Inconsistent compression across all cylinders CB750C DOHC

[baldjeff]

Hi all, I'm a new member here. With all this time at home I finally had a chance to get to a couple of projects. I just did a compression test on my '81 CB750C, wet, cold, and I got these results in, cylinder 1: 145, 2: 150, 3: 167, 4: 142. From high to low thats more than 17% so I know that is not normal but I don't know what it means.

To give a little context, I bought this bike a while ago without any history or maintenance records. It was to be a donor bike for a cafe project but I like her the way she is so I really just want to clean her up and ride. The bike has been sitting for a while, I can't say for sure how long but it's probably only been started a few times in the last several years but I don't think it was ever ridden. I put some Marvel Mystery Oil in each cylinder, rotated the rear wheel a bunch then let it sit for several hours. I just wanted to get the pistons lubed up since its been a while. I then ran the test and got the results listed above. I can't do a warm compression test because it doesn't run. Would you guys suggest a top end rebuild at this point? Thanks in advance for your help!

 

[amc49]

No, but valve adjustment is called for as nobody ever did them. It then lowers the compression. The valves tend to close up to no compression. The engine can be run until compression drops to 120 ish and then they will be getting weak. 100 psi is a dead cylinder.

On setting valves shoot for .005" instead of the book .003", the engine will thank you for it. Basically you throw away at least .002" of any clearance number you get as it is not real with the engine running, why .003" is too close, it actually ending up around .001". The .005" number ends up at .003" real world and fine. The engine can and will burn valves that are at .002", they are held open part of the time. Why so many don't run right when you get them.

 

[baldjeff]

No, but valve adjustment is called for as nobody ever did them. It then lowers the compression. The valves tend to close up to no compression. The engine can be run until compression drops to 120 ish and then they will be getting weak. 100 psi is a dead cylinder.

On setting valves shoot for .005" instead of the book .003", the engine will thank you for it. Basically you throw away at least .002" of any clearance number you get as it is not real with the engine running, why .003" is too close, it actually ending up around .001". The .005" number ends up at .003" real world and fine. The engine can and will burn valves that are at .002", they are held open part of the time. Why so many don't run right when you get them.

Wow super informative! thanks for your help!