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Discussion Starter · #1 · (Edited)
Cold Compression

My CJ360 was running OK but not great when I brought it out of storage in September. Once I got it home I drained the oil and started stripping parts off the frame. Before I take out the engine I figured it would be a good idea to test the compression, so I borrowed a compression gauge from the local parts store. The instructions for the gauge say to do the test when the engine is warmed up, but since the bike is no longer in a running state I did the test anyway with the engine cold (a little above freezing). I got 120 psi in both cylinders.

The original post says optimal compression is 185, minimum is 135. Would a warm engine make a big difference in my compression? What about testing on dry cylinders? (Hopefully kicking it over a few times without oil in it doesn't damage to the engine.) Does 120 sound like a healthy engine, given the conditions of the test? Thanks for your thoughts/opinions.

And, after moving the question to it's own topic I answered:

With 120 using the kickstarter on a cold dry engine and given that it was running "OK", I would just proceed with whatever plans you have for the bike. I wouldn't tear the engine down unless you were planning on doing that anyway.

I moved your question to it's own topic in the 360 section of the forum. You'll get more appropriate responses that way.
 

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My CJ360 was running OK but not great when I brought it out of storage in September. Once I got it home I drained the oil and started stripping parts off the frame. Before I take out the engine I figured it would be a good idea to test the compression, so I borrowed a compression gauge from the local parts store. The instructions for the gauge say to do the test when the engine is warmed up, but since the bike is no longer in a running state I did the test anyway with the engine cold (a little above freezing). I got 120 psi in both cylinders.

The original post says optimal compression is 185, minimum is 135. Would a warm engine make a big difference in my compression? What about testing on dry cylinders? (Hopefully kicking it over a few times without oil in it doesn't damage to the engine.) Does 120 sound like a healthy engine, given the conditions of the test? Thanks for your thoughts/opinions.

With 120 using the kickstarter on a cold dry engine and given that it was running "OK", I would just proceed with whatever plans you have for the bike. I wouldn't tear the engine down unless you were planning on doing that anyway.

I moved your question to it's own topic in the 360 section of the forum. You'll get more appropriate responses that way.
Thanks JT. The fact that both cylinders have equal pressure is also encouraging. There's a bit of oil seeping from the head gasket, so I'll probably go as far as changing it but no further.

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Oops. This question was asked by Yellowheader, I just moved it and somehow seem to have adopted it. God I love computers!
 

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Did you check your valve clearances? If they are too tight, compression will be lower. My CB400 had 145 and after adjusting the valves, it went up to 155 cold as well.
 

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Did you check your valve clearances? If they are too tight, compression will be lower. My CB400 had 145 and after adjusting the valves, it went up to 155 cold as well.
No I haven't looked at the valves yet. I'll be interested to see if I can bring up the compression a little that way.

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If it's only a little above freezing the valves are probably 'tight' as the head will 'shrink' more than the steel bits. You did have throttle wide open? (if not, thats excellent compression, usually you get around 90psi with closed throttle)
 
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If it's only a little above freezing the valves are probably 'tight' as the head will 'shrink' more than the steel bits. You did have throttle wide open? (if not, thats excellent compression, usually you get around 90psi with closed throttle)
The throttle was closed so I guess I can be happy with 120. It'll be interesting to see what the reading is next time when I have more of a clue about what I'm doing :) That will be when the bike is put back together though - not for a while.

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