2000 Carvr 396MY w/7.4MPI Merruisers

[johnsearay]

Have a 2000 Carver 396 MY with 7.4MPI mercruisers with only 276/296 hours on the engines.
On the original sea trial for purchase, she would only turn 3400 rpm's on each engine, (they were identical), found out the risers and manifolds were in terrible shape, so had them replaced, hoping that would help the rpms. No help at all to the rpms, exactly the same as before 3400, but engines are running nice and cool. Had the injectors removed and inspected and 8 of them were totally fouled, replaced them all, changed the fuel filters and added fresh fuel, (fuel was low during initial runs). This time I was expecting a big change in rpms as the engines started easier and sounded smoother, but absolutely no change to rpms at all, still tops out at 3400.
The bottom is clean, the running gear is clean, the props are original, (24x25 4 bladed nibrals), gears are original, (Hurth 800A with a ratio of 2.8:1. Compression is good on all cylinders and oil samples were good as well.
Fuel pressure is being checked next, but doesnt seem that the failure could be so identical for each engine?
The hull had a moisture reading taken, was normal and there is no water in the bilge anywhere.
She does have the single lever controls, which seem to have excessive play, but my tech says they go all the way to the stops on the engines.
The weirdest thing is that both are identical, its usually one engine or the other?
Does anyone have any idea what we could be missing here? At 3400 she will not get on a plane and just plows and plows and nothing has made any changes?
Any help would be greatly appreciated?

[bud37]

Have your tech check the total timing, not at idle but under load to see what is going on.....without the timing going ahead those engines won't pull the skin off a grape.....that way you can see where to go next as it crosses something off the list and only costs time....

[bud37]

Let me add that there are many other reasons for these things, generally I would consider the bottom and running gear cleanliness, bad fuel and throttle linkage which you seem to have covered. Both engines the same is interesting, thats for sure. Just check the timing thing, as under 300 hours in 16 years is low, also who's to say the props were ever correct,could be as simple as over propped , do you have feedback from previous owner??Well thats it for me....

[Viper]

Have they hooked up any diagnostic tool to check for fault codes and other data that might indicate a course of action? I agree with all the points bud37 brought up, there are several things that will cause this symptom but they should be doing proper diagnostics rather than throwing parts at it $$$.

[johnsearay]

There have been no diagnostics done on the engines, including the timing. Will check that asap and see if we can get a computer on it as well.
The props are correct per Carver specs for this boat, my first thought was over propped as well, but dont think that is it.
As for the previous owner, he thought that was normal, so its been this way for awhile, no luck with the owner before him.
I appreciate the help and will let you know what the computer says and what the timing looks like.
Still strange to me that both are identical, would think its props for sure.

[mjk1040]

Our 1998 Carver 355 AC/MY was over propped on purpose to keep the Crusader 7.4's from exceeding 4,400 rpms. Can you exceed 3,400 in neutral? You stated the risers and manifolds were bad and replaced, did something travel down the exhaust and you have a blockage? Mufflers collapsed or bad? A mystery for sure, especially both engines are doing the same thing.
Mike

[tomschauer]

It sure sounds like you have to much prop. Just it jump to 3400 or take awhile to get there? If it takes a bit to get there I would check the pitch and diameter before digging deep into the engines.

[johnsearay]

Both engines go straight to 3400, pretty easily and then thats it, no more. With no load, both engines will turn 5000rpm before the computer kicks in. Timing was checked it is perfect. Exhaust hoses were all checked and there is no blockage or restriction at all.
The single lever controls were checked and they both go to the stops on the engine.
This is driving my tech and everyone we keep asking nuts.
I appreciate all the feedback and will post any results that we get.

[bud37]

A question for you....did your tech check the total timing while under load ( trying to get on plane)??...any engine regardless of timing with no load will spin up to max rpm.Still both engines the same, quite a puzzler!!!
I do think that if timing was going to spec while under load you may be over propped as has been considered by a few. I can say that on a boat I had before the props had been messed with and did not match what was stamped on them when taken to the prop shop for a tune up/ balance etc.....explained a few things. Who knows what was done in the past 16 years, more pitch equals more speed at 1500 rpm maybe that was the trade off for running at hull speed , just throwing out some ideas here...some guys in Florida had divers every month...

[mjk1040]

My money's on props, as everything else seems to be technically checking out and you say in neutral the engines will reach 5grand. I'm thinking one of the previous owners over propped this boat for low rpm and better fuel economy. I'm speculating since I didn't note the number of hours on the engines, and if the previous owner traveled a lot he wanted to get the best bang for his gas buck! Pull them and find out for sure. Mike