help with idle
 

I have a 91 420sel,with fuel issues.After replacing fuel dist. with used one,I had to cut tower to access mixture adjustment.According to post I read,I shouls have about 4 volts between #2and #3 at idle to give me a 70%duty.Before adjusting,I was reading 8volts and a lot off black smoke when stepping on the gas.I did adjust fuel mixture,no more smoke and idle drop down to 800rpm(1200 prior).How ever when I try to read the voltage,it fluctuate between 3.5 and 6.8 constently.The engine idle somewhat rough also.What do i need to do next to get a smoother idle.P.s.I dont have much experience and have no access to a mercedes mechanic,so please explain in steps I can easely understand and follow.Thank for everyone's help.