Won't run, not burning fuel

Hey all, got an '86 sierra w/ 350, 4 speed. I live in california so I got a snakes nest of vacuum lines. The truck was running ok (I just bought it last month). I have two thermal vacuum switches up front on/near the intake. One was cracked so I assumed it wasn't working. While taking it out, it broke in half. I couldn't find the exact replacement switch, but I found one close. It operates in a little higher temp range. The OE was 120*. This is 140*. Could that be it?
Anyway, I had to use it one day while waiting on the new part and as I got home (it ran horribly) it died. Gas was pouring out of the carb from everywhere. So today I put the switch in and while it did start, it soon started missing and died. Black smoke was billowing out the pipe and you could smell the gas. I didn't let it run long enough to see if gas was coming out the carb.
So I double-checked all the vacuum lines and couldn't find any problems. Lines are all good, no cracks. Just gave it a tune-up. I know it must be something to do with the emission system, but could be a carb issue? Any suggestions would be appreciated.

Either a clogged cat or the carb is bad and loading up, or it may have jumped time, all these things are part of normal diagnostics for what you describe.

Do you think it would likely have anything to do with that vacuum switch? I mean, as long as it's similar in temp range, a switch is a switch, right? I'm thinking that because the nipples were 1/4" on the old part and 1/8" on the new one, maybe it's not getting enough vacuum?

OK, so I put a new carb and intake on it since last post. Now it will start fine when its cold, run on high idle for about a minute and then try to die. I CAN keep it running by giving a little throttle, but if I give too much, POOF, it backfires thru the carb and dies. It barely idles once it kicks off high idle, which I guess it could be the curb idle needing to be set?

When it's idling on high, there's whitish smoke coming out the pipe. When I rev it up a bit, the smoke goes away, but you can smell the raw fuel very strong.

Presuming the timing is set correctly, what else could cause backfiring thru the carb? The truck ran fine until the vacuum switch broke (see above). It's been all downhill from there.

The Q-jet is a pro rebuilt unit, but not necessarily set up to run out of the box. Does this sound like a carb issue? Any help is GREATLY appreciated!!!!

Usually a back fire is a timing issue or too lean a mixture. You need to dig deeper into vacuum issues or out of time issues.

Well, it turns out it was timing. After umpteen unsuccessful attempts, I had my friend come over and give it a whirl. He did the same things I had done and after four or five attempts at dropping the dist. in on TDC, it finally took. So apparently it was just the dist. being a hair off. Thanks for your help.