Ok, that's a 'TIGER' block. Very old, in the 60s...
Fuel leaking from the carbs when trimmed up is actually quite normal, they all do it.
With any run quality engine problems I always like to start assuming nothing.
Very first thing I would do is a compression test. Check you have clean spark plugs and they are the right ones. Some of those engines used surface gap plugs, some used conventional electrode gap.
Then with the engine running, pull each spark lead off in turn. You should notice a rev drop about the same for each cylinder. Any that don't drop tells you where to start looking.
Fuel lines and filters.
DO NOT UNDER ANY CIRCUMSTANCES USE PLASTIC AUTOMOTIVE FUEL FILTERS! You're asking for an explosion. There's a VERY good reason they are banned by USGC.
A water seperating fuel filter, spin-on cartridge type, should be installed in the line between any built-in tank and the engine, and make sure the fuel lines are a MINIMUM 3/8". If you use remote tanks, ensure they are clean each time before filling with fuel... And use the right quantity of good quality 2-cycle oil. 50:1 is the right mix. I have had engines that refuse to run on certain brands of oil... The best I've found (as much as I hate admitting it) is Mercs' own brand, Quicksilver TC-W3.
With an engine that old, I'd look at checking the fuel lines thoroughly, and pulling the carbs and cleaning them out too... And putting a new set of diaphragms in the pumps.
Chris.......