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4.3L Mercruiser bogging down issue

J

jeremym8

New member
Hello

Recently been working on a 1991 Bayliner Capri with a 4.3 Mercruiser engine. Boat sat for several years and was difficult to start and idled very rough. I drained the fuel tank, replaced fuel/water separator filter, soaked carb with seafoam spray and treated fuel with seafoam, filled with non-ethanol gas and was able to get engine to idle well and come up to temp on the muffs and in the lake.

However, on the lake, the boat will quickly come up onto plane with about 3/4 throttle and run great for a few minutes. Then after a few minutes it begins to bog down. Sometimes it will then regain power but quickly bog again (sometimes several times). Otherwise engine typically bogs to the point of stalling. This also happens if throttle is pushed past the 3/4 point and it will cause it to bog out quickly.

Engine typically starts right back up fine and is able to drive back to dock at low speeds and sounds fine. Also, decided to anchor out and let the boat sit for about an hour. After the hour, boat seemed to have full power again for about 5 minutes (again not past 3/4 throttle), but then exhibited the same bogging.

From other posts it seems the list of possible causes is endless between plugs, wires, distributor, fuel pump, carb, etc. Looking for some insight on where to start rather than just throwing parts at it.

Thanks in advance!
 
Ayuh,..... It's runnin' outa gas,..... ya gotta figure out Why,.....

Hook a remote tank to the fuel filter/ fuel pump, 'n try it under load,.....
If it runs good, the boat side of the fuel system is where the problem is,.....
If it still runs bad, the problem is on the motor side of the fuel system,.....
 
bondo said:
Ayuh,..... It's runnin' outa gas,..... ya gotta figure out Why,.....

Hook a remote tank to the fuel filter/ fuel pump, 'n try it under load,.....
If it runs good, the boat side of the fuel system is where the problem is,.....
If it still runs bad, the problem is on the motor side of the fuel system,.....
Click to expand...


Thanks for your quick response. Just to confirm, should I connect the fuel tank directly to the fuel pump or should I still run it through the fuel separator filter?
 
Ayuh,..... I usually hook it to the filter, as on mine, it's hard piped to the fuel pump,.....
 
A carb that's been sitting for unknown years? would def benefit from a rebuild as it seems that there is something keeping the fuel from flowing. Could be sticky needle? Hard to say but it's prolly inside the carb. Is that a task you would be able to do yourself?
On the bright side, it does start and run so it would indicate a case of insufficient fuel when you push on the loud pedal, and not ignition or timing.
 
Last edited:
o2batsea said:
A carb that's been sitting for unknown years? would def benefit from a rebuild as it seems that there is something keeping the fuel from flowing. Could be sticky needle? Hard to say but it's prolly inside the carb. Is that a task you would be able to do yourself?
On the bright side, it does start and run so it would indicate a case of insufficient fuel when you push on the loud pedal, and not ignition or timing.
Click to expand...
The boat has been in my family for a while so know about the last 10 years of history. It has been started a few times every season for the last 3 years or so but had not been in the water. Also, 4 seasons ago it was in the shop for a similar hesitation that only happened at WOT. So yes a carb rebuild has been something I also assumed may need to be done. I've not done it before but seems there are plenty of "how-tos" available. However, wasn't sure where to find the specs for my particular motor as far as some of the settings when reassembling. Also is an ultrasonic cleaner a requirement or just a nice to have for a rebuild?

Also wanted to provide an update as I took a look at the fuel system last night. Removed fuel/water separator and it was full of fuel and looked clean. I temporarily routed past it though and hooked the tank uptake hose directly to the fuel pump. With the engine off, moving the throttle to full position twice, I see 2 strong streams of fuel into the carb each time. Boat started right up but after running for a bit the inside of the carb seemed dry. Is that normal? If I added some throttle I would eventually see small amounts of fuel at the bottom but it seemed very minimal. I also took a video of this but not sure if it is very easy to see.
 
My personal preference is the Gumout carb cleaner that comes in the one gallon tin with a little metal basket included for the small parts. This along with one or two cans of compressed air (keyboard duster) from Staples is about it along with paper towels.
I pull it apart as much as I can minus the throttle plates, and dunk all that stuff in the carb cleaner pot for an hour or so. Then you run the basket under hot water to remove the glop and dry the individual parts with the canned air (don't let anything fly off into a black hole in the universe!). You'll reuse almost everything, so no worries on the jets and emulsion tubes and such. Just be uber careful when removing the soft brass parts.
 
o2batsea said:
My personal preference is the Gumout carb cleaner that comes in the one gallon tin with a little metal basket included for the small parts. This along with one or two cans of compressed air (keyboard duster) from Staples is about it along with paper towels.
I pull it apart as much as I can minus the throttle plates, and dunk all that stuff in the carb cleaner pot for an hour or so. Then you run the basket under hot water to remove the glop and dry the individual parts with the canned air (don't let anything fly off into a black hole in the universe!). You'll reuse almost everything, so no worries on the jets and emulsion tubes and such. Just be uber careful when removing the soft brass parts.
Click to expand...

Rebuild kit was ordered today and is on the way. I've seen some recommendations to replace the float with a brass one but they seem to be hard to come by. Should the original be ok as long as its not full of fuel? Also, any thoughts on the video I posted? Does it seem normal? Debating on also replacing the fuel pump at the same time as the carb rebuild to rule out all fuel delivery issues.
 
One consideration on some of these older boats is that the fuel systems were not optimized for ethanol added fuel. Plastic float perfectly fine.
 
jeremym8 said:
Carb is rebuilt and reinstalled. Heading to the water now....keep your fingers crossed!
Click to expand...

Well no luck....Idle was great but had the same lack of power and cutting in and out when attempting to get up on plane. The few times it did get up on plane it ran a bit better once the engine was not working as hard but still was lacking power and bogging down. Don't have an easy way to test fuel pressure with the flared hard line from fuel pump to carb. Would it seem logical to replace fuel pump next and then test?

Also, noticed the motor did not want to go into neutral anymore. This was never an issue before...could I have messed something up during the carb removal/reinstall? Any adjustments for this I can do?
 
Ayuh,..... It's still starvin' for gas,......

Look, 'n see if the shift interrupt switch is still hooked up,.... If unhooked, it won't come outa gear,.....
 
Replaced fuel pump and took out last week. Same story, ran great then started chugging again.

Going to try and get a remote fuel tank and test that out. In the meantime, the only thing I haven't checked on the the fuel side is inside the tank. Should there be a pickup tube or hose in there that could have gone bad?

Thanks
 
I'm having the exact same problem. Replaced the fuel pump, alternator, ignition amplifier. Fuel water separator. Wondering if the fuel tank vent line is plugged up. I'm at my wits end on this.
 
nitrolouie2142 said:
I'm having the exact same problem. Replaced the fuel pump, alternator, ignition amplifier. Fuel water separator. Wondering if the fuel tank vent line is plugged up. I'm at my wits end on this.
Click to expand...

Yeah I've replaced all that as well except for ignition amplifier. I've blown air through my vent line and it seems to be clear. Also opened fuel tank and looked at pick up tube...screen was clear and tube looked in good shape. Ordered plugs, wires and cap/rotor since it can't hurt but still puzzled by what seems to be fuel issue. Working on acquiring remote tank to hook direct to fuel pump.
 
Have you checked the anti-siphon valve to see if it's collecting debris as fuel passes through it? You should find the valve located at the top of the fuel tank, typically it's screwed into an elbow at the exterior end of the tank's fuel pickup tube, and the fuel line will be connected to it.

This is an example of what you're looking for: https://www.marineengine.com/boat-accessories/fuel/033802-10
 
Checked the anti siphon valve, and the vent line over the weekend, looks fine. I was told there's an oil pressure switch that could be going bad. Is there one on this engine, could it be the issue? Pulling what's left of my hair out chasing this issue.
 
Am i looking for debris in the anti-siphon valve? Also, decided a tune up couldn't hurt given these strange issues so replaced plugs, wires, cap and rotor. Rotor was seized on pretty good with quite a bit of corrosion under the cap. Looked like previous cap was installed with no gasket (not sure if thats normal). Anyways, had 1 plug break apart on removal. Any chance that could cause my intermittent issue? Also noticed the sensor in the distributor cap is a bit rusty. From my research it seems like the engine would always run terrible or not at all if that was an issue but wanted to get your guys' opinion.
 
jeremym8 said:
Am i looking for debris in the anti-siphon valve? Also, decided a tune up couldn't hurt given these strange issues so replaced plugs, wires, cap and rotor. Rotor was seized on pretty good with quite a bit of corrosion under the cap. Looked like previous cap was installed with no gasket (not sure if thats normal). Anyways, had 1 plug break apart on removal. Any chance that could cause my intermittent issue? Also noticed the sensor in the distributor cap is a bit rusty. From my research it seems like the engine would always run terrible or not at all if that was an issue but wanted to get your guys' opinion.
Click to expand...

Did you run with a portable tank as suggested early on in the thread? I haven't seen your reply/result on that. I had this problem with my 95 350 Mag Tournament Ski. The boat tank was a big part of the problem. I don't care what the tank looks like, it's not likely you can see everything going on in there. Try the portable tank test and tell us what happens.
 
Did procure a remote fuel tank and went to the lake to test yesterday. However, wanted to see if my tune up (plugs, wires, cap, and rotor) had any impact on the bogging issue. The boat ran perfect...so never ended up testing on the remote fuel tank.

In the end, what seemed like a fuel issue ended up being a spark issue. Thought about doing the tune up from the beginning but wanted to be methodical and only change 1 thing at a time so I could find the true issue...would have been nice to have had it fixed 2 months ago but happy it seems to be working great now!
 
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