Moorron
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 posted on 29/7/07 at 06:54 PM |
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compression test done, help with results
My CBR1000F engine had been burning oil for some time, i got round to check the compression and the results are even but only 95 PSI.
The book suggests around 160 PSI.
So, i need to fix it and am unsure what to do. I would like to order the bits ready for me to start, but at this point im unsure if i need larger
pistons or just standard piston rings? any ideas? worth me taking the risk and ordering the rings only.
Also, any idea how much power i would loose with these results.
Sorry about my spelling, im an engineer and only work in numbers.
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flak monkey
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| posted on 29/7/07 at 06:57 PM |
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Worn bores is my bet. Which means a re-bore and oversize pistons and rings. Though I am not sure that you can buy oversize pistons and rings for bike
engines, I have heard a few bikers mention that they couldnt get them, but it is very probably dependant on the make and model.
David
Sera
http://www.motosera.com
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DIY Si
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| posted on 29/7/07 at 07:00 PM |
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How many miles has the engine done? It sounds, as Flak says, like worn bores. There are overbore kits for many engines, but not all. It may be worth
getting a new engine over the rebore, but that depends upon price.
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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Moorron
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| posted on 29/7/07 at 07:01 PM |
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david silver spairs show an oversized piston for the engine. But how much is a rebore?
Also, maybe talking poop here i have a spare engine but dont know its history. is it worth me taking the bore block and pistons out of it and stick
them in my engine. (swapping the engine is so much more work and also riskier then the above)????
Sorry about my spelling, im an engineer and only work in numbers.
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britishtrident
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| posted on 29/7/07 at 07:04 PM |
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Compression pressures that low normally indicate broken compression rings or burnt or bent valves however these faults usually only occur in one or
two cylinders, which makes me think perhaps the test is giving a spurious result..
-----
Did you have the throttles wide open and all the plugs removed when doing the test?
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DIY Si
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| posted on 29/7/07 at 07:05 PM |
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How will changing the engine be more work than taking it out to split it in half? And then still putting it back in!
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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BenB
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| posted on 29/7/07 at 07:13 PM |
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Have you tried putting (a little) oil in through the plug holes to help seal the bores? If that improves the readings it's worn bores... If not
it's something else.... Otherwise, was the engine hot / cold as stipulated in the manual for the reading of 160 (or whatever it is!).
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nick205
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| posted on 29/7/07 at 07:24 PM |
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I think British Trident may have the answer here!
If your getting even results at around half the specified compression it sounds to me like the test isn't being done correctly. You need to
have the engine at normal operating temperature, then (as above) remove all the plugs and test each cylinder by cranking on the starter with the
throttle fully open. You need to allow 4-5 revolutions to get max compression pressure. if you're using a dial type gauge then ask an
assistant to watch the gauge while you crank and let you know when the reading stops rising.
Puma Race Engines have an excellent guide HERE on compression testing an engine - well worth a
read.
Obviously you may alrady know this and be doing the test correctly and indeed have a seriously worn engine. However I didn't know when first
attempted it and spent a few weeks thinking my freshly rebuilt engine was no good.
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Moorron
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| posted on 29/7/07 at 07:50 PM |
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warmed up, all plugs removed and full throttle. Also i could see the guage from the drivers seat on all the tests. I am positive its the correct
result, just trying to see what needs doing before i take the car off the road.
i wont b splitting the engine out of the car, but leaving it in and taking it apart there. i need to do the cam chain tensioner anyway so its comming
apart. I am also turbo charging it soon and might lower the compression at the same time.
Sorry about my spelling, im an engineer and only work in numbers.
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scottc
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| posted on 29/7/07 at 09:14 PM |
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Is it possible the engine timing is out?
I know nothing about bike engines. But it sounds like the engine timing is out.
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DIY Si
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| posted on 29/7/07 at 10:43 PM |
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Is it possible to do it in place? Do you not have to split the box/crank case from the inside? I may be wrong of course, but most I've seen you
have to get at half, if not more, of the bolts from inside the gearbox half.
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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