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CB750 Low Compression and rebuild questions

JCM6767

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Peterborough Ontario Canada
I just bought a 79 CB750L and found out that the compression is low in all cylinders. 1, 2, and 4 were at 60, 3 was 90. Bikes turns well but will not start. Valves clearances are within spec (recent)

They guy who tested it told me the rings were bad but didn't do a test with oil.

Me being me, I took the engine out and proceeded to take the head off but am running into a slight issue. The valves seem to be stuck on the cylinder head. I've broken the gasket seal, it moves freely on one side and about .5mm on the other. I am assuming the guide pins are rusted/binding and I've put some PB blaster in and am letting it sit. Any suggestions on how I can leverage? I'm not a fan of using too much force, just don't want to cause any damage.

This is the first time I've gone this deep into an engine and from reading other posts I can see that the compression test with oil would have been a smart move. Since that's not going to happen at this point, can anyone point me in the right direction to inspect the valve, seats and etc...

Thanks in advance.
 
I just bought a 79 CB750L and found out that the compression is low in all cylinders. 1, 2, and 4 were at 60, 3 was 90. Bikes turns well but will not start. Valves clearances are within spec (recent)

They guy who tested it told me the rings were bad but didn't do a test with oil.

Me being me, I took the engine out and proceeded to take the head off but am running into a slight issue. The valves seem to be stuck on the cylinder head. I've broken the gasket seal, it moves freely on one side and about .5mm on the other. I am assuming the guide pins are rusted/binding and I've put some PB blaster in and am letting it sit. Any suggestions on how I can leverage? I'm not a fan of using too much force, just don't want to cause any damage.

This is the first time I've gone this deep into an engine and from reading other posts I can see that the compression test with oil would have been a smart move. Since that's not going to happen at this point, can anyone point me in the right direction to inspect the valve, seats and etc...

Thanks in advance.

Are you trying to remove the cylinder head from the cylinder block or have you done so? Which engine do you have SOHC or DOHC.?
 
79 is a dohc. Honda spec is too tight for the valves. The best thing would have been to run a compression test yourself with a known good gauge but like you said it doesnt matter at this point. Beings you havent been far into and engine before I would send the head to a reputable Japanese motorcycle shop. Automotive machine shops tend to be to sloppy for small motorcycle engine work. Once you get the head off then you can see more how the cylinder bores look. Let the PB work and keep moving the head around. Double check that you have all bolts out and make very sure you have a good service manual, not clymer, hayes.....Honda the only way to go. If your careful and fully aware of where you are prying you can pry a little bit around the edges just be sure not to gouge the sealing surface.
 
Update - got the everything apart but it wasn't easy. I used some plastic trim removal tools that I had and was able to work it off. Cylinder bores look very good, only glazed and absolutely no scoring. Rings were toast, the bike was left outside for a while for sure but the bike had a few miles on it before that even happened. Thanks for the help
 
Keep in mind you HAVE to at least hone the cylinders if you are only going to replace rings. You should have the bores measured to make sure they are round and that the taper is acceptable. Valves and seats are an issue on these motors, they are typically burned or worn from lack of maintenance and wrong valve clearance
 
Perfect - thanks for the info. I most certainly was going to hone the cyclinders and check the valve seats (+fix if needed) but I will have measurements done to be on the safe side. Valve clearances were really good, only two were a touch out for spec. I realize that down't mean the seats aren't damaged so I will be taking them apart to be sure though
 
hello. I recently bought a 76 cb750 chopper/bober and have same issue with compression. but it starts every time(kick) electric was removed. anyway. I bought a complete rebuild kit from CB750supply. and just pulled the motor now sitting on wrk table. pulled of valve cover.. but don't want to get to deep, before asking if I should pull the head out. I have been Restoring vintage cars/motors for a long while. but this is my first chance at a bike this build is crazy and was very un safe yet together. FRANKINSTINE. any how please help with any advise. thank you
 
hello. I recently bought a 76 cb750 chopper/bober and have same issue with compression. but it starts every time(kick) electric was removed. anyway. I bought a complete rebuild kit from CB750supply. and just pulled the motor now sitting on wrk table. pulled of valve cover.. but don't want to get to deep, before asking if I should pull the head out. I have been Restoring vintage cars/motors for a long while. but this is my first chance at a bike this build is crazy and was very un safe yet together. FRANKINSTINE. any how please help with any advise. thank you

Bobber/Chopper
 
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