you are correct in your assessment, the primer bulb is simply an air pump. When priming, the primer bulb sends a shot of air to the float bowl. some fuel is displaced - this travels up the venturi tube and some fuel enters the air stream in the venturi area.

Most probably the needle and seat has worn over the years or perhaps some dirt has entered this area. To solve, your carburetor needs to be removed for cleaning and replacement of the needle and seat plus other replaceable parts. Test the carburetor by inverting - attempt to blow through the fuel inlet to see if there is leakage.

You should also remove the fuel tank. Dump the fuel and flush out any dirt. (I usually spray the tank interior with carburetor spray and draw this out with my vacuum equipment.) Dry the tank with an air hose. Clean the plastic air filter housing.

When reassembling, use a new fuel hose and new gaskets for the carburetor and air filter housing.

Check the oil quantity and inspect the oil to make sure fuel did not leak through the engine - entering the crankcase.

Get rid of the old fuel by pouring in your vehicle. If it looks bad, ask some of the local repair shops if they have a collection point.

1. are the fuel tank caps on these vented or non vented?

2. and, if you take the air filter off and look down into the carb, is it normal for there to be a shallow (1/8 inch) puddle of fuel in the carb "hole" / side nearest the primer bulb? - when engine is off of course?

Thanks