Thank you @Zed7! Mechanics here are pretty smart at guessing things. Lols. Even if I go to the dealer, just the same conclusions.I can't see your mechanic's homemade air filter working properly. If you are hard up, use motor oil to oil the foam. NOT chain lube. Chain lubes stiffen up so they don't get flung off. I would think chain lube will just clog the filter or quite possibly get sucked into the carbs gumming up the slides. Just a theory. Probably just clog the filter.
Earlier you said you said you had a full tank but was gone by afternoon. Was that from riding it? Or did it leak to the ground?
If neither of those there's only one other place for it to go. In the crank case. Did you sniff the oil in the crank case for a smell of gas yet?
Fuel systems in motorcycles are a finicky thing, especially the carbs. Any change from the way it was designed and it will happily give you headaches forever. Or until it's done right.
I don't mean to sound insulting but that mechanic friend of yours...