Did some work that required removing the dizzy. After that truck suddenly started idling really low. At first I thought it was good, cuz truck used to idle kind of high. But then at the new low idle truck seemed to idle rough. I figured something happend to the 220k miles dizzy in the process of taking it out and putting back in, so I ordered a new one. Installed the new dizzy and set the timing to zero BTDC with the EST wire disconnected. Engine purred like a kitten and thought I had solved the problem. Then I shut her off, hooked up the EST wire, started her up and she idled real low and rough just like with the old dizzy. When I hooked up the EST the timing at idle moves from 0 BTDC to 16 BTDC. I disconnected the EST wire and advanced the timing to 16 BTDC by turning the distributor to see if it would cause the same poor idle as the EST plugged in....and it does. So it seems like it is the advanced idle timing that makes my engine unhappy. I thought most engines liked advanced timing at idle...and 16 is pretty tame..why would that be causing such poor idle in my engine? Where should idle timing be with EST connected? Is 16 BTDC right?