Max RPM? 40hp Mercury Classic

[anderso8623]

hi all! I have a new to me, 1987 24’ pontoon with a 40 HP mercury classic I assume is the same or close to the same year. I am looking for Max RPM and possible prop adjustments based on the data I have. My main goal is top end speed as I do not want to run WOT all the time.


The two toons are 24” diameter. The prop is a MI Wheel propeller and the only markings on it are on the hub, it reads MI Wheel on one spoke, C01 13 on the second spoke and Vortex on the third spoke. Not sure on the diameter of the prop as it is not obvious in any markings

It is hard to read the serial number on the motor but it looks to be 00070343. I think it is an 1987-1989 Mercury Classic 40HP. WOT is 4,300 rpm and it pushes the pontoon 14mph.

Please let let me know if I have left anything out.

jason

 

[anderso8623]

 

[anderso8623]

Here is a pic of the serial number on the motor

https://photos.app.goo.gl/TKnd1pREv9hXK4Ny7

 

[jimmbo]

That's the 44 cubic inch 4 cylinder? 5000 - 5500 is the WOT RPM range for that engine

 

[anderso8623]

Thanks Jimmbo. It is the 4 cylinder, not sure on the cubic inch. I figured 5-5500 but I can not find anything on this engine online. It’s like they never made it! Lol.

 

[anderso8623]

So I propped down to a 9 pitch, a difference of 4 pitch, to get mor RPM and only saw a 3-400 RPM increase. I am now turning at 4,600 rpm, That does not sound correct. This is what the local Mercury dealer recommended.

Here red is my next question, there are two throttle stop screws that are adjustable. Is adjusting these an option to increase RPM?

Other than prop adjustments, do I have any other options? I would like to have it running in the normal wot rpm range. I think the speed question is dead in the water.

 

[anderso8623]

PS the new prop is a mercury black max 12 1/4 diameter 9 pitch 3 blade aluminum

 

[Scott Danforth]

Pontoons react different than planing hulls

first, you have to get the motor height correct. in too deep and the motor lugs a bit more. a hydraulic jack plate is the best option with variable water lines like a pontoon.

keep raising the motor with the load you normally run until the prop starts to ventilate, then drop down just a bit

then play with props

 

[Texasmark]

As Walleyehed used to say, (He is posted up in the sitckies at the top of this section).....get your setup corrected then dial in your prop. Rule of thumb is 150-200 change in rpms per inch of prop change......all others things constant.....and changing prop mfgrs. or models changes other things.

One adjustment would be idle set and the other probably timing. You can look into the throats of your carbs and see if the butterflies are wide open, parallel to the venturi when the throttle is maxed out. Timing would have to be instrumented to get that correct.

I'm not a toon guy but 14 mph on a 24 footer with 40 ponies sounds pretty good to me.
Last question is: Can you believe your tach?