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kdtg

Leak Down Test Results

10 posts in this topic

I've got a 1995 XR600R that is having some issues and I took it to a local bike shop. I just heard from the shop and they had a 35% leak down test result. He said that was quite a bit but not terrible for this bike. I'm having trouble with the cough and die at idle syndrome. Could a poor leak down test contribute to my problem?

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I used to be an auto mechanic, and I seem to remember that 5 to 15% leak down were good numbers. I would expect an air cooled motor to have higher numbers based on the increased clearances needed to allow for expansion, so your 35% is probably still pretty high. Keep in mind that these numbers are based on a warm motor, and will increase quite a bit when the motor is cold due to contraction of the parts allowing a looser fit, as well as the reduction of engine oil around the rings if the motor has not been started recently. With a leak down test, the mechanic will typically use a stethoscope to listen at the air intake, exhaust , and crankcase (oil fill point on wet sump engines, crank breather hose on dry sump). By listening carefully at these three points, the mechanic can pinpoint whether the leak is occurring from the piston rings, intake, or exhaust valve seats. Sounds like your shop can do a bit more to narrow down the cause of these numbers and help point to where the problem lies. I would be concerned about a 35% leak down number, but there are many factors other than low compression that could be causing the problem you describe.

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I don't know if they listened with a stethascope or not I'm going to pick it up tonight and will get to ask the tech some more specific questions.

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35% is a little too much and the compression pressure is low. If the shop hasn't done it already, I'd go for a valve adjustment in hopes that one of the valves is a little tight. If that doesn't help, new rings and a valve job should do the trick.

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How is a compression test done with an auto-decompresion on a stock bike ?

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Consider too that cough and die is usually related to a lean condition.. Be sure and check for air leaks and a pilot jet partially clogged, along with the valve adjustment.. 155 is not great compression, however, it should run for sure.. Furthermore, whatever shop did the work should have already checked for a tight valve if they were at the point to even consider doing a leak down test.. You check the valve clearance before you do a leak down test... :applause:

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