wot rpm not reached

[vics95]

I have a 1995 mercury tracker 75 2 stroke and a wot I'm only running around 3200-3500 rpm and about 27mph. I just got the boat and don't know for sure if the prop has been changed. Does anyone know what would be the correct pitch to run to obtain the correct rpm at wot?

 

[BobGinCO]

Re: wot rpm not reached

It doesn't quite work that way.

Whatever pitch you're running right now, it's too much.
You will need to lower your pitch (and maybe your diameter), until you hit the sweet spot for your boat/motor combo.

Running that low, I would want to check the compression in the motor. It seems awful low, and it's an old motor.

 

[jestor68]

Re: wot rpm not reached

Go to the Mercury Marine web site and use their prop selector to determine what prop you should be running, and compare it to what you have.

Be advised that their model years only go back to the year 2000, and assumes a good healthy motor developing full horse power. If your motor has a few years on it and is likely down a few points on compression, take the pitch they give you and drop one size to account for your motor's age. That usually works out quite well.

Also on their web site; in the engine section, are tests of all the various motors on like size boats for you to look at to see what size/type props were run on a 75 Optimax(in this case) on a boat like yours. Again, use caution in following their size exactly, since the motors tested are new, and yours may not be able to pull the same pitch. If the motor tested was using a 19 pitch and turning good rpm, you probably need a 17 pitch.

You want your motor to turn between 4750 and 5250 rpm.

 

[vics95]

Re: wot rpm not reached

Go to the Mercury Marine web site and use their prop selector to determine what prop you should be running, and compare it to what you have.

Be advised that their model years only go back to the year 2000, and assumes a good healthy motor developing full horse power. If your motor has a few years on it and is likely down a few points on compression, take the pitch they give you and drop one size to account for your motor's age. That usually works out quite well.

Also on their web site; in the engine section, are tests of all the various motors on like size boats for you to look at to see what size/type props were run on a 75 Optimax(in this case) on a boat like yours. Again, use caution in following their size exactly, since the motors tested are new, and yours may not be able to pull the same pitch. If the motor tested was using a 19 pitch and turning good rpm, you probably need a 17 pitch.

You want your motor to turn between 4750 and 5250 rpm.

Ok thanks. The prop now is a 21 pitch

 

[jestor68]

Re: wot rpm not reached

What sort of boat is it on?

 

[vics95]

Re: wot rpm not reached

What sort of boat is it on?

1995 bass tracker pro 17

 

[GA_Boater]

Re: wot rpm not reached

Of course the prop needs to be changed because your new to you boat isn't reaching WOT. Do you know if the motor is running properly? The first thing to check before you start throwing props at it, is the motor. Especially a new to you boat. Good luck and welcome aboard.

 

[jestor68]

Re: wot rpm not reached

A Tracker Pro 170 (16' 8") is rated for 50 HP.

A Tracker Pro Team 175 (17' 7")is rated for 75 HP.

What does the capacity sticker say?

 

[vics95]

Re: wot rpm not reached

Sorry its the pro team 175 with the 75 mercury tracker 75 on it

 

[jestor68]

Re: wot rpm not reached

A quick trip to the Mercury prop selector reveals something disturbing. And Tracker agrees.

The selector says your rig should spin a 21 pitch Laser II at 5000 rpm @ 40.1 mph. Tracker estimates the top speed at 40 mph with the 75. These numbers represent a 7% slip; certainly reasonable using a Mercury Laser II prop.

You likely have motor issues if those present rpm and speed numbers are accurate; which they appear to be, since 3500 rpm with a 2.33 gear ratio and 21 pitch equals 28 mph at a 8% slip.

A compression test is in order(to begin with).

 

[vics95]

Re: wot rpm not reached

Ok. So what comes after the compression test if it's bad?

 

[sublauxation]

Re: wot rpm not reached

It would take a whole lot of pitch change to get 2000 RPM. Either your tach and speedo are wrong or there's something else going on. Is the prop in good shape? Could it be a bad hub? Otherwise Jestor is right, something isn't right with the Merc.

 

[jestor68]

Re: wot rpm not reached

Ok. So what comes after the compression test if it's bad?

Like new compression on that motor would be in the range of 130-135. All cylinders need to be reading within 10% of each other; the higher the number the better. If you have a bad cylinder(s), there is no option but a rebuild/replacement.

If it passes the compression test, then start looking at ignition and/or fuel delivery issues.

 

[aerobat]

Re: wot rpm not reached

and of course look also for the simple things - e.g when you give full juice on the remote, do the carbs also open fully ?

 

[vics95]

Re: wot rpm not reached

It has a hydrofoil stabilizer on it also would that make much difference in the performance?

 

[frantically relaxing]

Re: wot rpm not reached

Problem with 2-strokes is it's hard to tell sometimes if you have a dead cylinder. The 115 on our Party Cruiser is a 4 cylinder, and with a dead #4 it was only able to reach about 4200 rpm (out of about 5500 it should've been). Your engine is a 3 cylinder, one dead cylinder would result in approximately the power you have. You may just have a bad spark plug or wire, or a dirty carb. Check your plugs to see if one looks different than the other 2. Wetter than normal indicates a plug not firing, dryer indicates a lean fuel condition...

 

[vics95]

Re: wot rpm not reached

Could anyone tell me the steps for a tune up that I could do. I'm fairly mechanically inclined but don't know a whole lot about boat motors. I'm getting a repair manual shortly to have on hand. And would anyone know the correct plug to run in this motor?

 

[vics95]

Re: wot rpm not reached

Problem with 2-strokes is it's hard to tell sometimes if you have a dead cylinder. The 115 on our Party Cruiser is a 4 cylinder, and with a dead #4 it was only able to reach about 4200 rpm (out of about 5500 it should've been). Your engine is a 3 cylinder, one dead cylinder would result in approximately the power you have. You may just have a bad spark plug or wire, or a dirty carb. Check your plugs to see if one looks different than the other 2. Wetter than normal indicates a plug not firing, dryer indicates a lean fuel condition...

I pulled two plugs and they were fairly wet but I'm not sure how they are supposed to look in this type motor.

 

[jestor68]

Re: wot rpm not reached

An easy way to check for a dead cylinder is to; with the motor idling, pull the plug wire(s) off one at a time and look for the motor to start "missing" when you pull the plug wire off. If you get to a cylinder that shows no miss(no change) when you remove the plug wire, that cylinder is dead. Most likely the coil for the cylinder is bad, or the ignition control module could be bad.

 

[steelespike]

Re: wot rpm not reached

Plugs should be dry and tan/brown. You need to note what cylinder they come out of and their condition.
Wet plugs should smell gassy. The wet could be water which may have droplets,rust and the plugs may be exceptionally clean.
A good start on a tune up would be a compression check. Should be at least 100 psi and within 10% between cylinders.
Get us the numbers and we'll go from there.