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Cummins 4BTA lags

Old 07-04-2019, 06:23 PM
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Default Cummins 4BTA lags

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?
Old 07-04-2019, 07:19 PM
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A $20 subscription to boat diesel .com is the best money you will spend, there is a wealth of information over there Good luck.
Old 07-04-2019, 07:29 PM
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The "aneroid fuel control" is what controls pre boost fuel .
Old 07-04-2019, 07:45 PM
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Have you thought about checking the boost psi and comparing engines? A bit of corrosion in the exhaust housing can cause low boost even though everything seems in good shape.
Old 07-04-2019, 08:36 PM
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Originally Posted by prowlersfish View Post
The "aneroid fuel control" is what controls pre boost fuel .
The AFC has more than one possible adjustment in it. There is the star wheel that can adjust boost fuel delivery, and the pre boost fuel control screw. I am looking to see if anyone with marine engines has touched that one.
Old 07-04-2019, 08:39 PM
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Originally Posted by Aliboy View Post
Have you thought about checking the boost psi and comparing engines? A bit of corrosion in the exhaust housing can cause low boost even though everything seems in good shape.
Both engines have been hooked up to gauges and tested. All within specs.
Old 07-05-2019, 04:57 AM
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Originally Posted by boatmanmark View Post
The AFC has more than one possible adjustment in it. There is the star wheel that can adjust boost fuel delivery, and the pre boost fuel control screw. I am looking to see if anyone with marine engines has touched that one.
Bosch Pump inline pump??


Old 07-05-2019, 07:44 AM
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Originally Posted by prowlersfish View Post
Bosch Pump inline pump??
Yes, "P" pump
Old 07-05-2019, 07:26 PM
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I would stay away from the hot rod ticks . Are you getting any black smoke while it lags ? If not I would adjust the star wheel some more . What are your WOT rpms ? 250 HP ?
Old 07-05-2019, 08:29 PM
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I would return all the fuel pump settings to factory spec, then have a real good look at fuel supply, and exhaust/turbo system.
there is also fuel pump timing to check, as well as exhaust temps at each port.
Simple injector crack test too.
check out sbmar.com
Old 07-05-2019, 09:35 PM
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Originally Posted by prowlersfish View Post
I would stay away from the hot rod ticks . Are you getting any black smoke while it lags ? If not I would adjust the star wheel some more . What are your WOT rpms ? 250 HP ?
WOT is 3200, no smoke. Star wheel is about as far as it can go. yes, 250 hp.
Old 07-05-2019, 09:38 PM
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Originally Posted by nuffaredy View Post
I would return all the fuel pump settings to factory spec, then have a real good look at fuel supply, and exhaust/turbo system.
there is also fuel pump timing to check, as well as exhaust temps at each port.
Simple injector crack test too.
check out sbmar.com
All systems gone over this winter. Pumps done, timing set to marks. Since power comes on late in the rpm range, looking at all the info I can find it would appear the aft is not set right for pre boost fuel delivery.
Old 07-06-2019, 05:11 AM
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Originally Posted by boatmanmark View Post
WOT is 3200, no smoke. Star wheel is about as far as it can go. yes, 250 hp.
3200 in gear ? If the star wheel is as as far as it goes and no smoke ( assuming you went the right way I am betting you did) make sure the air tube to the AFC is not restricted or leaking . You could have a bad Diaphragm .
Old 07-06-2019, 07:58 AM
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Originally Posted by prowlersfish View Post
3200 in gear ? If the star wheel is as as far as it goes and no smoke ( assuming you went the right way I am betting you did) make sure the air tube to the AFC is not restricted or leaking . You could have a bad Diaphragm .
Thanks Paul. Very little info around on these, I will dig into it soon. I did soap test the tube and it was good, no air leaking.

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