what streets said about the throttle stop is usually what causes that problem on all gm cars in the seventies and up, but from the pic it looks like your running an edelbrock and I love those carbs I have never had that problem with them unless your fuel pressure is through the roof and its flooding out the vent or something I doubt its that if its a holley it they always piss out the vent tubes even with good needles and seats and proper fuel pressure. You can check this by taking the air cleaner off making sure the choke is open and just shut the engine off it you see a bit of smoke or haze from the carb after its off you know somewhere raw fuel is leaking down in the intake. As far as cheap tests
I would try cooler plugs and better fuel cheap start to find out what the problem is may be even a cooler thermo stat would help too if its running hot any idea on compression ratio forgive me if I am blind and didn't read it also how often or hard do you drive if you said it was running fat and sooting up carbon deposits could be doing it either being hot and igniting the fuel and or raising the compression ratio. Some say you can burn them out by gettin it on the road but the best way is to either take it down and decarbon the combustion chamber some with will say leave it together and use the walnut shell blaster which I know is a gm thing or may be a healthy shot of Seafoam in your fuel tank may clear some of that up. hope this helps...I have never heard of the backfeeding ignition one but I have had problems with aftermarket ignition doing all sorts of things I never would have checked that...hmm the fan spining made enough power to keep the engine running? I heard this somewhere else before weird oh well.