If your CO is above 100 you have a problem
if you have a unstable CO you have a serious problem.
If you start to overfire a furnace your CO will go over 100, O2 goes under 6%, or you will exceed the delta t or flue temps.
If you cannot control draft then you shouldn't adjust gas pressure.
I guess it's something I have to study to wrap my head around. There's hardly any training in our area too far north, and know one else I talk to seems to know much, plus I don't know many techs anyways. I have been doing service for going on 3 years mixed with installs with little help. Thanks for some direction anyways, it's probably better than learning from some "good enuf" tech.