High rev but slow speed

[huslayer]

Hey Folks, I found many threads similar to my issue abut none is my actual issue.
I've 2004 Mercruiser 3.0L, Alpha 1 gen 2..

The boat would rev up to 7K RPM WOT and only would get up to 25 MPH..
I changed the prop and hub thinking it might be spun and even 20" pitch from 17" and still no hope..

I changed the oil, oil filter, gas filter, plugs, gear lube, cleaned the carb.. but no hope.

could be the coupler? never smelled rubber or anything!!

I did a compression test and all 4 cylinders read 120...

the boat runs fine, but soooo slow, 7MPH at 4K RPM, oil pressure is less than 40 PSI, around 30 PSI, could that be any issue too?


any idea?
thanks

 

[cionjay]

Im not an expert, but it could be stuck valves or an exhaust restriction. try a cylinder leak test with an air compressor. it is like a compression test but in reverse. you can pick one up at harbor freight for about 40 bucks. Low oil pressure could be a bearing problem or worn journals on your cam shaft. Is your engine making any type of abnormal noise?

 

[ondarvr]

If you hit 7,000 RPM's you could do damage right there.

The next part, if you were at 4,000 RPM's but only going that slow, it has nothing to do with low oil pressure or anything esle with how the motor is running. The weak link is somewhere between the engine and the prop.

 

[Maclin]

The tachometer may not be reading right, and probably is NOT if it says 7k at anytime for any length of time,

Is this a recent issue? Has this boat always acted like this for you. New purchase?

 

[huslayer]

Im not an expert, but it could be stuck valves or an exhaust restriction. try a cylinder leak test with an air compressor. it is like a compression test but in reverse. you can pick one up at harbor freight for about 40 bucks. Low oil pressure could be a bearing problem or worn journals on your cam shaft. Is your engine making any type of abnormal noise?

Hi, thanks for your help, no, I don?t have any weird noise at all.
what would be the regular or normal oil pressure?

 

[huslayer]

If you hit 7,000 RPM's you could damage right there.

The next part, if you were at 4,000 RPM's but only going that slow, it has nothing to do with low oil pressure or anything esle with how the motor is running. The weak link is somewhere between the engine and the prop.

Yep I was thinking about the coupler, I wanted to hear other's ideas before going to a mechanic.. thanks for stepping in.

 

[southkogs]

New issue on a boat you've had for a while? Or is this a new boat and this issue came with it?

Welcome aboard?

 

[huslayer]

New issue on a boat you've had for a while? Or is this a new boat and this issue came with it?

Welcome aboard?

I just bought that boat in June and the issue was there, I thought that's normal and it's underpowered boat, it's a 20" sea fox 204CF, but after some research I found that I'm wrong and I've an issue that needs to be fixed!!

thanks

 

[huslayer]

The tachometer may not be reading right, and probably is NOT if it says 7k at anytime for any length of time,

Is this a recent issue? Has this boat always acted like this for you. New purchase?

Yes always acted that way, and Yes new purchase, I suspect the tach theory too, I'll try to get another one to test with.. Thanks

 

[ondarvr]

The first thing would be to check the tach, it's probably wrong. And then go back to the original prop and see what you get.

 

[southkogs]

How heavy is the boat this is in? What type of boat, I guess I should ask?

The Bayliner 1950 I just got hold of is about 2,000 LBS with a 3.0l Alpha one. 3200RPM and we're doing about 22 MPH just up on plane with 5 people on board.

 

[huslayer]

How heavy is the boat this is in? What type of boat, I guess I should ask?

The Bayliner 1950 I just got hold of is about 2,000 LBS with a 3.0l Alpha one. 3200RPM and we're doing about 22 MPH just up on plane with 5 people on board.

I think it's around 2500 with Gas, it has a cabin

 

[huslayer]

The first thing would be to check the tach, it's probably wrong. And then go back to the original prop and see what you get.

The tacho's settings were wrong, it was on 8 cylinder not 4, that's why the numbers were doubled.
I tested the boat yesterday and it was doing 25MPH with 3 ppl on 4K rpm, max was 4100 RPM and it was screaming.

old prop was destroyed in the process of removing it, the hub was melted around the shaft.

I guess I'll buy a smaller prop then, but does it make any difference??

 

[littlerayray]

A 20' cuddy cabin with a 3.0 merc your underpowered iMO I would start looking for minimum v6 to push that boat even a v6 is a little small

 

[huslayer]

A 20' cuddy cabin with a 3.0 merc your underpowered iMO I would start looking for minimum v6 to push that boat even a v6 is a little small

Yep but that's not an option at the moment

 

[thdrduck]

A 20 ft cuddy with 3 people doing 25 and he needs more motor? I must be missing something, seems 25 is pretty respectable for a boat that size. I bet a good cruise would be about 3000 rpm and about 20 mph. Just my thought, I'm not a speed freak anymore.

 

[southkogs]

Are you sure that's not the dry weight of the boat? An '02 Bayliner 21' Cuddy weighs in at 3,200# with a HP rating of 220.

3.0L Merc is 135HP which does sound low for a boat at 2,500#. I would think that once you load it, 25 - 30 MPH is pretty reasonable. Not sure on the prop part ... I'm still gettin' Mercs figured out