Johnson 25 to 35 hp conversion

I recently got a 1980 Johnson

I recently got a 1980 Johnson that I would like to convert to a jet 35. Can you share your thoughts on the conversion to a 35? Is this just a carb change? Thanks!

"Hi David,
I'm pretty s

"Hi David,
I'm pretty sure the earlier 25s cannot be upgraded to 35hp, largely due to exhaust restrictions. The 35 had through-hub exhaust, while the 25hp did not. Simply dropping a 35hp carb on the 25 will likely result in poor running characteristics...

- Scott"

"David,

I don't think t

"David,

I don't think there is a jet kit available for a 1980 25 hp. I feel that if you're going jet, you are going to end up looking for a used one. It will likely be cheaper since the jet kits cost more than your 25 cost new, plus you can sell your 25 and use that towards the jet. I don't know how the market is on used, but in areas where they are popular, you should be able to find one.

Good luck
Rick"

"Sorry a big quote here: But

"Sorry a big quote here: But this is the thread that got me thinking it might be possible. Any other opinions or is Scott right on the money? Thanks.

Message


Charlie M.
Posted on Tuesday, February 25, 2003 - 11:51 am:

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As I am slightly short of power I would like to upgrade a 25 to 30 or 35. Weight and transom strength will not allow a 40. Any opinions?


Jon
Posted on Tuesday, February 25, 2003 - 12:12 pm:

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What's the boat rated for? Are you planing on buying new, or finding a particular used engine? An increase of 5 HP really won't mean much in performance, you're going to have to got to at least a 35 for any noticeable difference. If you have weight concerns, it's going to have to be a 2-stroke as well.

A few ideas:
Nissan 40 HP 2-stroke 3 cyl - 160 lbs
Mercury 40 HP 2-stroke 2 cyl - 175 lbs
Johnson 50 HP 2-stroke 2 cyl - 200lbs

The Johnson 50 HP is also available for the same weight in a 40 HP. There really are not any new 35 HP's out there, you could find one (mabye new or used), but I'd just worry about weight - if a 35 & 40 HP weigh the same, may as well get the extra 5 HP.

Jon


CharlieM.
Posted on Tuesday, February 25, 2003 - 02:21 pm:

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The boat is rated at 25hp. When you put on a Jet Pump, you loose about 30%, so 40 is over limit (28.8hp). 35 (24.5hp) would be great, but none are available new. That's why boosting a 25 (common motor) is in play. Thanks much!


Jupiterzeke (Jupiterzeke)
Posted on Tuesday, February 25, 2003 - 02:25 pm:

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Jon;If you put John/Rude same year 1990 35/30hp carb and intake manifold on your 25hp you have added that much horsepower as that is the only difference between a 25hp and a 35hp with no increase in weight! [email protected]


Johnnielund
Posted on Tuesday, February 25, 2003 - 03:27 pm:

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Charlie; I have a 1990 25 hp Johnson that the previous owner had converted to a 30 hp by a very reputable marina here in northern Illinois. I have a 16? Lund Explorer and am very happy with this boat/motor combination. According to Joe Reeves the 25 and 30 have the same block, cyl head, crankshaft, rods, pistons, leaf plate, leaf valves etc. The differences between the two are as follows:

The throat hole venturi on the on the carb and manifold is larger on the 30 than the 25.

30hp is hit at 5500 rpm. 25hp is hit at 5000 rpm.

The cam, which is attached to the magneto armature plate, is different. The 30hp cam is # 326876. The 25hp cam is # 334750.

The orifice at the extreme top of the carburetor is different. The 30hp is # 39 (.039) # 334568. The 25hp is # 30 (.030) # 317473.

The high speed jet, located at the center bottom portion of the float chamber is different. The 30hp is #65D (.065) #327541. The 25hp is #51 (.051) #331567.

I hope this information is helpful to you. It is my understanding that trying to increase hp is usually not a good idea, however I think in the case of this particular motor it works well.


CharlieM.
Posted on Wednesday, February 26, 2003 - 09:01 am:"

"Dave,

In later models, the

"Dave,

In later models, the 25 and 35 share the same mid-section and lower unit, but not so in 1980. I do feel there may still be a manifold and carb which will allow you to boost your hp, but it still does not change the fact that you can't buy a jet pump for your 1980 motor.

Sorry,
Rick"

"Thank you Rick! I won't

"Thank you Rick! I won't TOTALLY give up hope on the carb change, but I guess the jet is just not going to happen. Honestly, I bought it thinking it would!

Is it difficult to convert to an electric start? If you are getting the picture here, I need a winter project, and came across this motor CHEAP, and it runs well."

"Hi David,
I think if it we

"Hi David,
I think if it were mine, and it didn't fit my purposes, I'd clean it up, tune it up and sell or trade it towards what I did want. If it's in good shape to start with, then it'll have a fair amount of value left, even at 25 years old.
As for adding electric starting, you can probably find a kit at a dealer - for almost as much as the motor is worth. Better off finding a junker/clunker with the parts you need and transplanting them.
Things needed would be the starter motor, mounting bracket, flywheel (if yours doesn't have teeth already) solenoid, and related wiring. A nice bonus would be a charging system, which would be the charging armature plate and the rectifier.
Good luck!
- Scott"