4.3L Mercruiser Stalls Under Load

[enjoimark]

I have a 2005 Larson 180 with a 4.3L Mercruiser Alpha/Bravo carbed. The boat was was used as a rental on a marina since new. It was pulled out of the water in working condition, sat for 2-3 years, and given to me.

She starts right up with muffs on in the driveway. Idles fine, revs all the way up to 4k RPM fine. Put it in the water, cruise around 5mph or so just fine. Throttle up to try and plane, and she gets about halfway up and then bogs way down... if I keep the throttle in the same position, she'll die. If I move the throttle to neutral, stays running. We can't figure it out and before IO replace anything else I thought I'd come here to gather some opinions.

Here's everything I've done so far:

When I first got it, before the first try on the water:
Replaced all spark plugs
Removed fuel line from inlet on carb, checked fuel pump operation (pumped fuel)
Changed oil

First time we took it out, it started right up with no problems. Let it run for a bit, then went out. Once we got past the break, I opened her up about halfway, half planed, and did that for about a minute... then went ahead and went for WOT. It tried to get on plane but just before it did, I noticed the RPM's droppping a bit and then coming back up, then dropping again and coming back up... then all of a sudden she just bogged all the way down, died, and did not want to start. I opened the engine cover and noticed water had sprayed everywhere. Got towed in.

That turned out to be a cracked rubber boot on the right side exhaust. Checked to see if water got in the carb, didn't see anything. Replaced that boot, went back out the next weekend.

Same thing happened except now it ran pretty rough even at idle.

Drained fuel tank (it was full when I got it, 26 gallons I believe) then filled back up with new gas. The gas was almost bright orange... replaced Fuel Filter/Water Separator (there was water in the filter, about half). On second thought wished I would have done that at the beginning.

Took her back out, ran much better at idle and 5mph... but same thing happens when trying to go any faster but now she won't die if you take the throttle back down to idle.

Had carb rebuilt by a mechanic. Took her back out, ran even better at idle and 5mph (apparently what I thought was "running good" was not at all lol) and thought for sure that would fix it... same problem. Boat absolutely hates to go past 5-10mph and I've never gotten her to plane.

My next step was going to be the fuel pump... but it's in a screwed up spot and before I spent my afternoon cussing at that, figured I'd see what you fine people thought.

Thanks for the help!

 

[alldodge]

You drained all the fuel, but there could be some crud in the tank. Did you try and flush it out with something?

Need to check fuel pressure and also check the antisiphon valve on top the tank.

 

[enjoimark]

I wasn't sure what to try and flush it with. It is pretty hard to get to, and the top cover inside the boat only reveals the fill line and vent line. I can't see the actual fuel line (I believe)... I'll have to look again. I know they put one of the hose clamps on the fill line on upside down so I can't even get a tool on the thing to loosen it. When I siphoned out the fuel, I did it by removing the vent line.

What should the fuel pressure be? Does it change according to engine RPM and load?
I'm not real sure where the anti-siphon valve is, but I'll look for it. How do you test that valve?

 

[alldodge]

The pressure should be between 4 to 7 psi at all rpm's

The antisiphon valve is on top the tank and looks like a barbed fitting. Your rubber fuel line attaches to it. The valve has a ball and spring inside it, and the purpose is to keep fuel from siphoning if the fuel line brakes or leaks.

There are different sizes
https://www.iboats.com/shop/moeller-anti-siphon-valve-3-8-barb-3-8-npt.html

 

[Rick Stephens]

I wouldn't get too surprised if after everything else checks out, the carburetor still needs attention. Most common job that needs redoing if the rebuild wasn't done with a great deal of care and at least a little knowledge of what the goals are when rebuilding a carb. Every passageway needs to be cleaned and proven free of any debris or blockage. Float level set to tedious precision. Check balls and throttle pump booster replaced and passageways perfect. Details are everything, and often a quick once through doesn't accomplish anything.

 

[B Rad G]

I would also double check the fuel lines for cracks. Especially where they are clamped to fittings .

 

[enjoimark]

I wouldn't get too surprised if after everything else checks out, the carburetor still needs attention. Most common job that needs redoing if the rebuild wasn't done with a great deal of care and at least a little knowledge of what the goals are when rebuilding a carb. Every passageway needs to be cleaned and proven free of any debris or blockage. Float level set to tedious precision. Check balls and throttle pump booster replaced and passageways perfect. Details are everything, and often a quick once through doesn't accomplish anything.

Well that’s unfortunate to hear, as a boat mechanic did the work. They had no way of testing the boat in the water... so would this be something I mention? I don’t know jack about carbs, that’s why I took it to them. It wasn’t one of those “fix this boat no matter the cost” type of trips, I specifically said to rebuild the carb (and they also said that could be the issue, but it also could be something else). I took the carb to them so they didn’t/couldn’t check anything else.

 

[achris]

.... Throttle up to try and plane, and she gets about halfway up and then bogs way down... if I keep the throttle in the same position, she'll die. If I move the throttle to neutral, stays running....

This screams 'carb' (or at least fuel system).... every time...

Just to rule them out, I would also check the engine cylinder compressions, and the initial timing and make sure it's advancing properly...

Chris........

 

[Rick Stephens]

The tough part of fuel system problems is they generally don't limit themselves to one component, like the carb. Boats are expensive to maintain, and that bites. But it is a fact that can't be gotten around. If your fuel gets dirty, or gets some water in it, then things start happening, from anti-syphon valve getting plugged up, to fuel lines, filters and carburetors getting plugged and gooey. All it takes is a little water in the tank and a little time. Throw in gasoline with ethanol in it, and you have a huge mess that isn't fixed until all of it is cleaned up.Water and ethanol were made for each other and combine instantly. The boat sat for several years, the entire fuel system, most especially the carburetor, needs going through. That's why the boat was given away, if I had to guess. It costs a bunch to go through all of the fuel system on a boat. I've seen floors ripped up and the fuel tank removed to get it clean enough to use again reliably. Every fuel line and hose replaced, and the carburetor rebuilt or even replaced in worse cases. (I replaced my carburetor when it was allowed to hold rain water for a couple years by the previous owner, I lucked out on the fuel tank)

Anyway, the point is that you have to look at those things, take the fuel gauge sender out and inspect the inside of the fuel tank, look inside the carburetor bowl, see the inside of fuel lines, to know what you have that is good or bad. The chips will fall where they fall. If it is plugged up somewhere, it needs to be cleaned up and or replaced.

 

[enjoimark]

Ok. Sounds like I just need to take it in and have everything looked over. I don’t mind spending the money, I just hoped it was something fixed easily. Just my luck lol

The boat ran fine when it was taken out of the water, just replaced with a newer model. My father in law runs/consults a few marinas and it was one of those “it’s been sitting for a while, it’s probably gonna have a few issues, but here ya go get it out of the parking lot” type of deals. Other than this problem, it’s a really nice boat especially for free. I am lucky to have it. If it takes a few thousand to get it going again then so be it!

 

[achris]

...The boat ran fine when it was taken out of the water,...

Everything runs fine... until it doesn't.

I've had people bring me TVs that 'ran fine' until the smoke got out...

Ask a Jeep owner how his Jeep was running just before it caught fire and burned to the ground... It ran fine....

Just saying...

Chris.............

 

[enjoimark]

Everything runs fine... until it doesn't.

Well, yea... obviously lol

 

[wahlejim]

Have you tried running it on a remote tank? Do you have a fuel/water separator / Filter (if not, install one)? pour in a ziploc bag and let us know the contents.

You mentioned bright orange fuel, that was probably in everything. If you had the carb re-built and ran it again with bad fuel, time to at least clean it if not re-build it again. You mentioned you brought the carb to the mechanic. Did you clean out the filter at the carb?

 

[enjoimark]

Have you tried running it on a remote tank? Do you have a fuel/water separator / Filter (if not, install one)? pour in a ziploc bag and let us know the contents.

You mentioned bright orange fuel, that was probably in everything. If you had the carb re-built and ran it again with bad fuel, time to at least clean it if not re-build it again. You mentioned you brought the carb to the mechanic. Did you clean out the filter at the carb?

Have not tried a remote tank.

It has a fuel filter/water separator and I have changed it, and checked it's contents twice since. As the first post says, there was water in the old one. Hasn't been any since.

The carb was rebuilt about a week after the bad fuel was replaced with new fuel.

I personally didn't clean out the filter at the fuel inlet of the carb, but I know the rebuild kit included a new one and I was told the new one was installed.

 

[wahlejim]

I think the next step is the remote tank. It will isolate the problem. If it runs as it should, its a tank/line problem. If it still doesn't run properly, its carb/fuel pump problem.

 

[enjoimark]

Excuse my ignorance, but where in the world would I get a remote tank setup at?