Possible intake gasket leak.

quote:

Originally posted by shovelhead:
Possible intake gasket leak.

Agreed, check vacuum.

Also put a scan tool on it and check your fuel trim and O2 sensor.

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First thing that comes to mind when one bank is affected is restricted exhaust. Is that boat running check valves in the exhaust to keep water out?. I'm no boat mechanic, just fleet trucks.

Since one bank of the V8 has the issue, we must consider causes affecting four cylinders. Fuel injector pulse width is commanded from a table of numbers based on engine speed and load. Adjustments to those pulse widths are made from calculations including sensor inputs. Some sensors are singular, such as manifold absolute pressure. Some sensors are bank-specific. In vehicles, there are often two oxygen sensors on the exhaust manifold of the driver and passenger banks.

Oxygen sensors are killed by water intrusion. Being a marine application, I wonder if one bank exhaust manifold was subjected to water, while the other was not. For example, in a backing maneuver, where the boat was moved into a following wave, did a check valve fail?

Of course, the wiring harness to one sensor might also have fractured, or melted.

If you have to remove the O2 sensor, pay a few dollars for the correct sensor socket tool.

I don’t believe marine engines like this have O2 sensors because they are wet exhaust - the water going through the engine exits via the same exhaust.

The manifold looks like this - linked only as an example.

https://www.ebay.com/itm/19399...GEAQYASABEgI20PD_BwE

Lower https://www.ebay.com/itm/17499...UEAQYASABEgLajPD_BwE

I’m curious if one bad injector can cause the entire side to foul. I can’t picture how the air flows on this one from photos.

There’s an upper plenum and a lower plenum. All the injectors are hidden in the space between.

Have you confirmed you have healthy Ignition/Spark on that Cylinder Bank of the Engine?

quote:

Originally posted by nhracecraft:
Have you confirmed you have healthy Ignition/Spark on that Cylinder Bank of the Engine?

It is a traditional distributor with a single coil. I think they are getting the same spark as the others but something is causing excess fuel to dump on half.

I do need to check the wiring harness. I was hoping to narrow it down before separating the plenum to do that in case I need to do any testing while I can start it.

Are the plugs fluffy black, where the black flakes off or slick black where you have to wipe it off?

Fluffy, too rich, slick, oil intrusion.

quote:

Originally posted by shovelhead:
Are the plugs fluffy black, where the black flakes off or slick black where you have to wipe it off?

Fluffy, too rich, slick, oil intrusion.

More like wet transparent black that evaporates quickly and smells like gas. The inner ceramic is just dull black.

Sounds like fuel related. What sensor monitors the right bank injection signals?