Fuel FIlter? Low RPMs

[NautiDog]

I have a 2007 Sundancer 310 with two, Merc 350s. We took it out for the first time this year and while it ran very smooth, it would not get over 3800 RPMs. It seamed like the last 1/3rd of the throttle did not change anything. We couldn't get it above roughly 25 knots either. The fuel filter says june 2019 on it, which would mean we put about 40 hours on the engines last year afterwards. No black smoke at all.

Ideas? Could it be the fuel filter? Please note: I have always had the marina service the boat...I have very limited knowledge of the mechanics of a boat. -Thank you.

 

[Highliner]

A clogged fuel filter would normally present itself as a spit/sputter/stall type scenario. Is the bottom clean?

 

[Seapig]

If it’s been in the water all season, unused, I’d second the “bottom clean?” Question.

Fuel starvation would likely present as a surge or near stall under load that improves as you back off the throttle.

Assuming she’s a twin engine and if both engines maxed out at 3800, it’s almost a dirty bottom for sure

 

[scrumper]

Wow - can a dirty bottom cause that much of a drop - around 1,000 rpm?

I have a similar issue with my twin 6.2 MPIs - cannot get over 3800 no matter what, should be at 4800-5000. I was thinking I was overpropped - both engines have exactly the same problem. But just a dirty bottom? Mine is filthy, for sure.

 

[Snapon24]

Wow - can a dirty bottom cause that much of a drop - around 1,000 rpm?

I have a similar issue with my twin 6.2 MPIs - cannot get over 3800 no matter what, should be at 4800-5000. I was thinking I was overpropped - both engines have exactly the same problem. But just a dirty bottom? Mine is filthy, for sure.

Definitely. I wasn't making rpms after I didn't leave the slip for 6 weeks. I could barely turn 3K. Bottom had some slime but not too bad. I hesitated because I didn't think that was the issue but I had a diver come clean it and I was back at 4K. It doesn't take much to really affect rpms.

 

[scrumper]

Ah cool good to know. I've started calling round for a diver.

EDIT: Have found a diver, and have booked a cleaning. I'll report back when the job done.

 

[scrumper]

Thought I'd update. The diver came on Friday and I took the boat out the next day. With full tanks, I got her up to 4050rpm but no higher. In every way the boat was happier though - popped right up on plane and took very little time to get up to those revs.

I'll try again with half tanks but I don't think I'll get up to 4800. I may still be over-propped.

However I strongly recommend you get a diver to clean the bottom per Snapon24's recommendation! Best $100 I've spent on the boat. Next season I'll have him come monthly.