Compression

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Compression - 6/3/2006 3:56:42 PM

waz57

Posts: 389
Joined: 5/22/2006
From: Gladstone, Queensland Australia
Status: offline

THe other day I thougth that I would do a compression check on my CBR. But only to find out some disturbing readings?????? now I couldnt figure why the motor still runs very sweet and to have such high an dlow compression ???
Until I just read a reply to " Loose Nut " Now DAD went on to say when doing a compression check you need to open your throttle wide open to get a more acurate reading has anyone out there fallen into the trap of not opening the throttle like me ? as I thought it was strange to have such a wide degree of compression.

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RE: Compression - 6/3/2006 4:06:24 PM

dad

Posts: 298
Joined: 11/6/2005
From: Pittsburgh Pennsylvania
Status: offline

What you saw is EXACTLY what I'm talking about.

I know of a few engines that have been torn down due to bad, but inaccurate readings. One guy on a British board tore the same engine down twice.

Try it again with the butterflies open and you'll get the true reading.

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RE: Compression - 6/3/2006 4:16:23 PM

waz57

Posts: 389
Joined: 5/22/2006
From: Gladstone, Queensland Australia
Status: offline

Thanks for that I thought that I had bought real lemon of a bike as I bought it from a very small bike shop who was selling it for the previos owner in Brisbane and I had told them I live 600 Ks away and was coming down on the friday to ride it home on the saturday ( Couldnt believe I told them that !! ) and when I saw the compression difference the other day I just couldnt understand why it went so hard and wasnt idling rough, so I will do another compression test today and I am sure like you said it will come up to reasonable levels in a 4 cyl.

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RE: Compression - 6/3/2006 4:40:11 PM

dad

Posts: 298
Joined: 11/6/2005
From: Pittsburgh Pennsylvania
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The factory manual says 149-206 PSI @ 400 RPM is the reading you should get. I'm guessing that's the speed they figure the starter turns the engine. As you probably know, it's big variations that you're really looking for moreso than the actual numbers. Also, if it cranks uniformly with the kill switch engaged, it's probably fine.

And don't feel bad. You're not the first to do this. You can do an auto engine without the butterflies open and still get readings that are useful, even if a little lower than actual, but not the bike.

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RE: Compression - 6/3/2006 4:44:56 PM

waz57

Posts: 389
Joined: 5/22/2006
From: Gladstone, Queensland Australia
Status: offline

When I think back I did the same stupid thing with my outboard it had variations of up to 40 PSI but once i gave it full throttle they cam eup to 155 PSI

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