Topic: 1980 CB750K Weird Low/No Compression Problem

[Gbpatterson44]

So for a while now I have been trying to get my 1980 CB750K to run. So far I have ruled out fuel delivery and electrical related causes. However when I checked my compression today I found that 1st and 3rd cylinders were reading zero psi. The 2nd cylinder was reading low compression at 60-70 psi, and the 4th cylinder was reading pretty close to normal compression at around 110-115 psi.

When I took the exhaust gas temp yesterday when it was idling at full choke and with me manipulating the throttle to keep it running. The 1st and 3rd cylinders were reading at around 290 degrees, the 2nd cylinder was reading at 190 degrees, and the 4th cylinder was reading at around 420 degrees. I’m beyond confused at this point because I don’t understand how the two cylinders with no compression could have a higher EGT than the one with low compression, or honestly how I was getting it to start at all.

Today I took the head off and I don’t see any obvious signs of the valves being damaged and I don’t think the head gasket blew out (however I don’t really know how to tell on either of those). I was mostly looking for obvious indications of what would cause no compression. Any help at all with this would be great! I’ve been running circles with this bike for some time now and I really want to get it running so I can sell it. Thanks!

[HondaMan]

This just sounds like the valves are not sealing. It could be rusty valves or valve seats, or even bent valves. The fuel will fire in the cylinders regardless, so you will still see elevated temperatures on the exhaust.

[scottly]

So for a while now I have been trying to get my 1980 CB750K to run. So far I have ruled out fuel delivery and electrical related causes. However when I checked my compression today I found that 1st and 3rd cylinders were reading zero psi. The 2nd cylinder was reading low compression at 60-70 psi, and the 4th cylinder was reading pretty close to normal compression at around 110-115 psi.

When I took the exhaust gas temp yesterday when it was idling at full choke and with me manipulating the throttle to keep it running. The 1st and 3rd cylinders were reading at around 290 degrees, the 2nd cylinder was reading at 190 degrees, and the 4th cylinder was reading at around 420 degrees. I’m beyond confused at this point because I don’t understand how the two cylinders with no compression could have a higher EGT than the one with low compression, or honestly how I was getting it to start at all.

The obvious answer would be that the compression test was flawed.

[bryanj]

Isnt a 1980 a dohc?

[Gbpatterson44]

Isnt a 1980 a dohc?

Yeah it is but I just wanted to know what people that know more than me about these bikes thought about it.

[Gbpatterson44]

This just sounds like the valves are not sealing. It could be rusty valves or valve seats, or even bent valves. The fuel will fire in the cylinders regardless, so you will still see elevated temperatures on the exhaust.

There a way to check that for sure? Just by looking at them they don’t seem damaged and I can’t really see a difference in how they are seating. Would I need to get the seats redone or just get new valves?

[bryanj]

Neither stir it out and test them properly. With cams out and head off put kerosene into the ports and see if it leaks past the valve seats

[BenelliSEI]

You have the head off? Cams are out? Turn the head upside down on your bench and fill each combustion chamber with varsol (spark plugs installed). The leaking valves will let fluid pour into their respective ports.

[low-side]

I had a '78 GL that actually didn't run too badly with bent intake valves on the left bank.  When I couldn't get a decent idle no matter what I did I found the issue with a compression test and physical inspection.