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I dont think you can do an accurate compression test during cranking because of the decompression release on the cams.
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Old 10-12-2004, 07:37 PM   #3
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nope.. not accurate... but it needs more than that... my buddies had 150 lbs...
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Old 10-12-2004, 11:56 PM   #4
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the decompression pin on the exhaust cam COULD be stuck possably. Clean it and oil it and get the sand or junk out of it.

But here is what I think happened. When you did the cam mod, somebody moved or touched a shim bucket and one of you shims is up at an angle and it just doesnt run right. Just check your valve spacer distances and make sure, if they are all right, then not to worry about that.
also you may have rotated the exhaust cam the wrong way, that will really make it weak.

You have to rotate the cam CLOCKWISE one tooth at Top dead center the dot is up by the distance between two chain pins.
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Old 10-13-2004, 03:22 PM   #5
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would you beleive that the cammod reduces compression by 50 lbs while its not running...?

it does... i have over 130 right now and the rings are fresh and not seated yet...
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Old 10-13-2004, 03:37 PM   #6
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I think with the cammod, you have also changed the timing of the decompression pin. This would cause harder starting.

So everything is back to normal?
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Old 10-13-2004, 07:59 PM   #7
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dunno bout normal yet... my compression issue is solved... gotta get it together and running again... i dunno why i was loosing power tho...
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