Combustion adjustment on Lochinvar Noble NKB-080N

GroundUp

I've installed several dozen Lochinvar Noble boilers, both heat-only and combi units. Combustion has always been spot-on from the factory, even with converted units which surprised the heck out of me. I finally got one which was really low on C02 both on low and high fire, so I adjusted the screw counterclockwise to raise the % as the manual suggested. This made it worse, so I started going clockwise 1/8 turn at a time. I was able to get 4.75 turns out of it until it was tight, but even at that was only 8.68%. Now, the manual calls for a goal of 9% but lists 8.5-10.5% as acceptable so I left it at that BUT for future reference, what can I do to help this if I'm unable to dial it in to an acceptable range? Gas pressure started at 12" locked up and 7" during operation. I dropped it to 10/4 and raised it to 14/9 at the end trying to raise that C02, with zero change. I'm not a service guy but an installer so I apologize if this is a really dumb question, but I'm stumped... All other parameters were perfect.