2007 250 engine's stall - Fuel Starvation or Vapor Lock ?

Got a question for you guys - my brother is running (2) 2007 Yammi 250's with 300 hours and has been having the same issue for 2 years; either engine will suddenly die under way. The offending engine's priming ball will be totally sucked in. Not sure if it’s related but sometimes it happens after he refuels - can be either engine. Happens randomly.

Remedies tried
- New fuel lines from tank senders thru to engines including fuel priming balls
- All new fuel filters within each engine as well as water / fuel separator.
- Fuel tank polishing 2x ( after the first cleaning, an engine stalled and the guy came back did it again )
- Perko fuel filler with an integral breather vent in cap. This all seems to function - We checked the breather cap’s ability to breathe when closed - it does.

After 15 hours of use, it just happened again..

Engines are professionally maintained - the Yamaha guy thinks it's something in tank - he's at a loss...

Vapor Lock Theory
The fuel filler and the vent line are both located at the very back of the fuel tank which seems like a bad idea. I can see how with a 3/4 to full tank that the vent and fuel fill lines would be submerged under fuel while under way ( the gas would all be aft ). This could create a vapor lock as the fuel would be obstructing the ability of the vent to breathe. And, its possible that the engines fuel pump would suck the primer ball in, starving for fuel.

However, you would think that once the boat was stopped - the fuel level would normalize. And, when the fuel cap, which also houses the vent line breather were opened, the primer ball would fill back up - this is not the case. The primer ball stays sucked in. The only thing that remedy's this has been to wait and sometimes, infrequently if you turn off and re-start the other motor - the ball will normalize.

Fuel Starvation Theory
- something randomly clogging the pickups at different times even though the fuel tank has been polished twice
Water Fuel Separators filter size too small . Possibly switch the 10 micron water / fuel separator to a 20 micron one

Any ideas ?

ideally your fuel filter should sit between the bulb and the motor. The tanks need to have an independent breather, it sounds like it doesn't?

I see you replaced all fuel lines including the pick lines that run down into the tank. There isn't by any chance small fuel filters on the end of pick-up lines that sit in the bottom of the fuel tanks is there? Some manufacturers put those little filters on the ends of those pick-up lines and if someone were to replace the fuel lines they may stick those stupid little filters on them again. If they are there, pull them off or cut them out. I had a friend who chased a fuel starvation ghost for months that would come and go. It turned out to be those stupid little filters on the pick-up lines (that NEVER, ever should have been put there as there is no need for them). While underway they would gradually clog with particulate in the tank until they caused a fuel-starved shut-down. Then, once shut down with gas sloshing around for a while some of the the crap that clogged the filter would fall off and all would run well again for a while until it found its way back and created the same issue over and and over. Check for those stupid filters and get rid of them if they are there... There are plenty more, larger filters throughout the system that will deal with anything in the tank.

Thx for your input --

DeeJay -- The OEM set up is fuel tank... filters... fuel primer balls... engine... The tank has an independent vent linen from the tank that vents through a Perko, dual purpose vented gas cap. The fuel vent seems to function properly when the boat is not moving ( not sure who moving )

Deep -- Could not change the pick up lines - they're welded in place. However, a root rooter type device was used to clean each pick up and was driven through the bottom of the pick up ( so, no screen or filter ).

Hmmmm. I don't think your theory that a vent in the back of the tank could be submerged by fuel preventing it to vent is the issue. Having learned the hard way long ago with a 6 gallon portable tank that I filled and failed to open the vent on prior to running I created a vapor lock and the suction/vacuum that the engine created in the tank was unbelievable before it shut down.. When I discovered the issue and opened the vent I could hear the air rush in and the bottom of the metal tank almost popped like a can... If there's a vent regardless of being under fuel the suction created by the engine and fuel pump will easily sick air down through a vent line submerged in fuel and bubble up through the fuel.

That being said.. now I wonder if it the bulb could collapse and somehow seal off flow before there is enough vacuum in the tank to pull air down through the vent line submerged in fuel? Something is going on that is restricting fuel flow from time to time in front of those bulbs that keep being sucked in.

You have two fuel lines, two bulbs, two filters heading to two separate pick-up lines in the same main fuel tank right? If that's the case a vapor lock in the tank would effect both lines and fuel bulbs at the same time. If not, I kind of agree with the pro... there is something in the tank that is finding its way to one of those two pick up lines from time to time and clogging it but not completely clogging it as with time fuel seeps around it, gradually fills the void as the fuel bulb want to expand and once the vacuum is gone whatever got stuck there falls away in the sloshing fuel (until it happens again). I problem with the vent built into the common fuel fill cap would create the same issue with both engines and fuel bulbs at the same time.

It's not an optimal solution but I am wondering if you put some kind of a quick-disconnect or manual vent in each line between the fuel /wate filters and the tank when the issue presents again you could quickly eliminate the vacuum that developed releasing what "may" be sucked on the end of that pick-up line, (maybe back-blow through it) re-connect and go on your way. There's no way open that tank and really see what could be inside. Ahh the joys of boating.

Im glad it's my brother's boat ��
But, I'm the more mechanical of the two of us... I'm challenged to find the problem. I agree it's got to be a random thing in the tank. Nothing else makes sense. The tank baffles make it impossible to scope the tank and after 2 tank polishes, you'd think it would be clean...

What other options are there for cleaning the tank ? It's under the deck with only access plates so removing the tank is not an option.

on most boats the vent is at the bow end of the boat for this very reason. You may want to check the vent to make sure it has not formed rust on the screen and effectively closed it up. The cap likely helped the situation but the tank is set up to use the vent in it. Did you try to run the engine on a portable tanks with fresh gas when this happens and if so does it run or is the problem the same? you also should check the fuel injector pump filters external and internal. This should be done after every issue with possible gas contamination.