Won't go over 4,000 RPM

[Kconleyk]

Recently bought a 99 Bayliner trophy 21' cuddy cabin with a mercruiser 4.3 I/O.

Single dad wanted something affordable to take my daughter on.

Today was third time out on it, we cruised (Chesapeake bay of Maryland) and had a blast for two hours. It has ran fine the first two times out as well. Right before we headed back we hit a very shallow cove and turned up a lot of mud for a minute before we got out of it. I tilted the motor up and worked are way out of it. Then when in normal depth lowered the engine back down. On the way back today we were cruising at 4,500 Rpms and all of the sudden it reved up to 5500 then down to 3500 then back to 4500, and kept doing this. It didn't have any power of 4,000 Rpms so we road at 11 mph the whole back, it ran fine, started up fine when I tried to turn it off and back on, idles perfectly.

I was going to change the fuel water separator filter, but supposed to go back out tomorrow morning after her soccer game with a few other parents and their boats.

First time boat owner, besides bass boats, growing up my dad was on bass masters tour but never had a salt water boat before.

Thanks

 

[Chris1956]

Was the boat going 11 MPH at 4000RPM? If so. you spun your prop, or chewed the daylights out of it.

I doubt changing the water separator will fix it.

 

[Kconleyk]

Was the boat going 11 MPH at 4000RPM? If so. you spun your prop, or chewed the daylights out of it.

I doubt changing the water separator will fix it.

Was like 3,700 and between 11 and 16 mphs is there a fix for a spun prop or just a new one?

 

[Chris1956]

You can get a prop with a fixed hub, rehubbed by a prop shop. If the prop is newer, it could have a modular hub. You can buy a new hub for that kind of prop and install it yourself.

 

[Kconleyk]

So I am shopping for Props, and what is on there is currently a 14.5 R 21 Pitch

All I can find is 14.25 R 21 Pitch, in aluminum

What difference would that cause? My neighbor is a retired fireman that now captains boats and said he thinks instead of plaining at 3,800 RPMS on normal conditions I might be at 4,000 RPMS but should be fine

 

[Chris1956]

Normally the OEM chooses the prop diameter. You Choose the prop pitch. If the old prop had a 21" pitch, and you liked it's performance, choose another 21" pitch.

 

[Kconleyk]

Hey everyone,

So update, bought and replaced the prop, launched today and ultra high tide after storms to make sure i didn't spin a prop from shallow water. First time launching just me and my daughter (9) and she did great helping tie the boat up and hold it while I ran to get truck.

So first two hours boat ran perfect, got on plane quicker than ever, hit higher speeds than previous normally at 28.2 at max was up at 30.9. Crushing speed at 4,200 Rpms at 28.4 mph.

We rode out did some site seeing and rove for about 2 hours. Turned around to hide back, never had the boat turned off, we floated on neautrel for about five minutes while she grabbed us drinks and a snack.

Heading back, same thing as last weekend (last weekend we thought it was a spun prop, but worked perfect for two hours then did this as well), started in got up on plane and the boat kept losing power, then reviving real high then going back to correct Rpms and mph, then drop down and up again. Kept losing Rpms then over revving then holding tight for a few then the same over and over again.

I'm lost, it ran so good with the new prop I thought I had fixed it.

Took us a long time to get back to the ramp.

Fresh gas, new prop, under 2,000 Rpms it seems to run fine, only when you add throttle and only after about two hours of running perfectly.

All gauges show oil temp, battery, etc as good.

Any ideas?

 

[Chris1956]

You should repost this in the MerCruiser forum. Those guys are experts and can tell you if it is the coupler or something else.