1979 KZ650 oil spilling from crankcase

Hey guys, I'm new to this forum and would really love some help getting my KZ650 up and running. I've done some work since I bought it, and have gotten it running, albeit not great. I have been very tentative about running it for very long due to this issue.

While it runs, there is a large amount of pressure in the crankcase, that causes a lot of oil to be pushed out of the crankcase and into the venting tube that connects to the air box. I recently changed the oil, putting in about 3.5 liters.

My thoughts are that it might be either bad valves or piston rings, as it seems to be a compression leak into the crankcase is causing this problem. (it is tough to tell for sure, but there might be some gas that is mixing with the oil that is leaking out.)

Any thoughts guys? I would appreciate any help. Thanks!

Bad piston rings/sleeves are definitely what I'd be thinking about, after I made sure that the oil was about midway on the sight glass with the bike level and having been sitting for a bit. Would consider trying a leakdown test to isolate the source of the problem. Have you compression tested it before, if so what were the results? Might also give some insight into whether or not there's major leaks somewhere, though the leakdown would be better for finding exactly where.

Did you ever figure this out? I am currently having the same issue.

How did you remedy the leak. For me it seems to start when the bike gets up to 3000 Rpms