671 TIB RPM problem

[TedIII]

I have a 1988 38 ft. conv. with 671's rated for 485 HP. During a survey there was one bad injector on each engine so prior owner had all injectors replaced. It originally had M110 injectors and they were replaced with 7255's per Detroit. The boat used to be able to get 2400 RPM WOT with the old injectors. Now I cant get past 2000 RPM and cant get up plane. Rack is adjusted properly, linkage and cable move all the way, and I dont think there are any fuel restrictions. At 2000 RPM it does not blow any black smoke and there is more movement left throttles in the before it hits the stops. Any ideas?

[Boatsb]

Is the bottom a mess? That can loose you 500 RPM's

Also who checked the engine? Maybe there is something the mechanic missed or made a mistake with.

[TedIII]

The bottom is clean as far as marine growth but it does have many layers of old paint including the coat I put on in the spring. Soda blasting is on the list for this winter. The boatyard says the checked all adjustments after the new injectors were installed. Possibly its yhe governor causing the problem.

[Boatsb]

See if they have a relationship with a DD expert mechanic who will go over it again.

Over the years we have all had problems but the good ones will call in the expert for a second opinion before it gets to late.

Was the Governor adjusted? Could there be something preventing the rack from opening all the way? I have heard of things being missed by really good mechanics that the next guy finds.

[TedIII]

That's one thing I am trying to do now. I even called Covington trying to solve this problem and they told me the same thing. Finding an "old school" mechanic who knows DD engines is harder than I thought.

[Boatsb]

Where is the boat? Someone here probably knows a good detroit guy in the area.

[TedIII]

It's in New Bedford, MA and Fairhaven Shipyard in Fairhaven, MA installed the injectors.

[MikeP]

Do a no-load WOT test. That will tell you if it's a governor problem. (I'm betting it's not.) If the engine can hit the specified RPM then the governor is doing its job. A no-load test is the first step in engine troubleshooting.

Odds are that if the boat used to perform a certain way and now no longer does, AND the bottom/running gear is clean, it's the result of whatever was changed...

As noted, dirty running gear can easily cause the problem - our boat would not exceed 1800 RPM and 10.5 knots with barnacled shafts/props. Dove boat, cleaned shafts/props and the next day it turned 2300 RPM and 17+ knots.

[Boatsb]

It's in New Bedford, MA and Fairhaven Shipyard in Fairhaven, MA installed the injectors.

years ago when I was working on Long island ( the 80's) I used Mac Boring for all the detroit stuff. They may still have the guys on staff but It's been a long time.

[TedIII]

I can get 2500 RPM no load. Could the many layers of bottom paint cause that much drag? It has spots where paint has flaked out and causes an uneven surface. There is no growth and the problem has been there since I first put the boat in the water and everything was growth free.