Seeking a solution to a problem I have come across with the engine unexpectantly cutting out. I am running a Yamaha 8Hp 2 stroke which has an exhaust vent situated below the engine cowling (see pic). Have a hunch that the smoke being emitted from the upper exhaust outlet is collecting within the engine well and is smothering the engine through its air intake. This has happened only when running with the breeze or stationary with no breeze therefore no flushing out the exhaust fumes. Did cause some concern when the engine just cut out coming into a ramp one day. Has anyone else come across this problem and better still does anyone have a solution. I have considered rigging an exhaust pipe to remove the exhaust fumes from the engine well.
I had a similar problem especially when idling or going downwind, so it sounds similar to your problem. My motor did not cut out but ran very badly. That exhaust outlet is only used when idling otherwise the exhaust vents through the propeller.
I fixed it by making a cover from a piece of PVC electrical conduit fixed into a hole drilled into a flat piece of timber. Then glue dense foam rubber to the underside of the timber to make a more-or-less airtight seal against the motor. I held it in place with one of those multi coloured tie-down straps around the motor leg and up behind the leg hinge to stop it sliding down the leg. Then some flexible black reinforced pipe from the PVC pipe to a plastic skin fitting fixed through the transom. I was concerned that the heat would melt the foam rubber but it seems to be OK and I have used it for many years.
There were issues with Magnums outboards running poorly at low speed due to the exhaust being recycled at low speed. The Magnum had an enclosed well that did not ventilate the exhaust at low speed.
At speed the exhaust goes out the propeller and vents away from the well. At low speed the exhaust lack the pressure to push out the propeller and vents from a low speed exhaust on the back of the leg.
The RL24 well is open and reasonably vented so I doubt that a rough idle would be caused by the exhaust, its more likely to be simply the water back pressure in the leg causing the engine to fluctuate.
I have a smaller 5hp 2stroke and I run it on 75:1 mix Castrol Biolube to reduce smoke, it runs rough at idle but that is just the 2 stroke.
If you are having trouble with the Yamaha then Yamaha sell an optional exhaust outlet barb fitting that screws into the low speed exhaust in the leg and allows connection of a silicone rubber hose which can be lead outside the well. Getting proper stainless steel clamps for the silicone rubber is near impossible in Australia, use a nylon tie wrapped twice around the rubber hose which should clamp it adequately on the barb.
Alternatively drill some holes in the front of the engine cowl near the engine air inlet to increase the flow of fresh air to the engine away from the well. The engine cowl is designed for outboard mount where temporary submersion can occur, since it is in the well, a few extra holes would not be a problem.
I too have a problem such as you describe, the previous owner fitted a stainless peice of tube to the exhaust port which unfortunatly fell off!! Since then the problem has occured. (typical mechanic in found a way to clear the engine before moving off) I shall now seek a Yamaha barb and try to fit it to my Mercury