engine stalls at any rpm shortly after start

‘04 185 Sport with 4.3 carb tks, serial 0W011342. Boat only has ~150 hours on it.

Hoping somebody can help me with diagnosis and has seen this before.

At the end of the last season, engine stalled on me at full song. Cranked but no start for a few minutes without stalling almost immediately. Eventually i got the motor fired and I limped back to dock at idle. Anything more would stall engine.

current symptoms:

Engine starts almost immediately. Runs maybe 20-30s at idle, stalls. The higher the rpm, the faster the engine stalls. When the engine runs, it does run very smoothly.

Feels like the pump initially gets some gas into carb and then eventually the engine consumes more than it gets and stalls?

Not a carb engine guy, I grew up with various kinds of mfi/efi.

things checked:

Fuel in tank is ok (clean, stabilized and not separated, no water)

I’ve swapped fuel filter several times, no difference. No indication of dirt or water.

Pump runs, don’t have correct adapter for my fuel pressure gauge to check pressure.

cleaned carb barrels and anything I could reach without disassembly from top with carb cleaner, looked pretty clean beforehand, even more so now. Squirting carb cleaner into vent slot(?) at the top made the engine run for quite a while, but I’m now wondering if that might’ve been just on carb cleaner or mostly on carb cleaner.

Flushed out flame arrestor with brake cleaner. Pretty filthy, weighed about half of before cleaning.

questions:
I suppose a fuel pump can get weak rather than die outright. But is that a common failure mode for these low pressure pumps?
If pump is bad, where can I source a reliable pump that doesn’t quite cost what mercury wants for it. Model, brand, source. What is a reasonable price?
Does it make sense to remove fuel line from carb and check the filter basket? I suppose a pump could spit crap out in failure mode?
What else should I be looking at? Do I need to pull carb apart? Not experienced with carbs, so, would love to avoid that if possible. Or is this non issue and straight forward rebuild? Can I clean the insides without having rebuild kit on hand?

Please help. Any thoughts on this would be much appreciated.

thanks,
Christian

PS: I have the SELOC manual for this engine but I really would like to get my hands on the correct mercury shop manuals for this. Which one(s) do I need and from what yr? Where from?

Looking at description you provided, it is a '04 with only 150 hours? is it a 2bbl or a 4bbl carb and what make is it? Rochester, Carter, Holley etc. I would be that a 18 year old boat with 150 hours on it needs a lot of upgrades / replacements on the fuel system, just due to age and lack of use.

What type of gas you using? standard pump gas for cars or marina type non-ethenol gas? Ethenol can effect fuel system hoses go South, fuel pump diaphragms and valves dry out and fail and the carb gaskets dry out and start leaking. THere are a number of replacements fuel pumps without the crazy price tag. Carter, Airtex, Holley, CV and Sierra, to name a few.

Start by getting your hands on a Mercruiser shop manual for your engine (I have only small and big block V-8 manuals on hand). Someone here may have one or they can be found on the net. I have the Alpha Gen 2 drive manual if you want it.

Thanks, yes, 150 hours, previous owner garaged and babied this boat until it became ours 2.5 yrs ago. 2bbl and looks like Mercarb(?) when comparing the serial # to parts diagram.


I pulled the strainer basket (4) out of the carb fuel port. Perfectly clean, no staining. No varnish on spring (5) or anything else I could see. I expected the o-ring (28) under the cap (6) to show some kind of aging, but there's nothing, looks fine. The fuel line to the carb looks new (certainly not 18 yrs old) and factory.

When I removed the nut holding the hard line to the cap (6), there was no fuel present. I was prepared to catch leaks and nothing. Is that expected? I'm assuming the latent engine heat would encourage this to evaporate? Or should that be sealed? What does the spring (5) control exactly?

Some more investigation. Can the low oil pressure switch cause this behavior with the fuel pump shutting off after start if the pressure doesn't build? Fits the pattern. I guess i'm going to climb back in the boat.. I hear the fuel pump running initially before cranking the engine (with the engine cover out of the way, inaudible otherwise), which makes me think the pump is ok. Is that reasoning sound?

Thanks,
Christian

When removing the nut, there should be some fuel present. ‘Makes me think the pump is on its way out. #5 spring holds the filter against the inlet nut. Without that, dirt can merely flow out around the filter and into the carb.
You may be onto something with the low oil pressure switch, so don’t discount it just yet. However, the electric fuel pumps are usually the first to go.

Fixed! (I think) Found a local source for the low oil pressure switch (87-864252A01) and replaced the switch just now and test ran the engine. Running fabulously! Let's hope this works at the dock as well as it does in my driveway!

To recap, what is happening is that the fuel pump is turned on with the starter solenoid and pressurizes fuel for the carb to prime. As soon as the engine is started / no longer starting, the low oil pressure switch should close as the engine is making oil pressure. In my case, this switch failed open and that means the starting procedure energizes the pump and then when the low oil pressure switch is supposed to take over, the pumps stops and the carb is is drained after 15-30 seconds of running and stalls. That's why I had repeatable run times at idle or faster rpms which all ended in a stall. And I also know now that if you squirt enough carb cleaner into the bowl, you can run the engine for a while on that alone. *grin*

I think the reason there wasn't any fuel coming my way when I pulled the inlet nut is because everything was sucked dry when the engine stalled. And who knows, maybe evaporated with latent engine heat.

Hopefully this can help somebody else in the future.

Glad you found it.
My bad on thinking, for some reason, it was a mechanical fuel pump, instead of electric.

No worries. You're excused.