Outboard Exhaust in Lazarette

An update on the issue of outboard exhaust in the lazarette.

I had been told by Yamaha that the exhaust pressure relief ports were only used when the motor was idling or at low throttle. Af higher speeds the exhaust was sucked out of the prop. The intimation was that when running at full speed there was little exhaust exiting those ports (located just under the engine head at the back of the motor).

This year I ported those exhaust holes through the transom. To my great surprise, when operating the motor at the dock (no forward motion). the exhaust was maximized at full speed. This means that the exhaust (when there is no connection to the outside world other than upwards and through the open lid), when operating at full speed, and with the lazarette top open, is pulled upwards right past the air intake for the engine.

The problem is worst when the motor is running full out, when the air intake and the exhaust out flow are both maximized.

For those that are having problems with plugs fouling, or low engine power, this information should be helpful and give you something to look into.