mercruiser rpm problems

[bluewaterboater]

I bought a new boat recently and need some help. I took it out for the first time recently and it runs extremely high in rpm if I push the throttle down. It will run over 5500 rpm if I let it. I have been told that it should not be run past about 4800. It was a bit wavy when I went out so I couldn't test it too much. I was easily able to get to 45mph but it was revving very high. I did not push it....

The boat is a 1995 Blue Water Riviera bow rider, 19' long (2450 pnds) with a 4.3L 4barrel engine. The outdrive is an alpha one but I cannot find a stamp for the gear ratio on it. I am running a 14.25" x 21 pitch aluminum prop.

Is it possible the outdrive is mismatched to the motor? I googled the serial number and it does come in at the right year as the boat. Will changing prop size help me? Any other advice would be greatly appreciated.

thanks

 

[Don S]

Re: mercruiser rpm problems

Is it possible the outdrive is mismatched to the motor?

It's VERY possible that's the problem. Many DIYers put the wrong gear ratio drive or lower unit on a boat because they don't realize it makes a difference, and/or they got a "Great Deal" on the drive right or wrong.
Another possibility, your tachometer is set wrong or not working properly. Check it first before doing anything else.

 

[QC]

Re: mercruiser rpm problems

I totally agree with Don but have a question.

I was easily able to get to 45mph but it was revving very high.

How are you getting your speed? GPS would make the final data set usable to determine if all is good.

Welcome aboard!!

 

[bluewaterboater]

Re: mercruiser rpm problems

Thanks for the quick input! This is great!

@ Don S. - how would I determine what the outdrive gear ratio is? I have not been able to find it stamped in the side. I do have the serial number.

@ QC - I just got the speed with the speedometer. Like I mentioned it was a wavy day so I didn't spend too much time out there. I could tell however that the rpms were too high right away. I want to do my best to figure out what equipment I have first and then how to correct the problem.

 

[Don S]

Re: mercruiser rpm problems

On a 15+ year old boat, I wouldn't expect the SN or even the gear ratio on the drive to be right. If it's not marked at all, I would bet money it was replaced. What with is the big question as externally, they all look alike.
You need to pull the drive, put it in gear while turning the ujoint shaft in the same direction the engine turns. Mark the prop shaft and ujoint shaft. Then turn the ujoint shaft until the prop shaft makes 1 complete revolution. and see how much past 1 revolution the ujoint shaft turns.
If it turns 1.98 revolutions, it's for a 3.0L engine.
If it turns 1.6 or 1.8, it's for a V6
If it turns 1.5 turns, it's for a V8

 

[Bondo]

Re: mercruiser rpm problems

Ayuh,... Test the tach, against a known shop tach....
If it checks out Ok,...
Put a 23" prop on it....

It's Got whatever it's got for Gears,...
'n it's still within the range of available props....
Live with it....

 

[Don S]

Re: mercruiser rpm problems

Ayuh,... Test the tach, against a known shop tach....
If it checks out Ok,...
Put a 23" prop on it....

It's Got whatever it's got for Gears,...
'n it's still within the range of available props....
Live with it....

Not exactly, changing from a 21 to 23 is only going to change the rpm 200 to 300 rpm 400 at best. Still not enough to bring the rpm down from "over 5500 if I let it".

 

[Bondo]

Re: mercruiser rpm problems

Not exactly, changing from a 21 to 23 is only going to change the rpm 200 to 300 rpm 400 at best. Still not enough to bring the rpm down from "over 5500 if I let it".

Ayuh,... 25" props are out there, but considerably harder to find...

Merc had 'em a few years ago under the QuickSilver line...

I've got a Highliner 222, with a 350, 1.65:1 gears in the stable, that has 1 on it, double cupped....
When he finally Blows the drive, I'll put a 1.50:1 on it...
At this point, it gets 'im across the river, 'n he don't wanta spend the cash...

 

[HT32BSX115]

Re: mercruiser rpm problems

Howdy,

You may indeed have a 1.98:1 drive if your RPM and speed numbers are correct.

Using a prop-slip calculator: http://www.go-fast.com/Prop_Slip_Calculator.htm

1.98:1, 21" pitch, and 5500, I got 47 MPH (with 15% slip....typical for most planing hulls with s single prop.)

I agree with Don, a 23p prop might not do the job, but a 24p 4 blade might. And with a little additional hole shot added.......

Find a prop shop that will allow you to try a prop before buying (they may require you to pay up front and allow you to try different pitch props as long as you don't "prang" one!!)

 

[bluewaterboater]

Re: mercruiser rpm problems

I think I am going to first look a bit closer for the gear ratio numbers and then take it for another ride to get better readings. There are only 289 original hours on the motor so it is still possible that it has the original outdrive (I hope

 

[Don S]

Re: mercruiser rpm problems

.

 

[bluewaterboater]

Re: mercruiser rpm problems

Thanks Don S. I will check it again in the light. I do know that my serial number is on the port side on the bottom of the sticker, just below where your diagram shows. I looked behind it for the ratio but did not see it. I will have to check again.

Thanks for all your help!

 

[bluewaterboater]

Re: mercruiser rpm problems

I did a bit more checking online since I am worried about my gear ratio. I found this in the Mercruiser Sterndrive Reference Chart - Alpha I Gen II 1.50/1.47 1991 - 1995 0D469859-0F679999 . My sterndrive Serial number fits in to that list. Mine starts with 0F - it's dark out right now so I can't check the rest but I do know it fits in there. Would this be a positive sign that it is the correct ratio then??

Thanks

 

[90stingray]

Re: mercruiser rpm problems

Get a flashlight and get the entire number so you will know.

 

[HT32BSX115]

Re: mercruiser rpm problems

If you want to determine your ratio, pull your spark plugs and use wrench to turn the engine.

Have someone count prop revs while you turn the engine , counting crankshaft turns.

Whip out your calculator and divide prop turns by crank turns =ratio ........(1.5:1 is 15 engine revs divided by 10 prop revs etc)

You can also pop the drive cap and count teeth too..........

 

[bluewaterboater]

Re: mercruiser rpm problems

Update: I took the boat out today on the water with a mechanic. He hooked his tach on and we confirmed that mine is off about 300 rpm on the high end. He also believes that the drive is a different gear ratio then it should be but we cannot find the stamp. He believes if I change my prop pitch to 23 I should be in the spot where I want to be.

I will update you when I get a new prop. Thanks so much everyone for all the help!!

 

[bluewaterboater]

Re: mercruiser rpm problems

Final update: I changed the prop to a 14" x 23p and took the boat for a ride. I performed at 4800 rpm wot right where it was supposed to be. I hit 49mph with a little bit of chop.
Thanks everyone for your help!