1989 20 HP Johnson power upgraded to 25 HP for zero dollars

[Linhardt]

https://www.tinytach.com/gasoline.php

This will get your RPM for you. From what I have seen it is very simple to use, but I have never used it myself. I think it is as simple as wrapping the red wire lead around the spark plug wire and grounding the other wire, or something like that. Check around and you might find it cheaper on other websites.

 

[Siguz]

Linhardt » 29 May 2014, 11:12[/url]"]https://www.tinytach.com/gasoline.php

This will get your RPM for you. From what I have seen it is very simple to use, but I have never used it myself. I think it is as simple as wrapping the red wire lead around the spark plug wire and grounding the other wire, or something like that. Check around and you might find it cheaper on other websites.

Very neat!
I'll see if I can find it in Canada. I hate ordering stuff like this from across the border because of warranty and shipping hassles.
Thanks

 

[Stefan]

Linhardt » 56 minutes ago[/url]"]https://www.tinytach.com/gasoline.php

This will get your RPM for you. From what I have seen it is very simple to use, but I have never used it myself. I think it is as simple as wrapping the red wire lead around the spark plug wire and grounding the other wire, or something like that. Check around and you might find it cheaper on other websites.

Thank you for the info... I am going to talk to a buddy of mine who is into all of this stuff and see if he has one.. If not, I'll go make the purchase

 

[Johny25]

Linhardt » Today, 07:12[/url]"]https://www.tinytach.com/gasoline.php

This will get your RPM for you. From what I have seen it is very simple to use, but I have never used it myself. I think it is as simple as wrapping the red wire lead around the spark plug wire and grounding the other wire, or something like that. Check around and you might find it cheaper on other websites.

Correct ^^^^ I have this exact tach for my smaller motors and it works great and very easy to use and worth the $50 bucks or so. I bought mine off of ebay so I know you can get them in Canada.

 

[Johny25]

So you have an 89' 20hp which you turned into a 25hp correct. The 20 and 25hp have same recommended RPM range 4500-5500 RPM in that year. My advice would be to get real close to the 5500 RPM when you have the lightest load you typically run in your boat. And don't worry if you turn 5600-5800 it won't hurt these motors. In fact the 30hp runs at 5800 and it is just different carburetor.

Your gear ratio is a 13:28 also known as 2.15:1

Get a tach and we can help you get the correct prop. But by looking at the info you already have given me I think you may be OVER propped already.

If you are doing 38kmh (23.5mph in my language lol) with a 2.15:1 gear ratio and a 13 pitch prop then it is HIGHLY likely you are only running at around 4700-4800 RPM. These motors produce the 25hp at 5000-5200 and run way better over 5000k

 

[Siguz]

Got a tach. Will test tonight or tomorrow.
Which cylinder do I test.. 1 or 2?

 

[Siguz]

Johny25 » 30 May 2014, 02:14[/url]"]So you have an 89' 20hp which you turned into a 25hp correct. The 20 and 25hp have same recommended RPM range 4500-5500 RPM in that year. My advice would be to get real close to the 5500 RPM when you have the lightest load you typically run in your boat. And don't worry if you turn 5600-5800 it won't hurt these motors. In fact the 30hp runs at 5800 and it is just different carburetor.

Your gear ratio is a 13:28 also known as 2.15:1

Get a tach and we can help you get the correct prop. But by looking at the info you already have given me I think you may be OVER propped already.

If you are doing 38kmh (23.5mph in my language lol) with a 2.15:1 gear ratio and a 13 pitch prop then it is HIGHLY likely you are only running at around 4700-4800 RPM. These motors produce the 25hp at 5000-5200 and run way better over 5000k

So.....
I used an Innova 3568+ timing light/Tach
As Instructed I put the 'clamp' on plug wire #1, and connected it to the battery.
Of course I was running with the hood off but I didn't really have a choice.

I'm supposing that this is wrong but the RPM read as high as 9940 numerous times.......
If this is true shouldn't the motor have blown up by now?
Did I do something wrong? Am I missing something in the equation?

https://plus.google.com/u/0/106046188811399299593/posts
This is a link to the video of the tach going crazy.

What do you think?

Thanks

 

[rusty503]

What it looks like is that it is reading the engine as a 4 cylinder. I read the manual and didn't see a way to change that.

 

[Siguz]

What it looks like is that it is reading the engine as a 4 cylinder. I read the manual and didn't see a way to change that.

That makes sense.
So I guess that means that 1/2 of the reading is the answer.

Thanks very much!

 

[Siguz]

I think I'll try a few more runs tomorrow.
The lake was quite rough tonight.

 

[Stefan]

I am going to get a tach tool .. this is interesting.

 

[Johny25]

if it was reading double than your motor would be running 4970rpm which makes sense and would give you a 17% slip ratio.....which is not great. Get a tiny tach and get a good reading. I can assure you that if you buy a 15 pitch you will lug that motor and toast it eventually. You are likely going to end up at an 11 pitch to get RPMs up to par. You must be running a good load in that boat. What kind of boat is it anyway?

 

[Siguz]

Johny25 » 31 May 2014, 00:02[/url]"]if it was reading double than your motor would be running 4970rpm which makes sense and would give you a 17% slip ratio.....which is not great. Get a tiny tach and get a good reading. I can assure you that if you buy a 15 pitch you will lug that motor and toast it eventually. You are likely going to end up at an 11 pitch to get RPMs up to par. You must be running a good load in that boat. What kind of boat is it anyway?

Last night I did the run with my wife in the boat running through 2ft waves.
I'm going to run it alone with all my gear today. When I'm alone in the boat the motor has to be at it's lowest trim position to prevent severe cavitation. With a passenger, it runs best up one trim point.
I will measure RPM in both scenarios if the water is calm enough to do so.

What kind of boat?
--- It's a Starcraft. No readable VIn tag so I don't know the year or model. 14.5ft long

Stefan:
What's the pitch of your prop?

 

[Siguz]

Ok
5295 was max RPM
Trim up with 2 batteries full take of gas and fishing gear.
What do you suggest?

Earlier I used cavitation improperly. Should have said porpoising.

 

[Johny25]

Is this how heavy your boat is the majority of the time or do you add more weight usually?

Your numbers come out as follows.....(assuming you are doing 23.5mph still?) you gave RPM and no mph/kph reading so I am going to assume here.

10X13 prop (aluminum?), with a 2.15:1 gear ratio at 5295 RPM and 23.5mph gives you a slip of about 22% which is NOT great by any means. Better to be around 15% on these smaller motors.

My suggestion is if you are going to run any more weight in the boat you definitely have to go down in pitch. A 10X12 prop should get you around the 5500 mark with current setup. Your holeshot will improve and slip will likely drop a bit also. Your speed will not increase though.

If you had bought that 15 pitch you would have lost 400rpm and lugged your motor out. likely not gaining any speed and having a slip number in the upper 20's

Note: this is all speculation as every prop manufacturer has different blade design, rake and cupping which effects RPM and performance. If you go to a stainless prop you may get better performance and RPM with the same pitch prop. But if it has aggressive cupping then it will lower your RPM for same pitch.

I run a 10X14 solas saturn stainless on my 14' Lowe and my boat is pretty heavy with the decking I put in. Sounds like your boat is even heavier and you would not be able to turn a prop that high in pitch.

 

[Siguz]

Johny25 » 04 Jun 2014, 16:57[/url]"]Is this how heavy your boat is the majority of the time or do you add more weight usually?

Your numbers come out as follows.....(assuming you are doing 23.5mph still?) you gave RPM and no mph/kph reading so I am going to assume here.

10X13 prop (aluminum?), with a 2.15:1 gear ratio at 5295 RPM and 23.5mph gives you a slip of about 22% which is NOT great by any means. Better to be around 15% on these smaller motors.

My suggestion is if you are going to run any more weight in the boat you definitely have to go down in pitch. A 10X12 prop should get you around the 5500 mark with current setup. Your holeshot will improve and slip will likely drop a bit also. Your speed will not increase though.

If you had bought that 15 pitch you would have lost 400rpm and lugged your motor out. likely not gaining any speed and having a slip number in the upper 20's

Note: this is all speculation as every prop manufacturer has different blade design, rake and cupping which effects RPM and performance. If you go to a stainless prop you may get better performance and RPM with the same pitch prop. But if it has aggressive cupping then it will lower your RPM for same pitch.

I run a 10X14 solas saturn stainless on my 14' Lowe and my boat is pretty heavy with the decking I put in. Sounds like your boat is even heavier and you would not be able to turn a prop that high in pitch.

Thank you very much for your help.
I'm grateful for your knowledge and guidance.
Weight? I've actually decided to eliminate the trolling motor and extra battery.... save about 100lbs.

We are now thinking about doing the 35hp upgrade.

Other than carb and manifold, are there any other hard components that need to be changed?
..............Should I refer back to your original post, Johny25?

 

[Siguz]

Change of plans...again.....
We're going to keep everything as is and buy the prop.
Found a 10 1/8 x 12 from michigan wheel for $113 from a local boat shop.

Thank you all for your input.

I'll repost once the new prop is installed and run.

 

[Johny25]

Your welcome Siguz......and no, other than the carb and intake you are good to go. IT will actually make it a 30hp though. Not a 35

 

[Siguz]

Prop installed.
Topping out at 24 mph / 38 kph.
Motor sounds healthier now and jumps up onto plane much more quickly.

Are fuel pumps a common problem with these motors?
I think mine is starting to fail.....

 

[Stumpalump]

Siguz » 28 Jun 2014, 10:17[/url]"]

Are fuel pumps a common problem with these motors?
I think mine is starting to fail.....

If I have this right you basically put a 25hp intake on a 20 hp motor and you are starving for fuel.?.
The 25 may have the same carb or carbs but your main jets are sized for 20 Me thinks....You may be starving for fuel because the main jets are too small causing a lean condition. Swapping jets and putting a fuel pump rebuild kit are simple. The book may even list a different part number for the jets. You let in a lot more air and no more fuel unless I missed that in this long thread. Did you squeeze the bulb when it acted up?