I just got a komatsu 15 gasoline forklift. It starts up and has plenty of power. After it warms up it starts to die out. I van be full throttle and it gains and loses power. Is it bad rings? The line from the valve cover to the top of the carb. It blows out "air" when idling from the valve cover. I disconnected the tube to show you where I mean.
Can you describe dying out ? Like your loosing power and it stutters and shakes like a fuel issue or does it continue to run ok but just has no overall power. Get a cheap compression tester and pull Your plugs and do a cold compression test. Record those numbers. Then run it till hits the point where it falls out. Check the compression again and compare it your base line. Faulty rings should be easy to rule out. You could also do a leak down test as well. Both tools are pretty Cheap if you don’t got them. I think what your showing in the picture is the crank case ventilation system and it should pulse air and run it back through the combustion cycle or to atmosphere. Check your plugs while you got them Out. Plugs can tell you a lot about a gas engine. Are you burning oil at all ? I would also Check your ignition system. Pull your cap and check your cap and rotor etc. You could be loosing it there as well. I’d start simple and check your basics first.
Engine looks fairly clean. Have you had it long enough to know much about it, or did you buy it like this? Looks like an overcomplicated Jap carburator, probably an automatic choke? no manual choke control? Take a can of starting fluid, and when it gets nice and warm and is barely running, shoot some starting fluid into the air intake and see if it runs beautifully for a split second. Start with a little shot and work up to see how well you can get it to run for how long. That will give you a good idea if it's fuel or ignition. That vent is not an issue, unless it's slobbering oil all over with the air coming out, doesn't look like it.
Think maybe you PCV valve is stuck closed. From pic of hoses and valve cover it appears PCV valve hose to carb to recycle vapors. Hose from valve cover up to air intake horn allows fresh air to valve cover. If PCV valve stuck, you are not getting the cycle of fresh air from top to valve cover to be vacuumed out and back to carb to be fed and burned. Just and observation from the pics.