Hi. The loose clamp on your crankcase breather hose is not a problem. If your bike is similar to my '79 1000, you have some folded up mesh screen
inside your crankcase breather cavity. This screen can rust and you may get some rust seeping into the airbox. I've never seen anything beyond a "staining" enter the airbox, certainly nothing that would clog the jets,
but I suppose it could get worse than I've seen. If you have rust, I would take a better look at your tank. That's most likely where the rust is coming from. I'm not familiar with the design of your float needle valves, but a worn o-ring would cause a rich mixture, not a lean problem.
I know this problem is frustrating for you and time consuming, but you need to check or re-check several things. Some suggestions have been made, but you don't always respond if you have actually checked out the suggestion. We keep "assuming" something was checked and we start trying something else. Earlier you said the carbs were clean, but now they have rust. This makes it hard to troubleshoot. I'm sure the rust was a surprise to you.
For a problem that feels like lower rpm fuel starvation, lean sparkplugs, is intermittent, and considering the rust, this is what I would check.
Remember, I don't have a perfect understanding of your style carbs.
Inspect the tank. If any rust, Kreem coat it or do similar to remove/stop the rust.
Clean the gas cap vent.
Clean/check the petcock operation. Is ANY fuel entering the vacuum line?
(This would indicate a damaged diaphragm and it would not always fully open the petcock.)
Remove the fuel filter, at least temporarily, until the problem is solved.
Take the carbs apart and clean everything, blow out the passages, pay close attention to the pilot circuit and the floatbowl venting.
Check all parts in the carbs, especially the moving parts, floats/needle valves/ throttle valves/pistons and diaphragms.
Check the intake boots/manifolds/o-rings, if OK, grease the o-rings with hi-temp axle grease and tighten the clamps good, install the vent tube(s) and CORRECT size fuel line.
Synch' the carbs.
If all this is done correctly, I can't think of anything else that would give you a lean condition as you describe. I would still double-check the electrical connections too.
Good luck, keep us up on this! (I love a good mystery.)
