Trying to achieve my motors optimum RPM

[Keegan.MW]

I just spent a short part of my winter replacing the powerhead on my motor and I just got out on the water for the first time to test it out today. Everything seems fine except when I hooked up the tach at WOT it only read 4500rpm. My manual says the operating range is 5000-6000. I would love to be as close to 5500 as possible. Here are my current boat specs.

Prop- Solas 13 pitch 3 blade aluminum in brand new condition (10 or 11" diameter not sure)
Boat- LoweLine 18' flat bottom
Motor- Johnson 35hp 1978 seahorse

I have no way of knowing exactly how fast I was going but I would guess around 25mph. Also I'm not sure how much my boat weighs. I could guess and probably get a good ballpark figure. I'll try and post a pic because you guys could prob guess closer than me.

I know I need to prop down to raise rpm but I'm not sure how low I should go? Any and all help is appreciated!

 

[Keegan.MW]

Re: Trying to achieve my motors optimum RPM

 

[MikDee]

Re: Trying to achieve my motors optimum RPM

Is the powerhead rebuilt, or new? If so, you need to break it in with a coupla tanks of fuel, before deciding on a prop. Remember, new engines are tight, and should be.

By the way the prop sounds about right 10"X 13", or 11"X13"

 

[Keegan.MW]

Re: Trying to achieve my motors optimum RPM

Is the powerhead rebuilt, or new? If so, you need to break it in with a coupla tanks of fuel, before deciding on a prop. Remember, new engines are tight, and should be.

By the way the prop sounds about right 10"X 13", or 11"X13"

It's a used powerhead in pretty great shape for being about 35 years old. I dont know how much action it has seen but from the looks of the outside of it, it's been used. Cylinder walls are in perfect shape and both cylinders compression are at an even 125psi.

I did have to swap out the crankshaft from my old power head to the new one however, so do u think it will break in over time and the rpms will increase after a few tanks of fuel?

 

[jestor68]

Re: Trying to achieve my motors optimum RPM

What was the tach reading at idle? Did it seem to read consistently(no jumping around) up to the max reading?

Assuming the tach is accurate, adjust prop pitch based on 150-200 rpm per inch of pitch reduction. Ideal rpm should be around 5500.

 

[MikDee]

Re: Trying to achieve my motors optimum RPM

It's a used powerhead in pretty great shape for being about 35 years old. I dont know how much action it has seen but from the looks of the outside of it, it's been used. Cylinder walls are in perfect shape and both cylinders compression are at an even 125psi.

I did have to swap out the crankshaft from my old power head to the new one however, so do u think it will break in over time and the rpms will increase after a few tanks of fuel?

No, not if it's been broke in once already. Are you sure your tach is right? IMO, it's not your prop, I think your engine needs some tuning.

 

[Keegan.MW]

Re: Trying to achieve my motors optimum RPM

Ok ya that is possible. The carb is close but needs fine tuning. I have a guy going out on the water with me next weekend to help me do that. I just wanted to make sure the motor was running ok before I brought him out there to do it.

 

[steelespike]

Re: Trying to achieve my motors optimum RPM

We need your wot rpm and gps speed to evaluate your setup.Some cells have a gps app.
A quick calculation indicates 4500 rpm 13" pitch with 12% slip nets about 25 mph.
You need to be sure the link n sync is set right that the timing doesn't advance too far
there probably is a stop and it may be adjustable.
If its a 78 it has a CDI ignition.

 

[Sea Rider]

Re: Trying to achieve my motors optimum RPM

A fast check : remove engine cowl, full throttle untill throtttle grip does not advance a bit, check if carb's butterfly has achieved a 180? flat opening, if so carb syc is ok, if not, there's a throttle restriction, engine will never run at wot, adjust if needed. Actual tach probably is not reading accurately, install an induction tach and check wot again.

Seleect a prop pitch that will give near it's max 6K wot parameters with you alone and light loaded boat, bear in mind that as soon you will start adding weight or passengers wot rpm will drop accordingly. If you are proped to run at 5500 rpm at wot when adding more weight/passengers max wot rpm will drop below 5500 rpm and could be going into engine lugging extreme.

Most OB Companies states best engine all around power performance is achieved near the top of the wide open throttle factory parameters stated for every engine brand.

Happy Boating

 

[Keegan.MW]

Re: Trying to achieve my motors optimum RPM

Ok I'll check on that and report back.. Thx guys!

 

[Keegan.MW]

Re: Trying to achieve my motors optimum RPM

Just checked the carb and at full throttle the butterfly is fully open 180 degrees. I agree with 6k rpm that makes sense. Now the hard part is finding it...

Steelespike, I'm not sure what CDI ignition means but is there anything I can check for? And about the timing what do I look for?

 

[Sea Rider]

Re: Trying to achieve my motors optimum RPM

If carb reaches full 180? at wot, why would you believe needs checking advance timing sync ? if nobody has played with engine's settings no need to as long it remains factory adjusted. What's the max engine HP that boat can handle ? Probably bit underpowered, if so, could maximize engine top prop trust by correct egine/transom height match and correct prop pitch match to give 6K rpm or near it.

CDI, is a solid state electronic device used on modern engines, as opposed to older mechanical platinum & capacitor method as found in old outboards and cars too. Don't remenber what CDI stands for ? discharge ?

Happy Boating

 

[jestor68]

Re: Trying to achieve my motors optimum RPM

If carb reaches full 180? at wot, why would you believe needs checking advance timing sync ? if nobody has played with engine's settings no need to as long it remains factory adjusted. What's the max engine HP that boat can handle ? Probably bit underpowered, if so, could maximize engine top prop trust by correct egine/transom height match and correct prop pitch match to give 6K rpm or near it.

CDI, is a solid state electronic device used on modern engines, as opposed to older mechanical platinum & capacitor method as found in old outboards and cars too. Don't remenber what CDI stands for ? discharge ?

Happy Boating

CDI; Capacitor Discharge Ignition.

 

[Keegan.MW]

Re: Trying to achieve my motors optimum RPM

The boats max is rated for 35 hp and it's a short shaft motor with a short shaft boat. the idle and wot was very consistent on the tach once I reached wot. Idle was 800 I believe. I just got a new 11 pitch prop in the mail today and and taking it out in a little bit. Hopefully it gets me close to 5500 at least. Ill let u guys know

 

[Keegan.MW]

Re: Trying to achieve my motors optimum RPM

Ok just got off the water and have some useful information. I achieved 5700rpm with the new 11 pitch prop which I'm fully happy with! I used a phone app and it said I got to 27mph. Also took the boat to a co-op and weighed it and dropped it in the water and went back and re weighed. So I have an accurate weight of 1340 with me and another person just now. Do all these figures sound right? The carb still needs to be fine tuned. Do u think dropping to a 10 pitch would be worth considering?

 

[MikDee]

Re: Trying to achieve my motors optimum RPM

Ok just got off the water and have some useful information. I achieved 5700rpm with the new 11 pitch prop which I'm fully happy with! I used a phone app and it said I got to 27mph. Also took the boat to a co-op and weighed it and dropped it in the water and went back and re weighed. So I have an accurate weight of 1340 with me and another person just now. Do all these figures sound right? The carb still needs to be fine tuned. Do u think dropping to a 10 pitch would be worth considering?

NO! Give your motor a chance to get used to the new prop, you may pull more rpm's once you tweak it, & use it more.

 

[Keegan.MW]

Re: Trying to achieve my motors optimum RPM

Ok Will do. Thx for the advice guys!

 

[dazk14]

Re: Trying to achieve my motors optimum RPM

Ok just got off the water and have some useful information. I achieved 5700rpm with the new 11 pitch prop which I'm fully happy with! I used a phone app and it said I got to 27mph. Also took the boat to a co-op and weighed it and dropped it in the water and went back and re weighed. So I have an accurate weight of 1340 with me and another person just now. Do all these figures sound right? The carb still needs to be fine tuned. Do u think dropping to a 10 pitch would be worth considering?

No, but you didn't comment on the performance characteristics of your new setup.

What brand/size prop did you purchase. I know you went from a 13" pitch to an 11" pitch and picked up 1200rpm if I'm reading the post correctly. Looking for diameter of each and brand/model of new prop.

There may be a couple/3 more MPH in that setup. Did you try trimming the motor out a hole or 2? Ventilation and porpoising eventually get in the way.

 

[Keegan.MW]

Re: Trying to achieve my motors optimum RPM

They are both the same brand prop. Solas Amita 3 aluminum. Went from 10x13 to 10x11. And yes I jumped up 1200 rpm which I did not expect out of just two inches of pitch change. Is this normal? Does it indicate anything I should be aware of? Also I do believe I need to trim up or down a hole or two because at wot there was a significant amount of water being sprayed by the motor. Is there a way to know whether I need to go up or down without just trial and error?

 

[steelespike]

Re: Trying to achieve my motors optimum RPM

Smaller motors will usually have a larger rpm change for a given pitch change. A 10 1/2" X 11" is listed as the standard prop
though the final size would depend on the final application. Your numbers are right on 5700,1.93 ratio,11" prop,27 mph delivers about
12% slip high normal. At 1300 lbs 27 may be about all you will get.
Your flat bottom probably has 5 keels pressed into the bottom for strength.These surely hinder top speed a little.
After having the motor apart Sync check would be normal though the carb does seem to be opening as it should.As far as timing I believe there is no timing per say but a limit to how far the stator can advance that limit may be adjustable and if it goes too far at wot could actually slow the motor.