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  1. #1

    Default 4.3L Mercruiser bogging down issue

    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!

  2. #2
    Join Date
    Dec 2006
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    Greenfield Me. USA
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    Default Re: 4.3L Mercruiser bogging down issue

    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,.....
    Any Grease is Better,... Than No grease at All....

  3. #3

    Default Re: 4.3L Mercruiser bogging down issue

    Quote Originally Posted by bondo View Post
    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,.....

    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?

  4. #4
    Join Date
    Dec 2006
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    Greenfield Me. USA
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    6,074

    Default Re: 4.3L Mercruiser bogging down issue

    Ayuh,..... I usually hook it to the filter, as on mine, it's hard piped to the fuel pump,.....
    Any Grease is Better,... Than No grease at All....

  5. #5
    Join Date
    May 2010
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    DC Burbs/Maryland Eastern Shore
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    Default Re: 4.3L Mercruiser bogging down issue

    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 by o2batsea; 07-15-2020 at 06:55 AM.
    Practicing the ancient art of ren-ching

  6. #6

    Default Re: 4.3L Mercruiser bogging down issue

    Quote Originally Posted by o2batsea View Post
    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.
    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.

  7. #7

    Default Re: 4.3L Mercruiser bogging down issue

    https://youtu.be/7slpQR6_O3A

    Here is the video I mentioned in last post.

  8. #8
    Join Date
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    Default Re: 4.3L Mercruiser bogging down issue

    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.
    Practicing the ancient art of ren-ching

  9. #9

    Default Re: 4.3L Mercruiser bogging down issue

    Quote Originally Posted by o2batsea View Post
    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.
    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.

  10. #10
    Join Date
    May 2010
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    Default Re: 4.3L Mercruiser bogging down issue

    One consideration on some of these older boats is that the fuel systems were not optimized for ethanol added fuel. Plastic float perfectly fine.
    Practicing the ancient art of ren-ching

  11. #11

    Default Re: 4.3L Mercruiser bogging down issue

    Quote Originally Posted by o2batsea View Post
    One consideration on some of these older boats is that the fuel systems were not optimized for ethanol added fuel. Plastic float perfectly fine.
    Sounds good, will update once carb kit shows up and I get it rebuilt.

  12. #12

    Default Re: 4.3L Mercruiser bogging down issue

    Carb is rebuilt and reinstalled. Heading to the water now....keep your fingers crossed!

  13. #13

    Default Re: 4.3L Mercruiser bogging down issue

    Quote Originally Posted by jeremym8 View Post
    Carb is rebuilt and reinstalled. Heading to the water now....keep your fingers crossed!
    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?

  14. #14

    Default Re: 4.3L Mercruiser bogging down issue

    Anybody with any ideas?

  15. #15
    Join Date
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    Default Re: 4.3L Mercruiser bogging down issue

    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,.....
    Any Grease is Better,... Than No grease at All....

  16. #16

    Default Re: 4.3L Mercruiser bogging down issue

    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

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