Mercruiser not giving boat enough power

[Matt DiMauro]

A friend of mine just had two new inboard mercruiser motors installed into his boat. Off the top of my head i cant remember the exact size motor but they have the 4 barrel carburetor in them. It’s running on a 33 foot boat. When they went for the test drive the motors don’t seem to want to get the boat up on plane. in order to do so you need to turn the boat from side to side and eventually will get up there. Any ideas on what could be causing this issue or any steps I should take to hopefully correctly diagnose the problem.

 

[Lazy Daze]

Let's start with some background and more info... How did it run when the older engines were still good... Which model boat and year... drivetrain style... old engine size... new engine size... bottom and running gear condition/growth... gear ratio and prop specs... final speed and RPM at WOT... proper use of trim/tabs...

The assumption would be that the engines are healthy - but based on the responses to the above, there could be an issue there. But I think we need to learn more about the rig and the engines, first.

 

[Bt Doctur]

Per LD, need more motor info, who installed them, who tested them, who set the timing, etc

 

[DWABoat]

Could be something as simple as incorrect props or as serious as underpowered engines.

It might take some effort, but these guys can really help you if you get the info they want.

 

[Matt DiMauro]

Hi guys thank you for the replies. The new motors are an exact replacement of old ones. We are running two 4.7 Mercruisers I/O V6. The new motors are running at 4400 RPMs while the old ones usually ranged from 4500-4800RPMs. Not to far off. That seems normal but the boat does not plane. it is completely dry of any unnecessary weight and would normally plane.

 

[Bt Doctur]

you are seriously under powered with the 4.3 V6. Ideally you should be running 6.2. And pushing the V6 to it`s limit results in a holed piston

 

[Matt DiMauro]

Regardless of the power on the boat as of now, with an exact replacement, the Monterey should be getting up on plane with no problem like it did before correct? which leads me to thing there’s a mechanical issue when installing the new ones. Any ideas on what to look for

 

[LDB270]

My 270da has twin 4.3’s and I’m gonna guess right around 10500lbs and is a bit smaller than your friends 33. I wish I had a bit more juice to get her up in plane but it does it just fine. Regardless, if your old worn out twin v6’s got her up on plane she should still be doing so now. Did you guys do anything to the drives or props? Any heavy smoke from the exhaust? Engines running hotter than before? My guess is it’s something obvious looking you right in the face. That’s usually what it is for me but I figure it out after spending money I dint need to. Good luck

 

[Tpitch20]

Check the prop hub. Might be spun.

 

[DWABoat]

Earlier he said his new motors are maxing at 4400 where the old ones were 4500-4800. When I spun a prop I was over-revving.

Did the mechanic change props to a higher pitch or larger diameter? You could have been running 17’s before and maybe he put on 19’s or something. You may be overpropped.

What does Merc say is proper WOT for those engines?