Started up the boat for the first time after the winter with muffs at home. Fired right and ran on idle for quite some time to make sure all was good. Went out to the lake to fish and whenever I idle for an extended period of time the boat will just shut down. Go to check the primer bulb and it's soft again, to the extent its the same as if I haven't used it for a few days. To get it started back up I have to go through the startup procedures again. Prime, choke, turns right over. I've changed the fuel line, primer bulb hasn't helped. Not sure what would cause this. 06 Merc 50hp 2stroke

Fuel Pump

Let me jump in on this one if you don't mind. I have an '06, 200HPDI that has just recently been doing most of the same thing. New powerhead last February and has been running fine. I don't get a ton of time on the water, but put some hours on it in March. Followed the break in instructions to the T!! Last 2-3 times I have gone out it dies while idle. Shift into gear and it runs fine. Recently done service, new belt, new VST filter/seal, high pressure filter, main fuel filter, both low pressure pumps, plugs and even changed the primer ball. Ball never went soft though, just did it because. Fires up like a champ, then dies after about 5-8 seconds. I have not been fully on the water yet since all the changes, just sat the prop in the water. Is there maybe a time frame for all that gas to cycle through all the parts??

In all the research I have done, I did read ONE post that says he had a blown cylinder.!!! Hoping that isn't the case since its a new powerhead.

water....

My Mercury 50Hp two stroke does something similar from time to time; especially if the battery has low charge.
Battery and fuel pump would be at the top of my list.
If the battery is more than 3 years old, replace it.
Wouldn’t hurt to change the spark plugs and filters while you are at it.
Keeping a battery maintainer on my starting battery when the boat is in storage has helped the most instead of depending on the motor to recharge the battery.

"Water"...... that really says a lot. Thanks for the insight!!

IF you knew very much about fuel tanks, you wouldn't need another word. But, I'll offer some education.

Scenario: Fuel tank filled to 20% capacity as an example. That means the fuel tank contains 80% air. Air contains a varying degree of moisture. The fuel tank is not sealed; there is a vent, otherwise it would vacuum-lock as you burned fuel. Gasoline has a high coefficient of expansion (due to temperature). During the day, especially a hot one, the gasoline expands, and air is expelled from the tank. At night, it contracts, drawing in cooler air (that's often moisture-laden) via the vent.

The moisture condenses on the sides of the tank, then runs down the side as droplets. Water is heavier, so it goes to the bottom of the tank, where it is trapped under the layer of gasoline above. Once there, it can never "evaporate back out." This cycle continues over an over, with any added moisture accumulating as a trapped substance. Given conditions bad enough, you could theoretically end up with more water than gasoline over time! Some people store a boat for long periods of time with the tank mostly empty, setting up this disaster. When the boat is bouncing around, the water and fuel become combined. When more calm, more water settles and the fuel system pickup sucks up more of this water that has settled. Water doesn't burn very well, just in case you weren't aware.

The issue is best avoided by filling the tank to FULL when a long storage period is expected. The gas should be properly treated with a fuel preservation additive to avoid other storage issues, but the simple act of filling the tank will avoid the water issue.

The OP should pull a quart of fuel via the existing fuel line and put it into a glass or suitable plastic bottle to let it settle. If water is present, it will become obvious by visual observation after a few hours.

So kids, check for water in the fuel before you do anything else, because why? Its free.

Pilots do it before every flight with the same attitude. Not only can it condense in the tank, it can be introduced when refueling. Fuel with any ethanol additive is even worse in accumulating water because the alcohol attracts and absorbs water - just great for an internal combustion engine, right?

Compression.... that was my answer.

Mine too! I have on 06 Opti 150. Scattered my powerhead 2 years ago when I got water intrusion in my oil tank. Put new head on and first 5-6 hours, everything was fine. Then it started wanting to die at idle. Within a few trips it would barely idle more than 3-4 seconds, but ran strong hole shot and top end. Took it back and initial compression check was fine. They then did a leak down and he said 4 of the six were bad. Luckily I was still in warranty and got another head. I have about 12 hours on this one and so far so good.

Was a method of water contamination ever identified? I run a 2-stroke with a tank, and prefer to avoid whatever happened in your case.

Drain the sumps. On Avgas aircraft engines that is a pilots required task as the water will be near or at the sump drain. Nearly 60 years of flying has kept that buried in my feeble brain. Jet Fuel presents a different issue.

The problem is even worse for ground-use fuel because of the added ethanol, which attracts moisture! I was always careful, but especially when buying fuel when away from home. I'd take a break & let the tanks settle for a valid fuel sampling to be sure I didn't buy water!

check all fuel connections and make sure you have zero air leaks, took me about 2 months to track down the same problem with a 175 fast strike.
went from the click on fuel hose to direct to the engine. connector would not stay tight for some reason and was letting air into the fuel line at the engine connection.