1996 merc/mariner 150hp too many RPMS!!!!

mbecke2

Petty Officer 3rd Class
Joined
May 24, 2009
Messages
94
So I just had my stringers reinforced and floor and foam replaced. I was all excited because I thought I was going to gain some speed because the floors and foam was water logged (extra weight). Well the boat used to go over 45 mph. Well much to my demise, I took it out and it ran great, but I was getting almost 7000 rpms and only going 43mph. I want to make sure my rpm gauge works before I jump to conclusions, but it idles at 1000 rpms so I think the gauge is working. I'm wondering if the timing is off because I had to replace a throttle cable and messed with the throttle setting. But Im sure I put everything back on the same way it came off. My question is "Is it possible that the boat is so much lighter that Im getting too many rpms at WOT and now I have to step up my prop pitch?" or is something else going on? I tried riding with 4 grown men (800lbs), 30 gallon live well with water, and 30 gallons fuel andd rpms came down alot (around 6200) and went about 41mph WOT. But with two people it get well over 6500 rpms and only go 43mph. I know this boat used to go faster bc when I bought it the guy said it went 50mph. What happened to my speed???? Is there an easy way to check WOT timing?
21ft skeeter bay (1800lbs) very light
1996 mariner 150 2.0 liter v6----gas guzzler!!
 

aussieflash

Lieutenant Junior Grade
Joined
Feb 5, 2011
Messages
1,004
Re: 1996 merc/mariner 150hp too many RPMS!!!!

I wouldnt cry too much with those numbers.Timing should be done by the book and to exact spec(mercury Manual).If you removed motor check ride height is correct.7000rpm sounds a little scary....maybe look at prop selection also.
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,589
Re: 1996 merc/mariner 150hp too many RPMS!!!!

With that hull you should be running about a 25 pitch prop, what are you running now?
 

mbecke2

Petty Officer 3rd Class
Joined
May 24, 2009
Messages
94
Re: 1996 merc/mariner 150hp too many RPMS!!!!

With that hull you should be running about a 25 pitch prop, what are you running now?

I have 21 pitch power tech ss prop. Do you think I can spin a 25? Seems like too much pitch to me.
 

BonairII

Commander
Joined
Jun 7, 2011
Messages
2,727
Re: 1996 merc/mariner 150hp too many RPMS!!!!

If you really are spinning 7k rpm with a 21p....you def can turn a 25p prop. That should drop your rpm about 800 rpm(200 rpm per inch)
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,036
Re: 1996 merc/mariner 150hp too many RPMS!!!!

Like the OP, I am skeptical about spinning that 25P prop. I recommend the OP use a GPS and a known-good tach and get a baseline. I run a boat with a bassboat hull, which is 1100# with hte motor. I can spin a 23P prop, but top speed is a bit better with a 21P prop.
 

mbecke2

Petty Officer 3rd Class
Joined
May 24, 2009
Messages
94
Re: 1996 merc/mariner 150hp too many RPMS!!!!

Like the OP, I am skeptical about spinning that 25P prop. I recommend the OP use a GPS and a known-good tach and get a baseline. I run a boat with a bassboat hull, which is 1100# with hte motor. I can spin a 23P prop, but top speed is a bit better with a 21P prop.

I have a good gps and I can go 43.5mph with two people and I went 41 with 4 people. The problem is I dont know if my tach is working properly. I guess I need to use a tach that is known good. But my speed should be faster....or maybe not. It is not a bass boat. This is a center console bay boat 8 ft wide. I think it should go around 50mph and I know I got it up to 47mph with a tempest plus that I borrowed from a friend. But can't be sure if Im really spinning 7000 rpms or the tach is faulty. I believe the tach is messed up or I hooked it up wrong. I had to re connect all my wires after I had the work done and I have had this tach act funny before on me. And the motor doesnt sound like it is over reving to me. Ill figure it out sooner or later.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,036
Re: 1996 merc/mariner 150hp too many RPMS!!!!

There is a prop speed calculator on the web. It may give you an idea if the motor is spinning at 7000 (which I doubt). You need to enter the stuff you know, prop pitch, MPH and gear ratio (likely 2::1), and guess at the prop slip (say 10%) and see what RPM you get.
 
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