You would have to weld the weep holes shut. At higher rpms with weep holes open (essentially an exhaust leak) the exhaust stream will suck in air (venturi effect) and add additional O2 to the exhaust stream. This will skew the O2 reading, causing a false lean reading.
In an EFI application, the ECM would respond to the lean O2 by adding unnecessary additional fuel.
Jay
Any air leaks would make the O2 sensor reading be incorrect.
The ECM would only add fuel if it was tied into the O2 sensor and set up in closed loop operation.
Closed loop would be the ideal but I've never been able to run that way because my Haltech E6Kwon't read a wide band O2 sensor.
I'm not really sure how many boat EFI systems run closed loop.
Ideally the fuel maps are set up so that in closed loop the ECM will only take away fuel and not add fuel. Closed loop only functions to about 70# throttle then you're running straight off the fuel maps anyways.