Cummins Marine Diesel Repower Specialists › Forums › Cummins Marine Engines › 4BTA-250 lagging behind

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  • July 4, 2019 at 5:21 am #74687

    Mark Tripi
    Participant
    Vessel Name: Keeper
    Engines: Cummins 4BTA
    Location: Long Island NY
    Country: USA

    I have twin 4BTAs in my boat and the port engine has always had a lagging behind issue, that I was able to correct somewhat by using the star wheel to reduce spring tension in the aneroid fuel control. But it also has been a weaker engine, seeming to drop more rpm when a single engine was put into gear. Idle speed is set evenly between both engines, I believe the problem may be in the pre-boost fuel control screw but I can not find any info on this other than online info about trucks modifications to 6Bs. All of this by weekend warriors that want to “roll coal” it seems. I would like to adjust that screw and see if it is the problem but don’t want to destroy anything either. Any ideas?

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  • July 18, 2023 at 10:42 am #151772

    Mark Tripi
    Participant
    Vessel Name: Keeper
    Engines: Cummins 4BTA
    Location: Long Island NY
    Country: USA

    I did

    Tom I did get to the bottom of it. I hooked up boost gauges for both engines and my port boost was way down. I put on a new turbo and all was great. I sold the boat last fall and am about to sail a 36 Tiara Open down from The Saint Lawrence River via the Erie Canal. Twin QSB 425s in her.Ā 

    July 3, 2023 at 4:52 am #150995

    Tom Jerome
    Participant
    Vessel Name: The Kraken
    Engines: Twin 99' 4BTA 250hp
    Location: chesapeake, va
    Country: usa

    Same issue

    Hey mark did you ever get this figured out? My port has been the same and we have the same boat. I saw a guy post here that he had the p-pumpĀ  rebuilt but also had a low rpm issue. Ive beebeen chasing that lag for awhile too. Any info would be appreciated.

    July 5, 2019 at 10:25 am #74735

    Mark Tripi
    Participant
    Vessel Name: Keeper
    Engines: Cummins 4BTA
    Location: Long Island NY
    Country: USA

    No black smoke, tachs indicate 3100rpm full tanks and 3 people, actual photo tach was 3200. Star wheel is adjusted out about as far as it can be. Port engine still lags, like it has done for the past 18 years or so I have owned the boat. I have been searching the net lately and saw some info on the p pump. The screw I am talking about is under a tin cover secured by two screws and is located at the rear of the AFC. From what I have read it can be backed off to help with throttle response. Go too far and you have a ton of smoke.

    July 4, 2019 at 6:02 pm #74704

    Tony Athens
    Moderator
    Vessel Name: Local Banks
    Engines: QSB 6.7 550 HP
    Location: Oxnard, CA
    Country: USA

    4BTA 3.9 250's

    What is a pre-boost control screw?

    Any black smoke during any parts of your operation ?

    Do your engines spin up easily to an accurate 3100 RPM when loaded for bear?

    Tony

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