Engine started running rough 350 TBI

[Patsfan34]

I bought an 87 GMC 4x4 2500 about a month ago. Was sitting for a handful of years in a barn at a local farm. I got a great deal on it and knew I could fix most of the stuff it needed. It's said to have 73K original miles.

First what i fixed or replace: new exhaust (cat back), new shocks, full tune up (plugs, wires, cap, rotor, air filter), radiator and heater hoses, all 3 belts, coolant flush and new coolant. Radiator had a tiny hole in the center so I bought some stop leak and it immediately stopped leaking. I also changed the power steering pump. i didn't buy a new one cause an engine I had in my garage still had one that I knew worked fine. I was having problems with the starter (it would only catch every other time) so i pulled it out and noticed the teeth were pretty chewed up. I actually had another starter hanging around out of the donor truck for my project so I compared them and they looked the same (different model numbers) but I bolted it in and the truck turned over perfect.
I figured it was time to take the truck for a ride to see how it ran. I took it out last Sunday and drive it around for about 30 minutes and it was flawless. This was before I switched the starter and it ran great, sounded great. This Wednesday after I finished with the starter swap I took it for another ride only 5 minutes and when I got home it was running terrible. It will not idle. When I turn it over it starts right up but I have to give it gas to keep it running or it will shut off. Could the starter have caused the problem? I checked everything again hoping maybe a ground came loose but nothing.

I pulled the air cleaner off to inspect the TB and I noticed the plugs seem to have melted a bit. Pic below. Obviously I am going to replace them. What could have cause these things to melt? I've had the air cleaner off a handful of times but never noticed the plastic on the plugs melted. When I started the truck today a massive cloud of white exhaust came out of the tailpipe and it was like that for about 30 seconds but then eventually cleared out. Sorry for the long post but I wanted you guys to have all the info. I even pulled the exhaust again just to make sure the cat wasn't clogged and nothing. Any help is appreciated!

Thanks, Patsy