Chevrolet Repair: valve adjustments, tappet noise, vacuum leaks

Question
my son has a 80's chevy truck w/350 v8,we are trying to adjust the valves,and get rid of the tappet noise but when we tighten down the tappets the engine starts to miss,also at idle it runs rough,we have set the timing and adjusted the carb,it runs good going down the road except for the tappet noise but at idle it runs rough,it acts like a vacumm leak but cant seem to locate,any suggestions?

Answer
Hello Bill,
There is a proceedure for setting the valves, which allows about half to be set, then rotate the engine one turn and set the other half. But basically, that gets the valves closed that are being set, so if you want to go cylinder by cylinder, just make sure they are both closed. Loosen off the rocker arm nut, then tighten it to where all the slack is removed from the pushrod. Then tighten another three quarter turn. Do it on each.
Vacuum leaks can be the EGR valve sticking, the brake booster, the heater a/c controls, and the transmission modulator valve if it has one. Also the vacuum canister, and the vapor recovery system.

Van