If you look at the Kawasaki Invader, you will notice the fuel tank sits higher than the crankcase (and carbs) which makes it possible for the fuel to siphon in through the carbs and fill the crankcase. The fuel pump (stock) looks taller than a normal Mikuni pump because it has and anti siphon valve incorporated in it. thus the fuel pump will not work until the engine creates enough vacuum to open the valve, which makes it hard to start if there is no fuel in the carbs. Thus the need for a primer...
However, since the primer bypasses the fuel pump. there is a check valve in the primer to keep fuel from going into the carb (which is lower than the tank) unless it is under pressure. If the check valve fails, you get gas in the crankcase, and the engine will draw fuel in through the primer and you get a high idle. Seems lately primers only last about a year or two.
Once the engine gets flooded with fuel, it strips the oil off the surface of the crankcase and crankshaft, and of course the bearings. This causes everything to run dry and damages bearings and most importantly the crankseals. after the engine dries out and runs a bit, you could end up with a blown motor from a damaged crank seal.
To help this, I always packed the seals with grease to give them a little more protection fro drying out. Also new needle and seats every year. And make sure you have the correct fuel pump.
Sorry just some rants from an old guy.