1976 Z750 twin

What did the small end of the connecting rod look like? Common for the small end to not get enough oil, overheat, gall the wrist pin, and then burn a bunch of oil because the piston cant rock in the bore correctly.

I got the bike from a friend so I don't know how it was ridden before.
Compression was lost during winter when the bike was not fired up for a month or two. Before that it was ok, I did a three week trip last year without any problem.
I don't know what caused valve leak. Honestly, I didn't do any measurementss on the valves. The clearance in valve guide was ok, without any play.
I don't really remember whether there was cross-hatch visible from top to bottom. It was certainly visible on the bottom part where piston rings aren't in contact with cylinder, but if I remember there were some signs of it on upper part also.
I've measured the piston ring gap when inserted in the cyl as the manual suggests and they were bellow service limit. I've also checked the new rings.
The airbox is nice and dry.

Please do interrogate me. Although I'd rather call this a 'constructive dialogue'  

I haven't checked compression now (I have to borrow the meter). It's definetly better than before- the kickstarter has a lot more resistence than before.


@DoctoRot it seemed okay. Also the pin slide out and back in with normal resistence.

PS please do excuse my bad English. I'm not a native speaker. And thank you for all the help 

At this point I don't think there is anything you can do other than remove the head and cylinders and check everything out. is it possible an oil ring ends got overlapped?

I agree that head removal and thorough inspection in almost inevitable.
Hmm if I remember correctly, the oil ring is a slotted type. Can a slotted oil ring overlap at all?

I forgot the 750 twin use a one piece ring, it cannot be overlapped.

Hi guys,Hello again after a long time. After a few years' pause, I’ve finally decided to continue working on my Z's issues.

After discovering that it burns oil badly, I replaced the 10W40 oil with 20W50 (Motul 3000), but the problem remained. I also replaced the valve seals with softer ones (I think silicone ones).After a compression test, I found that it was within the service limit, but after adding some oil to the cylinder and measuring again, the compression reading increased by about 30 psi.It seems I’ll have to remove the head and re-check the cylinders and pistons.
Here is my to-do list:

• Bore measurement (diameter, roundness, taper)
• New rings? If yes, hone the bores
• Cylinder head check (valves, seals)
• Pistons check (I read somewhere that there are holes under the oil ring that allow oil to flow into the piston and back down into the crankcase—could they be blocked?)
• Connecting rod check
• Reassembly with new gaskets
• Try a different type of oil (European equivalent of Shell Rotella, maybe a mineral diesel engine oil?)

What do you think? Is there anything else I should check once the head and cylinders are taken apart?Thanks in advance!Here are some pictures:

Left cylinder spark plug, compression reading before and after adding oil:
 


 

 

Right cyl:
 


 

 


 

Your "to do" list looks pretty comprehensive, a 25% increase in compression on the "wet test" suggests worn cylinders. I intend to run 20/50 Motul in my build once complete, you can experiment with different oils but they won't compensate for mechanical wear