Stefan
Well-known member
My outboard is a 20 HP Johnson, 1989, 2 Stroke, Model # CJ20CRCEM
I did some research and found out that the 20 HP models from that vintage carry the same powerhead and gear casing as the 25, 30 and 35 HP.
Difference between the 20 HP and 30 / 35 HP are larger intake manifold and carbs.
Difference between the 20 HP and 25 HP is the intake manifold isn't restricted on the 25 HP. In other words, the 20 HP is the exact same engine as the 25 HP, including the carb and intake manifold, it's just that the intake manifold is restricted on the 20 HP.
I double checked this on BRP's (Bombardier who took over Johnson / Evinrude) website. By using the parts diagrams for the different sized engines I was able to verify this.
By boring out the intake manifold on my 20 HP motor, to match exactly to the opening of the carb, I gained an extra 5 HP for no funds spent.
How did I do this?
First thing I did is carefully remove the intake manifold and carb, which is pretty easy to do. Just take your time. Next, I found a bottle cap that matched the opening of the carb exactly (a peroxide bottle cap with a diameter of 1.25 inches). I then seated the carb onto the intake manifold exactly how it would seat if mounted on the engine, by lining up the bolt holes, and then I sprayed some white lithium grease through the carb opening which provided for an outline over the restrictor plate.
This gave me a reference point with which to place the peroxide bottle cap on the exposed intake manifold with the carb removed, allowing me to outline with a pencil the exact area on the intake manifold that needed to be bored out in order to allow unrestriced airflow. Next, I used a very small file to carefully file away to the edge of the pencil mark, filing in even strokes. This was a slow process. To clean up the edges I used a dremel tool in order to smooth out the new opening. It is very important at this point to use compressed air, a rag, or whatever thorough method to remove from the intake manifold any steel shillings from filing to ensure that they do not enter the cylinders. This is also a great opportunity to clean up your intake manifold, paint it, and give your carb a good wipe down / cleaning. Finally, I reinstalled the parts.
Results:
Before - with the restricted intake manifold my boat was bogged down, did not plane and topped out around 25 km/h. Even worst with a passenger. I tried shifting weight around, reducing weight on the boat and many other makeshifts solution to get the boat to plane and cruise better.
After - with the intake manifold now unrestricted my boat planes with ease and cruises along at about 43 km / h. Planes with a passenger and cruises along just a tad slower with the added weight. Increased air flow made my engine easier to start and it now idles better too. Next will be to get a prop with a bigger pitch to squeeze even more speed, as the extra power will be able to turn a higher pitch. I could not be happier with the results, and the best part is it was done for zero dollars expended, only about 2 hours of my time. I get to my fishing destination faster, which means I save fuel because the engine isn't running as much and it is pushing a boat that is on plane rather than bogged down. I tested this for the first time this past week and I am still grining ear to ear.
Other Considerations - I had originally intended to upgrade my engine by purchasing a 30 hp carb and intake manifold, which would have cost at least $200 if I could find good, used parts. However, by discovering that the intake manifold could be bored open I did not have to go the route of purchasing new hardware, while still achieving the desired result. This mod gave me just the extra power I required. If you have a 20 HP motor of this vintage, why not do the mod, even if you don't need the extra power? If you do need the extra power, this mod might achieve the same result for you as it did for me depending on the size of your boat.
Hope this helped. Good luck, happy boating and fishing.
I did some research and found out that the 20 HP models from that vintage carry the same powerhead and gear casing as the 25, 30 and 35 HP.
Difference between the 20 HP and 30 / 35 HP are larger intake manifold and carbs.
Difference between the 20 HP and 25 HP is the intake manifold isn't restricted on the 25 HP. In other words, the 20 HP is the exact same engine as the 25 HP, including the carb and intake manifold, it's just that the intake manifold is restricted on the 20 HP.
I double checked this on BRP's (Bombardier who took over Johnson / Evinrude) website. By using the parts diagrams for the different sized engines I was able to verify this.
By boring out the intake manifold on my 20 HP motor, to match exactly to the opening of the carb, I gained an extra 5 HP for no funds spent.
How did I do this?
First thing I did is carefully remove the intake manifold and carb, which is pretty easy to do. Just take your time. Next, I found a bottle cap that matched the opening of the carb exactly (a peroxide bottle cap with a diameter of 1.25 inches). I then seated the carb onto the intake manifold exactly how it would seat if mounted on the engine, by lining up the bolt holes, and then I sprayed some white lithium grease through the carb opening which provided for an outline over the restrictor plate.
This gave me a reference point with which to place the peroxide bottle cap on the exposed intake manifold with the carb removed, allowing me to outline with a pencil the exact area on the intake manifold that needed to be bored out in order to allow unrestriced airflow. Next, I used a very small file to carefully file away to the edge of the pencil mark, filing in even strokes. This was a slow process. To clean up the edges I used a dremel tool in order to smooth out the new opening. It is very important at this point to use compressed air, a rag, or whatever thorough method to remove from the intake manifold any steel shillings from filing to ensure that they do not enter the cylinders. This is also a great opportunity to clean up your intake manifold, paint it, and give your carb a good wipe down / cleaning. Finally, I reinstalled the parts.
Results:
Before - with the restricted intake manifold my boat was bogged down, did not plane and topped out around 25 km/h. Even worst with a passenger. I tried shifting weight around, reducing weight on the boat and many other makeshifts solution to get the boat to plane and cruise better.
After - with the intake manifold now unrestricted my boat planes with ease and cruises along at about 43 km / h. Planes with a passenger and cruises along just a tad slower with the added weight. Increased air flow made my engine easier to start and it now idles better too. Next will be to get a prop with a bigger pitch to squeeze even more speed, as the extra power will be able to turn a higher pitch. I could not be happier with the results, and the best part is it was done for zero dollars expended, only about 2 hours of my time. I get to my fishing destination faster, which means I save fuel because the engine isn't running as much and it is pushing a boat that is on plane rather than bogged down. I tested this for the first time this past week and I am still grining ear to ear.
Other Considerations - I had originally intended to upgrade my engine by purchasing a 30 hp carb and intake manifold, which would have cost at least $200 if I could find good, used parts. However, by discovering that the intake manifold could be bored open I did not have to go the route of purchasing new hardware, while still achieving the desired result. This mod gave me just the extra power I required. If you have a 20 HP motor of this vintage, why not do the mod, even if you don't need the extra power? If you do need the extra power, this mod might achieve the same result for you as it did for me depending on the size of your boat.
Hope this helped. Good luck, happy boating and fishing.