# 25 to 35 hp Johnson Powerhead



## rktman (Aug 25, 2014)

Has anyone replaced their 25 hp powerhead with a 35 hp unit on a 2000 Johnson motor (jet). 
I'm think of doing the swap if everything fits well.
I have the larger carbs already on it.

Thanks


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## Pappy (Aug 25, 2014)

If you are running the 25hp jet then it probably already has the 35hp powerhead on it. Jet ratings are very low compared to their "propped" counterparts.


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## rktman (Aug 26, 2014)

I think this older model is rated at the motor, not the jet so its really a 25/18.


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## airbornemike (Aug 26, 2014)

I've got an 2002 30hp johnson jet, I want to have mine built after the fishing season winds down. How did changing the carbs help?

I plan on having someone do the work for me, porting, carb reeds......if I could get 10 mph more I would be happy. Let us know how it works out for you. 


[url=https://www.tinboats.net/forum/viewtopic.php?p=364412#p364412 said:


> rktman » Yesterday, 12:26[/url]"]Has anyone replaced their 25 hp powerhead with a 35 hp unit on a 2000 Johnson motor (jet).
> I'm think of doing the swap if everything fits well.
> I have the larger carbs already on it.
> 
> Thanks


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## Pappy (Aug 26, 2014)

Not in model year 2000. Engines were rated at the propeller or in your case the jet unit. 
You would have 18 on the cowling if it were the 25/18 jet.
In your model year the 35hp (outboard)was a 3-cylinder. 
Yours is the32 cu. in. 2-cylinder cross-flow.
For your engine you already had the correct 30hp cylinder and crankcase and the large intake manifold. Same as the 30hp.


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## rktman (Aug 26, 2014)

[url=https://www.tinboats.net/forum/viewtopic.php?p=364498#p364498 said:


> Pappy » Today, 09:19[/url]"]Not in model year 2000. Engines were rated at the propeller or in your case the jet unit.
> You would have 18 on the cowling if it were the 25/18 jet.
> In your model year the 35hp (outboard)was a 3-cylinder.
> Yours is the32 cu. in. 2-cylinder cross-flow.
> For your engine you already had the correct 30hp cylinder and crankcase and the large intake manifold. Same as the 30hp.



Mine is the 3 cylinder looper 30.5 c.i., 25 hp 10/1999 on the engine date tag...(had to look again)


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## rktman (Aug 26, 2014)

[url=https://www.tinboats.net/forum/viewtopic.php?p=364497#p364497 said:


> airbornemike » Today, 09:18[/url]"]I've got an 2002 30hp johnson jet, I want to have mine built after the fishing season winds down. How did changing the carbs help?
> 
> I plan on having someone do the work for me, porting, carb reeds......if I could get 10 mph more I would be happy. Let us know how it works out for you.
> 
> ...



The larger carbs dont really help on this model...


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## Pappy (Aug 26, 2014)

...............so much for the parts book helping me out...lol

Okay....now where were we.
You have the 500CC version which was the (outboard) 25hp. 
If you can find the 565CC 35hp powerhead you will definitely gain some. 
I have several of these 3-cylinder engines and several sets of carbs. 

Incidentally, the reason why the carbs did not help out is that the venture size on the two sets of carbs is identical and the 35hp carbs have larger jets so there would be no gain by going to them. The larger cu.in. 35hp took advantage of those venturis a little better than the smaller one did.


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## Boat2fast (Sep 3, 2014)

I had a 2001 35hp 3 cylinder Johnson. It did a funny thing. The stock plastic fuel tank heated up in the hot sun and blew up nearly round. The pressure inside forced the gasoline through the fuel pump(s) into the oil tank. The gas forced the oil to overflow all over the ground and left the oil tank full of mainly gas. The color of the liquid in the oil tank was still dark enough green that you had to look twice to figure out it wasn't oil. I forget exactly what tipped me off but I discovered the whole boon-doggle before running the engine again...dodging an expensive bullet. The engine was quite new at the time.


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## rktman (Sep 3, 2014)

That is odd, perhaps that's why everyone says to remove the oil tank and run premix.

As an update, I decided to go ahead and try a power-head swap. I found one on ebay that is supposedly in good shape. I will swap it, do some test runs, and update everyone. Right now, with just me and the wife we're hitting 16 up and 19 down with the 25 jet on my 1648 blazer. Hopefully I can gain at least 3-4 mph to help it plane out better.


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## Pappy (Sep 3, 2014)

Take good care of that powerhead. There aren't many of these engines around. 
Run good oil and good gasoline. Since a jet runs at or near WOT all the time I would add additional oil as well.
Oil's cheap compared to powerheads.
You're lucky I didn't see that powerhead first. Would have converted one of my 25s over!


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## Boat2fast (Sep 3, 2014)

I have lots of ignition/charging system parts from one(or more) of those 3 cylinder engines. There's another story that goes with 'em. The box of parts is for sale $175 + shipping. The story is free....

The short version is: When testing the ignition parts, use the _factory_ test probe. Using a nail in place of a test probe opens up the connector terminal to where it won't make contact any more when the connectors are plugged back together. Can't tell either from outside looking in.

Last time I ever make that mistake.


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## rktman (Sep 4, 2014)

Thanks guys. I will certainly run a good premix.
I'm not a super marine mechanic but I do have the Johnson manual so it go fine.
BTW, are there any special tools required for a power head swap? I didn't notice any just skimming through the steps.

Boat2fast, send me a PM pic of your electronic goods, I might be interested.


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## rktman (Oct 22, 2014)

Quick update, I got the 35 hp ebay powerhead swapped into my case.
Everything bolted up fine. I started it (in a barrel) and it seems to idle fine, though I couldn't rev it up too much (water shower in my shop). Next step is to take it on open water. 
My carbs are the 35 hp version though I'm not sure how much adjustment needs to be done. Is there a good baseline setting to start with?
Also, is there any timing adjustment that needs to be made?
Thanks


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## Jim311 (Oct 23, 2014)

Roller on the timing plate should hit the arrow/notch in the plate right as the throttle butterfly starts to open. Baseline setting for the low speed screw is bottom out the screw, then back it off 1.5 turns. It's going to need more gas than that to run in a swift current without sneezing, though.


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## rktman (Oct 27, 2014)

Update: I took the boat out and tested it in the same spot on the river as before (though with a little more weight - son vs. wife). I got 19 mph upstream and 22 down which is an overall gain of 3 mph. Not too bad but I will try to get some more out of it through some carb tuning and maybe look at the timing. 
Note that with the jet, it feels like I have enough power but not enough power transfer at higher speeds. I'm wondering if a larger impeller would help (mine is the small one 5 3/4").
Hope this helps if someone is considering the swap.


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## shawnfish (Nov 2, 2014)

rktman said:


> Has anyone replaced their 25 hp powerhead with a 35 hp unit on a 2000 Johnson motor (jet).
> I'm think of doing the swap if everything fits well.
> I have the larger carbs already on it.
> 
> Thanks


not sure what hp's were available in 2000 but the 20,25,30 and 35hp motors all have the same powerhead and the different carbs and or manifold intakes dictate how much HP the powerhead produces for those models


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## rktman (Nov 6, 2014)

shawnfish said:


> rktman said:
> 
> 
> > Has anyone replaced their 25 hp powerhead with a 35 hp unit on a 2000 Johnson motor (jet).
> ...



Incorrect, you may be thinking of the 2 cyl model.
Thanks though.


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## Lil' Blue Rude (Nov 6, 2014)

Your motor has the AM model jet so it's a medium size pump running the 5 7/8 impeller in it. Next size up is a 6 1/8. Might pull it but I have my doubts.


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## rktman (Nov 7, 2014)

LBR, you're correct. I checked again and it is a 5 7/8" impeller.


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## shawnfish (Nov 9, 2014)

rktman said:


> shawnfish said:
> 
> 
> > rktman said:
> ...



sorry...I should have read the whole thread, didn't know you were talking about jet motors or 3cylinders.... and yes I was thinking 2 cyl models...


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## rktman (Apr 25, 2016)

Quick update for future readers. 
I swapped jet impellers from a 5 7/8" to a 6 1/8" and the 35 hp powerhead pulls it fine. 
I can run 25 mph with 2 ppl up or down.
With just me and a small amount of gear I can hit 28 mph down and 26 mph up on a slow hole of water.
Now I'm fighting the infamous 3 cyl coil pack issue.


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