On my 86 when I had the stock Mikuni carb, there was a EGR sub valve on the base of the carb. It is actuated by linkage on the carb. It is located on the drivers side of carb at the base and to remove it to clean you have to remove the "C" clip holding the linkage pin and linkage to the metal valve plunger. Remove the accordion rubber protection boot and move the small linkage arm out of the way and slide out the sub egr valve metal plunger from carb.
plunger.jpg

These metal plungers would get carboned up a lot and stick open. Mine would even make the gas pedal feel like a binding linkage problem when it got stuck. In fact the EGR system on these motors is like a carbon making farm and causes the system to get clogged with black soot. I'm sure when you check the EGR valve you will see the build up. This will continue the entire distance of the EGR gases and needs to be cleaned. SO, through the intake manifold into the head and dumping into an exhaust port. Usually a coat hanger or wire brush tip rod cleans them out pretty well.
Your EGR should not be opening at idle and or WOT and your probably getting vacuum at these times and shouldn't.
If EGR is on during idle, stumble and even stalling will likely result. If EGR is on during hard acceleration, low power (from reduced air/fuel volume) is the result. At part load, mix advanced for economy, conditions that create high combustion chamber temperatures. Without EGR, these conditions not only create [NO.sub.x], they foster pre-ignition and pinging.

This is all for cleaner air to breath and making your truck engine eat it's own poo over again.