Buderus G115WS3 w/RIELLO F40-3 ... can't lower CO2?

I just installed a G115WS3 with a Riello F40 burner. When I first commissioned the boiler the Riello would not ignite... just kept cycling start-purge-start-purge... Called Riello support who suggested raising pressure to 185psi. I didn't have my gauge so I increased pressure until it ran. I could not get back to it for a few weeks but have now come back with my equipment. The pump pressure was at 240 with CO2 around 13.8%, Stack at 389, CO at 11ppm, and efficiency of 86.5%... I have tried to lower the pump pressure and Air setting (originally set at 4) but can't get the pump pressure much lower than 225psi. Anything lower causes the burner to drop into purge and eventual lock-out... even when I significantly reduce the air flow ( air setting less than 2.) I reset the electrodes, turbulator setting, etc. to spec but can't improve it.

I would like to get to a stack temp of 350F, CO of 11-12%, and efficiency around 87%. Any ideas what's causing the Riello to require such high pressure?