So, it took a while to get it all tried out and I've gotten myself into another adventure.
I took out the carbs again, inspected them and I could not see any flaw. Put them back together and tried to start, but it wouldn't fly. I waited a day or two until I could use someone elses battery to test that too, before I noticed fuel was leaking. My whole tank empty, and a fuel smell from the carter.
So I guess one of my floating needles is jammed which I need to clean. I do not have a vacuum petcock but a simple one. I'll drain the oil, have the engine for a while, put in new oil and filter, start without the spark plugs and hope for some better results.
Also, I'm still not sure if this will solve the original issue. I've ordered a new battery and I'll check the petcock as my tank is empty now anyways.
Any additional tips are still welcome (I'll be gone for the weekend but will then work on it again) and once I've found what it is I'll share it with you
.
Thanks!