Don't despair just yet, i chased this problem for a long time on my motor. I've replaced all of the exhaust covers, pressure tested the assembled block and done lower seals a number of times along with replacing the lower bearing carrier and i still get a clean bottom spark plug. see :-
viewtopic.php?f=7&t=5199&hilit=mercury&start=30 from some years ago.
The motor itself runs well so now i just give it a good dose of fogging oil when i get home and leave it at that. I think with mine its that the boat is quite tail heavy so at rest the motor is low in the water and at tick-over the exhaust pressure is not enough to push the water all the way down the inside of the leg so i end up with splashes coming back up. Your winner also carries the motor quite low so could be the same issue. I think you need to go after the cause of your stalling into gear first, get that sorted then see how it runs and give it a good dose of fogging in the bottom carb and plug after a trip out just to keep safe.

There is a possibility,there are two seals with the lips facing each other,if the bottom one,to keep water out,is accidentally installed the wrong way ,water will work it's way into the crankcase.The other is to keep the oil in the crankcase.

haventaclue wrote:There is a possibility,there are two seals with the lips facing each other,if the bottom one,to keep water out,is accidentally installed the wrong way ,water will work it's way into the crankcase.The other is to keep the oil in the crankcase.

This is very true it appears to be a common error that i have also made in the past, when i bought a good used bottom bearing carrier from the US it came with seals incorrectly fitted.