Cummins 6CTA engines down on speed

[inmotion]

Hi All i have a Carver 530 Flybridge with a pair of Cummins 6CTA engines that have been totally rebuilt .. I was seeing speeds of 21 knots at full throttle with vertualy full fuel tanks and a clean bottom… Took the boat out last week and with 1/4 fuel in both tanks and still clean bottom i am only seeing max speed of 15.8 knots at 2300RPMs… Carver state boat should max RPMs at 2650and top speed should see 24.1 knots.. So what am i missing… I have recently changed air cleaners.. all fuel filters… where should i start looking … could filters be restrictive… can injection pumps be bumped up somehow to improve Max RPMs leading to the missing 7-8 knots… any advice would be great… Regards Paul

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[Rob Schepis]

Have you compared speed at a given rpm now vs then?

This might seem like semantics to you but to say you are down on speed is wrong, that would suggest you are making rpms but lost a few knots.  What you have is a power/rpms issue:

Understanding Low Power Troubleshooting

[Steve Lewis]

Paul, I always try to start simple and work my way up the complexity levels on any issue.  Have you checked the linkages of the throttle system to make sure you are actually getting full deflection?  This COULD be as simple as a linkage issue.  Are you seeing dark smoke out of the exhaust? Check out Tony’s Tips and click on the “Learning your Cummins Marine 6CTA 8.3 engine” article.  Look at pictures 5, 7 and 15. In picture 5 you will see the throttle lever behind the white tag and then you get a clear albeit skewed view of it in picture 7.  Make sure you get full deflection of the lever as this could just need a cable adjustment.  Could be the throttle stop has come out a bit and that is very clearly shown in picture 15.   These are easy things to look at and rule out before you start looking at the more complex items.  Tony recommends having vacuum gauges on the fuel filter so you can see what is going on with fuel supply at the engine.  You would see restriction of fuel supplied to the engine there and can work back to find where the restriction is……..  If there is any. Any chance you have a fouled shaft or prop?  a line or something wrapped around things?  Any vibrations?  Let us know more info about what is going on with the boat when you have it running and it is only making 2300.  Vibrations, sounds, smoke and the like.  

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