Mercruiser 4.3L - Stumbles at 3000RPM

[Maverick7]

2004 Mercruiser 4.3L
Thunderbolt V
Alpha 1 Gen 2
Engine S/N: 0m670495
Crownline 180BR

I took the boat out of storage today. It ran perfectly fine for about 30 minutes at varying rpms along our ride. After the first stop, we took off again and I normally like to cruise at about 3000rpm/22mph.

Every 10/15/20 seconds, the engine would "stumble". What I mean by this is that it would be running smooth, and for a split second it would drop 2-400 rpm, then return to 3000rpm and continue on. I think another way to explain it would be imagine driving down the interstate, and every so often the driver would tap the brakes just a little, and continue on.

The strange thing was that it only happens at 3000rpm. Increasing to 3400 and cruising showed no issues, slowing to 2500rpm showed no issue.

Its almost like at that rpm an air bubble is going through the gas line and slowing the engine just for a moment. Im sure this isn't the case, but thats just another way to explain the phenomenon.

As stated previously, this was the first time out of storage for the season. The boat was winterized as it always has been for the last 10 years by me. New oil, oil filter, fuel filter, drive oil, etc. I do store the boat full of premium, ethanol free fuel over the winter. I can't imagine it being a fuel issue as I have always done this, and never experienced this issue.

Thanks

 

[Scott Danforth]

how old is the fuel?

when was the fuel/water separator filter last serviced?

have you checked the content of the fuel filter for water?

when was the carb last cleaned.

it sounds like either water in the fuel, bad fuel, or dirty carb.

 

[Scott06]

To the above would also take a look at the ignition components cap rotor wires and what do the plugs look like?

I would also be leaning towards a carb cleaning - very similar symptoms to my neighbors 4.3 some times fine sometimes stumble- rebuilt the carb and good to go

 

[ESGWheel]

In addition to the above something else to consider is making sure the entire air pathway is clear and free to flow. Here is the thinking: an internal combustion engine is really an air pump. Problematic air flow dynamics can come into play at one RPM but not another. In a boat motor, that is held at a constant RPM like you are doing, its possible to set up a harmonic that may be ‘just at the edge of being disruptive’ and when conditions are just right, it kicks in, causes that disruption but the disruption itself causes its demise. For a carbureted engine the differential pressure across the carb is what drives the fuel flow. So a ‘burb’ in that air flow can cause a ‘burb’ in the fuel flow. While this thinking it a little out there, I have in the past played with tuned exhausts on small displacement engines and it’s amazing what can be done > flow and harmonics matter. On modern cars with all their plastic intakes, you may have seen what look like large square bubbles along the intake. These are intended to preclude disruptive harmonics and keep the air flowing steadily. See pic for an example.

This is a long-winded way of saying this > make sure your spark arrestor is thoroughly cleaned. And while not so easy to check on the exhaust side of the engine make sure there is no partial blockage like a flapper valve that may not be fully opening.

 

[Maverick7]

how old is the fuel?

when was the fuel/water separator filter last serviced?

have you checked the content of the fuel filter for water?

when was the carb last cleaned.

it sounds like either water in the fuel, bad fuel, or dirty carb.

The fuel was premium ethanol-free fuel, filled last year at winterization (October). Iv always filled the tank full before winter storage, never had an issue like this.

Fuel filter was changed during winterization (October).

Iv never actually cleaned the carb, never thought I had a reason to honestly. Iv seen sea foam videos of them just spraying seafoam into the carb while the engine is running, followed by shutting down the engine and letting it soak for 15 minutes, followed by starting the engine and running it. Does this seem like a viable option, or is removing and tearing down the carb the only way?

My other thought was to dump a can of seafoam into the fuel and see what happens.

 

[Maverick7]

To the above would also take a look at the ignition components cap rotor wires and what do the plugs look like?

I would also be leaning towards a carb cleaning - very similar symptoms to my neighbors 4.3 some times fine sometimes stumble- rebuilt the carb and good to go

Iv never actually cleaned the carb, never thought I had a reason to honestly. Iv seen sea foam videos of them just spraying seafoam into the carb while the engine is running, followed by shutting down the engine and letting it soak for 15 minutes, followed by starting the engine and running it. Does this seem like a viable option, or is removing and tearing down the carb the only way?

I did change the spark plugs just to see, pictured are the ones I pulled out.

 

[Scott Danforth]

Does this seem like a viable option, or is removing and tearing down the carb the only way?

My other thought was to dump a can of seafoam into the fuel and see what happens.

there is no mechanic in a can.

seafoam is just naptha. nothing special. wont hurt....... wont help...... will make your wallet lighter

 

[Maverick7]

there is no mechanic in a can.

seafoam is just naptha. nothing special. wont hurt....... wont help...... will make your wallet lighter

I just want to clarify something.

When I said i replaced the fuel filter, I was referring to the fuel filter/water separator (looks just like an oil filter). However, after some research I now see that there is a fuel filter/screen that is in the fuel line, where the fuel line connects to the carb. Is this something I should check, and if so how? Im assuming I just loosen the nut and the metal fuel line can come off. Then remove the nut that actually goes into the carb, and the little screen/filter rests inside there?

 

[ESGWheel]

Checking / replacing filters should be part of yearly maintenance including this one

If this filter was clogged you would have issues at more than just 3k

If u are not comfortable rebuilding the carb which, I think is the next step, you can look into getting a rebuilt or new carb as it’s easier to replace than rebuild and can then practice on old one - just a thought….

 

[Scott Danforth]

The filter at the carb will pass small rocks. It's about a 100 micron filter. The spin on one is 11 micron. Look at the water separating filter....looking for water

 

[ESGWheel]

Look at the water separating filter....looking for water

I would think that water in fuel would cause stumbling at any / various RPMs, no?

Since only at 3k and intermittent is why I came out of left field that something seems, well, out of balance at that specific RPM

Trying to continue to learn so I can help… thanks.

 

[ESGWheel]

I did change the spark plugs just to see, pictured are the ones I pulled out.

Forgot to mention > the plugs look fine.

 

[Maverick7]

Forgot to mention > the plugs look fine.

I cleaned the filter/screen in the carb, there wasn’t much in their.

However, when I tried running it tonight it idled perfectly fine, but giving it any throttle it immediately bogged down and then stumbled its way up to rpm’s. So when throttle was increased, the rpm’s would eventually increase but stumbled badly to get there.

I’m wondering if anybody thinks this could be a failing fuel pump issue? Or do we still think it’s a carb issue.

 

[ESGWheel]

Well that does change things. Was this on the muffs or in the water and underway (i.e. under a load)?

Do you own a fuel pressure gauge? If not time to buy one. Something like this: link That way you can put in line and monitor as part of the troubleshooting.

And also see Scott's post 10, please rule out water.

 

[Maverick7]

Well that does change things. Was this on the muffs or in the water and underway (i.e. under a load)?

Do you own a fuel pressure gauge? If not time to buy one. Something like this: link That way you can put in line and monitor as part of the troubleshooting.

And also see Scott's post 10, please rule out water.

Tonight was with muffs on. I ran it out of the water(muffs), and thats when it stumbled to climb rpm's with any amount of throttle.

Last night was on the water. It seems strange to me that this would be a carb issue as it ran fine on the water for over 30 minutes. It was then all of a sudden that the issue started. And now on muffs, the issue is so bad that it wont climb rpm's without stumbling. This gave me the thought of a failing fuel pump.

As far as water in the fuel goes, i did remove the fuel/water filter and dumped the contents into a clear jar. It showed only gasoline, I was not able to see any amount of water in the bottom of the jar, not sure if this was the correct way to check it or not, I assume it works.

 

[ESGWheel]

No water > good. And yes the correct process.

I was going with it only stubbles underway and only at 3k and only occasionally which was really odd. And yes, this does sound like a fuel issue, but where? While it’s possible to be the fuel pump, these mechanical devices typically outlast the engines. More likely some issue with another fuel delivery element, like the anti-siphon valve. But to know if a “fuel to the carb” delivery issue need to know what pressures you are seeing. And if all good then next item to tackle is “fuel to the engine” via the carb.

And it sounds like you take good care of it so let me ask when was the last time new dist cap, rotor, wires, etc.?

 

[Maverick7]

No water > good. And yes the correct process.

I was going with it only stubbles underway and only at 3k and only occasionally which was really odd. And yes, this does sound like a fuel issue, but where? While it’s possible to be the fuel pump, these mechanical devices typically outlast the engines. More likely some issue with another fuel delivery element, like the anti-siphon valve. But to know if a “fuel to the carb” delivery issue need to know what pressures you are seeing. And if all good then next item to tackle is “fuel to the engine” via the carb.

And it sounds like you take good care of it so let me ask when was the last time new dist cap, rotor, wires, etc.?

You were correct,at first it was only underway and only at 3000rpm. But now things have changed and it happens anytime except at idle, which it seems to run fine.

Also, I dont think I have a mechanical fuel pump if thats what you were referring to, its part number 861155A3.

I personally think I take much better care of it than most people do with their boats, which i guess isnt saying much, but I do maintain it meticulously, hence why any issues, especially debilitating ones, are incredibly frustrating.

Dist cap, rotor, wires where replaced in 2021. Probably have 50-60 hours on them. They seem good, i took them off today, didnt see anything crazy going on.

 

[ESGWheel]

Also, I dont think I have a mechanical fuel pump if thats what you were referring to, its part number 861155A3.

With the carb'ed engine i assumed. So yes those can go. Need pressure gauge.

 

[Maverick7]

With the carb'ed engine i assumed. So yes those can go. Need pressure gauge.

I can order the pressure gauge for $22, or I see that an OEM fuel pump is like $150, or $25 for some Chinese knockoff. I wonder if I just get the Chinese one for the same price as the inline fuel gauge. It should run for at least long enough for me to confirm or rule out the pump issue. That way I don't have to find a way to splice that gauge into the very short and mostly metal fuel line.

 

[nola mike]

Did you check to see if you're getting a stream from the accelerator pump when you stab the throttle?

And I'm confused about the symptoms. It now only hesitates on muffs?

Last, did the symptoms get immediately worse after changing the plugs?