You need to stop trying to start this engine right now!
You need to, at the very least, remove the tank and have it cleaned out buy a radiator shop, or by yourself, or buy a new tank.
If you do not do this, you run the risk of having decades of varnish which has accumulated on the sides and bottom of the tank to be dissolved by today's blend of fuel (which is a very good varnish solvent), and cause it to migrate and accumulate in the fuel pump check valve and throughout the inside of the carburetor, and gum up the valve guides to the point where you bend push rods. A fuel filter will not stop this from happening.
Way too many, well meaning, but in too much of a hurry people make this mistake, and dread it later.
Carl
