When you say "clean the carbs", does that mean disassembled and cleaned internally or just externally? If you haven't taken them apart for a good cleaning, that's probably it. The crud in the air cleaner can be normal. The crankcase vent is routed there to run the crankcase fumes through the motor, burning them, rather than just venting to atmosphere. Some mist comes with those fumes but will be worse in a worn out engine. I wouldn't worry about that. If it's got problems warranting attention, they will show up in performance or oil consumption.
Remove the plugs and make sure they aren't wet from your previous attempts. If they aren't new, replace them. A shot of starting ether sprayed into the carbs or a small amount of gas squirted in, then cranking over should get a short burst out of the engine that would verify that the ignition and such are functioning and that it's a fuel problem. If you decide to introduce fuel straight into the intake like this, DO NOT do it while turning the engine over. With everything off, introduce the fuel, move all containers WAY away from the area, keep yourself away from the intake, and ONLY THEN try to start it. It can backfire through and light up anything flammable in the immediate vicinity. That might narrow your search dramatically. Good luck.
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