Need Ford help

[bumpster]

I know this is not an off brand forum, but I am at wit's end. 85 F*@# half ton, in-line 6, put old style carb on about 3 yrs ago to eliminate the last of the smog stuff. EGR is capped off and lines are plugged. All was well till suddenly it started to run way too rich, won,t idle, blows black smoke. If I take the brake booster vacum hose off, engine speeds way up and smooths out. Why does this seem backwards to logic, and what am I not finding?
Any ideas other than the obvious{burn it}, would be much appreciated. Thanks in advance.