1990 truck running too rich

[shell1990]

I have a 1990 GMC 1500 5 speed with a 305 v8. It is running too rich, when I took out the oxygen sensor it was black and all of the inside of the exhaust was black. The truck was originally equiped with a 350 but the previous owner put a 305 in to it. This is where I think the problem starts. Does it have to do something with the computer being programmed only for a 350? Do I have to get it recoded?

[clampjockey]

i just blew a 350 up in my truck and replaced it with a 305; the first thing the engine builder told me to change if i had problems with it running correctly (before calling him) was the chip in the cpu in behind glove box. he said the problem happens more often when going from a 305 to a 350 because the chip doesn't allow it to feed enough fuel; BUT he said it can also happen the other way around too, and thus youd be getting too much fuel. he also said that for some reason there's no difference between stock aftermarket 305 and 350 chips. go figure. lol