More throttle, less speed/rpm

[Smokin Fish]

I have a 2002 396 that I recently purchased and, as I understand, hasn't been run very much in the past two years.

I had the engines inspected by Cummins, oil changed, etc. Both starboard and port checked out and ran like monsters.

When I first took her out, I could easily do 3000 rpm and make 20 knots. The throttles were uneven when synced, but that didn't bother me too much at the time. Not to mention, the sticks were only at about 3/4 throttle. I had plenty more room to go.

As a few weeks went by, I noticed I had to push the throttles more and more forward to get more speed. Pretty soon, I had the "balls to the walls" and I was only getting 2500 rpm and making only 16 knots. And now the sticks are even when synced.

My marina mechanic thought for sure it was fuel filters (I thought it that was a good idea as she hadn't been run or cared for for two years) He changed the filters and took her out and experienced the same problem... but didn't know what else to do (great mechanic, aye?)

Anyone have experience with this problem or have an idea where to start looking for the problem?

[tomschauer]

Are you in the gulf? If so it doesn't take long to grow a bunch of stuff on the bottom. You would be amazed at how much speed that will rob.
It can happen inland also, but a bit slower to develop the growth.

[Smokin Fish]

No - fresh water. Even if the bottom was fouled, I should still be able to produce 3000 + rpm.

Thanks though for your input. I appreciate it!

[bud37]

Qr Bbpost Even if the bottom was fouled, I should still be able to produce 3000 + rpm.

Not necessarily........a fouled bottom and running gear will slow the boat down there fore not allowing the engines to make their rated rpm. There is not much in the way of prop slip so the slower you are going ( speed wise ) due to fouling the less rpm the engines make due to overloading.

This may not be your case here since the fresh water but still something to consider.

[Vagabond302]

I just had a complete bottom job done in January 2020, May 2020, I could not get the revs up of sped.

had a dived check the bottom since he was cleaning the one next to me. Very fouled on both bottom and running gear.

He cleaned it all $2.25 per foot, more revs again and about 9 more knots. I am in fresh , the st Johns River and barnicles have been real bad this year.

[Smokin Fish]

Wow. I may have that checked. Funny how it happened so quickly, if that’s the case. Thanks for sharing your story.

[km1125]

Qr Bbpost I just had a complete bottom job done in January 2020, May 2020, I could not get the revs up of sped.

had a dived check the bottom since he was cleaning the one next to me. Very fouled on both bottom and running gear.

He cleaned it all $2.25 per foot, more revs again and about 9 more knots. I am in fresh , the st Johns River and barnicles have been real bad this year.

Thanks for posting that. I wondered how often you guys have to get that done and how much it costs. Is the per-foot an overall length, waterline length or square foot?

[Smokin Fish]

Found out it was a restriction in fuel flow through the GPH gauge set up.

[Smokin Fish]

Well, it wasn't the GPH gauges either. Exploring bad fuel and revisiting bottom fowling.