A friend has a 80's Chevy pickup. It is a 305 with throttle body. It is running rough so he has changed the plugs, distributor cap, and wires. It was throwing an engine code for running rich and he can smell the gas as well. He noticed that 4 of the plugs were very sooty and they are every other plug on the distributor. Any suggestions?
Check for a vaccum leak. The throttle body base can suck air in addition to the vaccum hoses. Check the line to the transmission modulator too.
If you spray water or carb cleaner around the suspect components, the engine idle will change if you get the spray near a leak.
Take a look at the injectors as they spray at idle too, see if one is different or looks weak.
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It sounds like one of the injectors or an injector pod is leaking. Every other plug in the firing order matches the dual plane intake. each injector feeds 4 cylinders in alternating firing order.
Turn on the key (but not crank or start) and see if fuel is dripping from one of the injectors. If so it may be a bad injector or the O-rings that fit around the injector. (fuel pump should run for about 2 seconds at key on for pressure to build).
use a standard timing light to help you see the "spray pattern" from the injectors at idle and see how things change as you increase rpm, If you had a junk injector, you could tell by the spray pattern. Also, there are cup filters installed in these injectors, remove the tops and inspect the plastic filters for debris, if they're loaded with crap, you likely have a fouled injector.