You could take the solenoid output signal pin on the MCU and loop it back through another input... in the sketch add your air pump loop, invert the signal out to another output pin to the air pump driver... then the air pump will follow the solenoid... air pressure will rise as the throttle is opened, and lower as the throttle is closed... it would mirror the mixture curve though.
***EDIT***
I take it you are using both the fuel solenoid valve along with the air pump... still using the solenoid to control the mixture, and the air pump to control the tank bias pressure?
Last edited by John_M_; 11-27-2022 at 09:26 AM.