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Discussion Starter · #1 ·
G'day I'm new to all this but I recently purchased a 82 CB400n which has been half converted into a cafe racer.
I have done a compression test on it and got 110psi on both cylinders when cold. Is this ok?
It also is only running on the left hand cylinder. The right hand cylinder has spark and the plug is wet so I'm sure it's getting fuel. Has anyone had this problem. I have pulled the carburettors apart and cleaned a bit of debris out of them. I will be pulling them apart again to double check everything soon.
Any help would be much appreciated.
Cheers
 

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Compression tests are done with the throttle wide open, choke open (or carbs removed) and both plugs out. Crank until the gauge stops rising. Spec is 185 +/- 14psi so 110 is extremely low. In fact it probably shouldn't run.
As for firing on one side and not the other? Bad plug end cap or plug. NGK D8EA's gapped to .026" with 5K ohm resistance plug end caps. Could be a bad wire but that's extremely rare. After that you're looking at vacuum leaks or carb issues.
This is assuming that your retest of compression comes back in a workable range. 135 is about as low as you can go and get it to run, not good running but running
 

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Discussion Starter · #3 ·
Thanks mate. I didn't have the throttle open when I did the test. I'll give it another crack now
 

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Discussion Starter · #4 · (Edited)
I'm getting 125psi on the left and 130psi on the right after doing the test correctly.
I've got the right cylinder firing but only on idle after adjusting the pilot adjuster.
More playing around and hopefully I'll get there
 

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Did you CORRECT for the hose length/volume on the gauge you used?
A friend of mine used a Sears "Craftsman" gauge set that had 24 inches of hose/adaptor length in total, and was getting very low readings.....
I used my gauge and got a 170 PSI reading (low, but not unrunnable)....
Harbour Freight gauges are notoriously inaccurate as well......So DON'T panic YET....
Just the fact it was a cold test could add 10 to 15 PSI to the apparent reading alone......Before correction for the tools used.....
 

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To calculate the correction, you'll need to know:
Bore
Stroke
Compression Ratio
Compressed dome volume (can be approximated from the above three)
Actual engine CC's...
Hose length and diameter
and which end of the hose the check/pressure release valve is on ....
 

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I just did a comp test on my 400na, ready for an engine teardown. I got 125 and 127 hot. Not done a cold one. My engine runs 'fine'with this reading pulls hard and is responsive etc... (have gasket oil leaks hence the teardown to resolve) After a bit of research on here and other forums I found it isnt uncommon for the superdreams to be running that low even if it is a false reading as we all wont be using the same equipment.
Certainly not alone in having them RUN at that low a comp. I'm not saying that its an ideal or even desired range of compression to be running in, and I myself will be looking at rings, valves etc to get compression up, but just saying dont get too focused on just the compression as your main problem. Is your coil good?
 

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Squirt some motor oil down the plug hole, maybe a teaspoon, to see if the compression comes up. If it does, that generally indicates worn rings. If not, worn valves.
 
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