Before you read on I am no expert but I may have a solution to your tuning problem. Bausch makes an exhaust sensor that is fairly easy to mount to your manifolds and it gives you a "neat and pretty" little read out of how rich or lean your engines are running in real time! The whole package is incredibly cheap, I used to use one on my lil rice rocket street car, I'm surprised not to see them in more marine applications. Hope this helps you, Good luck.
RiverDave