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Author Topic: 400f compression test  (Read 1553 times)

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Offline andrsn

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400f compression test
« on: July 27, 2010, 10:15:25 pm »
I checked my compression today, does anyone know what the psi is supposed to be? I can't find it in my manual. All of my cylinders read 120psi. Someone once told me it is more important for them to be similar than for them to be in the correct range. Is this true? I also figured that since I used a cheap harbor-freight compression gauge, the low reading may be partly the tool too.
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1977 Honda CB550f
1979 Yamaha XS650
1990 Honda CB125T
1975 Honda CB400F
1976 Honda CB400F
1977 Honda CB400F

Offline TwoTired

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Re: 400f compression test
« Reply #1 on: July 27, 2010, 11:53:25 pm »
Did you check the engine FAQ?
Compression test numbers aren't absolute unless you use the same test equipment that was used to collect numbers that are printed in the manual.
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Lloyd... (SOHC4 #11 Original Mail List)
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KingCustomCycles.com

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Re: 400f compression test
« Reply #2 on: July 28, 2010, 04:20:25 am »
Many factors affect compression results.  120 is fine, especially since all cylinders are even.  I typically see 100 -185 on my old SOHC 750 engines.  Rechecking after running will change things, checking cold will be different, not opening the throttle all the way during the test, etc... I made a differential type tester so I could troubleshoot where the shop air was going and not just the absolute numbers.  The differential method is the preferred method for light aircraft.
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