Ty, is the plug wet, and is there surplus fuel in the cylinder/crankcase? Have you tested the ignition? If the answers are yes to both questions, the carburetor is at fault and that is what I will focus on below.

I think the primer-equipped Walbro pumps air into the fuel tank, so if you connect it backwards, it pumps fuel into the pulse chamber of the carburetor instead, which floods it. (Not sure about all that, have only read up on choke type, not primer type, in detail, but I read a short treatise on the perils of connecting the primer backwards on another site.) So, check that your primer is pumping air into the fuel tank, not fuel into the fuel pump chamber.

Have you checked the metering lever setting? That is fairly sensitive, and affects the mixture. The emissions carburetors appear to have an internal adjustable needle valve as well, which acts as a coarse mixture adjustment. Finally, the fuel pump pop-off pressure also has a strong effect on the mixture:
http://www.aerocorsair.com/id28.htm

On a carburetor with no external mixture adjustment, once it has become seriously fouled up, the only solution seems to be to go through the carburetor manual and adjust everything by the book. What model carburetor is it? The full manual is available as a free download for most of them.