Serial: 2B117805
2015 Mercury Pro XS 250HP TM2

Problem: The motor will crank and start but then quickly die.

Background: Ran the boat all over the lake the day before with no issues, start and idle were perfect with no misses or hesitation.
The next morning I went out and boat ran great just like it always does and about mid-day. I was heading to a spot and when I came down off plane to an idle it very sharply died, almost immediate when boat got off plane with boat still in gear. I tried to start it and it would start quickly like normal, but then die immediately - this is the first time ANYTHING seemed wrong, so the problem occurred so quickly. I fished that spot for a while and then when time to go it would start and then die again. So I gave it a little gas and I could get it to start and stay running at higher RPMS, but when I left off the gas to idle it died again. So I had to crank with giving it gas and then quickly slam into gear to get it to go. Once it was running past idle it ran totally normal. It felt a little bogged down during launch, but then mid and top end were completely fine turning over 5500 RPMS running 70mph down the lake. I had to start it and throw it into gear about 3 more times throughout the day. Every time it would run totally normal if I could get it past idle.

Troubleshooting: I thought it sounded like it was not getting correct fuel/air ratio only at idle so maybe it was the one of the tracker, air, or fuel diaphragm valves. I took all three out and inspected them. the tracker valve looked ok, with a pin hole sized hole (took me a minute to see) only on one side. I have had a tracker give me issues in the past but the boat actually idled, just at rough idle. and the tracker valve was torn much worse then.. so i didnt think it was the tracker valve. the fuel diaphragm looked great with no rips or holes at all. The air diagram valve looked to be in bad shape with rips and tears i could see, plus I could see a oily mixture on the outside of the valve. I bought the $300 diaphragm from a local shop and installed (Yes I bought the the tool to compress the spring after failing to compress the spring with just longer screws). I put old diapragms back in tracker and fuel since they looked ok. put everything back together and tried to start motor and it would quickly fire and then rough VERY rough for 1-2 seconds and then die... so no change.

I have the little square smart craft, i guess its the SC1000. I have not gotten no beeps or anything during this whole time. and when I start the boat it gives the single beep when you turn the key to signal everything is normal.

I do the annual service as recommended on this form, however in January I had a dealer do this for me... so my next step is the check spark plugs to ensure the correct spark plugs were used (IZFR6J) by dealer and that the gapping is correct at 0.043.

I run "Dons Cocktail" in just about every fill up and consciously try to conduct proper warm up and shut down procedures before and after each crank.

I have called around to dealers for service and they are all several weeks out, bad time of year with everyone getting their boat serviced.

Have you seen a similar problem in the past? what was root cause?