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Discussion Starter · #1 ·
Hello,
My CB 125 k5 (France) does not start. I did a cold compression check and one cylinder gives about 120psi which is good, and the other have no psi whatsoever.
I didn't quite understand how it could read 0, and then I added some oil in the cylinder as to check if that would seal the piston (maybe worn piston rings) and I would get some compression. So now instead of 0 I got a maximum of 210 psi.

Does anyone know how this could be? Could I conclude from this my piston rings are worn?
Is this the reason that the engine won't start? As I thought the engine could run on 1 cylinder, but maybe more difficult to start it on 1..
Any help appreciated!
 

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Sounds like you added an excessive amount of oil. Zero compression is unlikely to be caused by worn rings. If you have access to an air compressor you can do a leakdown test which will tell you exactly where your compression is going.
 

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Hung open / up valve... if the motor was stored for a while a valve was open, some corrosion may have built up on the stem not allowing it to close/seat. Do a "leak down test" at TDC and DBC... it will tell you where inside the motor you are loosing compression.
 

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CB125T with 11,000rpm red-line?
Left cylinder problem?
Someone let oil level get too low, if you caught it in time big ends should be OK.
Probably had a partial seizure or if oil was OK, clipped a valve and it's bent
 

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Always adjust the valves first when doing a compression test. It doesn't cost anything to do so and it is important. A valve without enough clearance won't seal properly.
 

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Discussion Starter · #6 ·
Thanks everyone, this is real helpful.
I have not yet checked the valves as the valve covers are completely stuck. I strip the metal as I try to open them and have tried everything but still they won't budge. So now I will take the engine off and will probably try to get them off by grinding/welding.
Yeah I think after these comments it's probably something with the valves.

Just wondering if stuck/bent valves on the left cylinder could also cause fuel to spit out of the left carb?
 

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If the intake valve doesn't seat.. yeah..
pull the plug, put the bike in high gear, either secure the rear wheel or place your front tire against a wall so the bike won't take off when you put air into the the spark plug hole (ask me how I know)..,
Add the air with the piston on the TDC AND DBC. Compression stroke.. if you have air coming out the exhaust.. exhaust valve is not seating.. same for the intake.. if you remove the oil fill plug and hear air blowing by.. your rings are a issue..
30/40 lbs should be more then enough to give you an idea..
 
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