Sven is starting you at the correct place. I'm sure you know much of what I say here, but some members might not....
First check to be sure your crankcase vents are working.
There is a thin vacuum (neg. pressure) tube going from the carb body to the front valve cover. Inspect to be sure it is connected.
Then look at the fatter (pos. pressure) tube that comes from the center of the rear cylinder to the carb housing. You'll find a plastic PCV valve in it..good idea to remove and check it to be sure it works ok. It is a check valve, so be sure not install it backwards.
My bike has a spike air cleaner so there is no place to connect this hose, so I installed a little chrome filter and attached the hose from the rear valve cover to that. Some guys run a hose straight down to drip on the ground. Around town that works out OK, but on the highway at 95 degrees and 80 mph, wind velocity atomizes oil and makes a hell of a mess.
My bike was having problems with blowby, and continued to blow some oil out of that breather tube coming from the rear cylinder (the positive pressure tube). Since that Arlen Ness filter was paper, I just watched and when it started dripping oil, I'd clean it and reinstall. The bike ran great, so I just cleaned the filter and rode it several thousand more miles.
Blowby comes from air being pushed into the crankcase. Usually that happens when rings wear and also cylinders become elliptical instead of round, so compression goes by the rings into the engine and out of the crankcase ventilation hose (the one with the PCV valve from the rear cyl to the carb base). Defective valves or valve seats lower compression too. A compression test complete with bleed-down can tell you a lot about that. Bleed down is when you put compressed air into the cylinder at TDC and see if it holds or not. All you have to do is listen for the air leak. Intake will make carb noise, exhaust will hiss in the tailpipe. Bad valve seats are usually related to heat, so ex. valve seats typically fail more often, and require a rebuild of the head itself. If the valve itself is not seating correctly, qualified mechanic can re-seat valves.
