WOT Merc 7.4 Horizons

Golfman25

Well-Known Member
Sep 12, 2009
7,584
IL
Boat Info
1998 370 Sundancer
Engines
7.4 MPI
So looking at a boat with twin 7.4 Horizons. Year 2000. 600-700 or so hrs. WOT Spec is 4400-4800. Only hits 4000 RPM, 30.7 Kts (35 mph). Owner reports cruise generally 3100 RPMs, 27 kts (31 mph).

A bit aprehensive as these are the engines with the overlapping valve train and subject to water ingestion issues. So what are the possibilities regarding the WOT?

Tune up?
Compression issues? Compression test not done.
Valves/Head issues?
Easy fix? Hard fix? Run forest run?

Thanks in advance.
 
what are they trying to push? Been in the water all of its life, seasonal? I've never had any of the 8 7.4 Horizons get past 4400 on a 370 Sundancer.
 
what are they trying to push? Been in the water all of its life, seasonal? I've never had any of the 8 7.4 Horizons get past 4400 on a 370 Sundancer.
340 DA. In Great Lakes, so seasonal. Not even getting the 4400.
 
compression = hp = speed under load.
both engines not getting to 4400? look external to the engines first.
With a 20 year old boat and original engines, the possibility list is long. Be thoughtful
Good luck. And if this is pre-sale, walk away if it doesn't look and feel right. It won't magically get better by buying it.
 
3,100rpm and 27kts? That 340 has been propped I dare say. That's hauling the mail at 3,100rpm.

First thing I'd do is ask about the prop work they've had done...
 
compression = hp = speed under load.
both engines not getting to 4400? look external to the engines first.
With a 20 year old boat and original engines, the possibility list is long. Be thoughtful
Good luck. And if this is pre-sale, walk away if it doesn't look and feel right. It won't magically get better by buying it.
It's pre-pre sale. Trying to decide if it's worth the trip. Issue was identified when it was last surveyed 3 years ago. Owner hasn't looked into it or corrected it as "it runs great."
 
First thing I would do is walk.

"Owner reports cruise generally 3100 RPMs, 27 kts (31 mph)."

My 330DA with twin 7.4s plowed water at anything less than 3400, propped correctly. Assuming the tach are accurate I would walk.

If you want to continue then I'd get a pre-sale engines compression check. 2 hours tops, $200~

If it's the heads they can be pulled and sent out to a shop, pretty common.
 
3,100rpm and 27kts? That 340 has been propped I dare say. That's hauling the mail at 3,100rpm.

First thing I'd do is ask about the prop work they've had done...
This is what I am thinking. Looked at the survey and they listed the props as 17x19. Checked a previous survey I had done on another boat with same engine package and props listed at 17x17. Plus, it's both engines.

They have only owned the boat about 2 years. Don't think they are familar with prop work. Didn't address the finding during the survey. So that makes it tough.
 

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