20,25,30 and 35hp mods

I know there are already some threads on here about modifying 20-35hp outboards. Just thought I would start a new one.

Here are some pics of a Johnson/Evinrude 3 cylinder 25/35hp I am modifying. Mine is a 25hp block. I hope to find a 35hp. I am starting with shortening the midsection 3-3/4”. It was a little trickier to cut down than the J/E 31.8 cu.in. towers I have cut down. The exhaust pipe and water passage are cast in on the triples. I haven’t yet figured out how to handle the water yet. I want the tower to look manufactured, so an external water pipe is out of the question.
The new motor will be run on my 11’ runabout. The current motor is a J/E 31.8 cu.in., bored 0.040” with wiseco pistons and a milled head, making 160psi. It runs 49+ mph. I still have some tweaking to do with that motor. Hoping to run 50+ mph next summer.

e-tec?

The 25 & 35 blocks are the same, 25 has thicker sleeve, smaller bore. You can bore out your 25 and install 35 pistons. Be sure to jet carb for 35. Glue joints in ports, especially finger ports, can be bad and block flow, be sure to clean them out. Make every effort to maintain complete exhaust system, it's more complicated than it looks. Lots of development in that system. If you decide to cut head be sure to maintain combustion chamber shape and min. squish of 0.030. Your going to get a lot more power than with your 31.8 twin.

No it’s not an etec. It is a 1996-2001 evinrude 25hp 3 cylinder.

That’s great information, thank you. I am concerned about what the reduction in volume will do to the exhaust tuning. I have not changed the pipe length. But I did open up the cavity where the relief hole at the top of the pipe bleeds into. It just happened that, that’s where I cut the tower.
I thought I read in another thread that the port timing was a little different between the 25 and 35. I will definitely clean up the ports and check the squish on the head when I get there.
Here is a pic of my boat. Just your average everyday 11’ plywood runabout. With power trim, 2” travel lift and trim tabs.
Look closely and you will see the midsection is shortened 4” and has open exhaust. I will try to find a better pic. The lower unit has 2.15:1 ratio. The water is at the top of the bullet now.
Attachment 72342

Got the midsection welded and ground today. I am not going to use the stock lower cowl, so I ground off a lot of extra material.
Attachment 72433
Attachment 72432

I have pulled the sleeves out of the block to make it easier to massage the ports. The glue lines from the lost foam process, obstruct the flow as nerstrom pointed out in another thread. I will probably bore the sleeves to the standard 35hp dimension. Attachment 72503

I am making some modifications to the lower unit as well. I may have the exhaust exit above the cavitation plate as I have done on a two cylinder motor.
Attachment 72504
I will also be blocking off the stock water intake and making new holes at the front of the bullet, as I have done before. Or blocking off the carrier bearing at the rear if I go with above cav plate exhaust
Attachment 72507

I will be blocking off the stock water pickup in the lower unit and adding holes at the front as I have done on my two cylinder. Attachment 72508
Or I may block off the back of the carrier and have water inlet holes in the bottom or sides, if I go with above cav plate exhaust. Attachment 72509

I have been working on the lower unit. Here are some pics.
I blocked off the exhaust passage at the cav plate. Now the exhaust will exit above the cav plate instead of through the hub. I used some devcon aluminum putty to build up the nose cone a little. I think it will be pretty and loud, and hopefully fast. I see there are some signature 10-1/2x20 choppers and 10-1/2x22 cleavers available. I would really like to see a 10-1/2x20 cleaver.
I can turn my current signature 10-1/2x18 cleavers and choppers at 6500 with my 30hp twin at 47-49mph.
Hope to run in the low 50’s with this motor.

what an awesome looking little boat

Arltralite111, If you are looking for a 35hp powerhead, you might give Tom Booth a call at Riverside Marine in Little Rock Arkansas. The location may not be feasible for you. That's where I got my 35hp Powerhead to replace my original 25hp.

Nice horns, speedfreak. I found a 35 power head. I measured the ports. The transfers and fingers are the same on both motors. The exhaust is 0.070” higher on the 35hp.

Quote:

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Originally Posted by Arltralite111

Nice horns, speedfreak. I found a 35 power head. I measured the ports. The transfers and fingers are the same on both motors. The exhaust is 0.070” higher on the 35hp.

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Are you talking about the ports on the 31.8 motors or on the three cylinder?

I have some older (76-94) motors in both 25 and 35hp and Im trying to piece together the internal differences between the two.

Arltralite111, How is your 35 triple build coming along? I have one, interested to see what you have learned and did for performance.

Has anyone figured out how to get around the rev limiter (set at 6200 RPM) on these little 1996-2002 25/35 3 cylinder 2 stroke mirage motors. CDI does not make an aftermarket power pack with a rev. limiter delete. Nor, can they delete it from the factory power pack if you send it to them.

Quote:

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Originally Posted by speedfreak

Has anyone figured out how to get around the rev limiter (set at 6200 RPM) on these little 1996-2002 25/35 3 cylinder 2 stroke mirage motors. CDI does not make an aftermarket power pack with a rev. limiter delete. Nor, can they delete it from the factory power pack if you send it to them.

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Does the ignition work like a 49/56 3 cyl, then you have unlimited packs.

Powerabout,
I don't think so. I did talk with CDI tech (Clark). He said something about "cut the orange wire and the orange wire with a black strip on it and replace the feed to the orange wire with a 12V source", or something like that. I have to dig into it a little deeper before I go cutting wires on an almost $450+ power pack. I would like to keep the "quick start" function if I can, it's a part of that loop somehow. Electronics is not my thing.

What kind of compression can these little Mirage 35hp 3 cylinders handle? Can they handle 170? I'm going to have the head cut/milled on mine to increase compression as I hear they respond well to increase in compression. Just do not want to go too far. My last compression readings/checks were: Top 145 psi, middle 147 psi and bottom cylinder 150 psi. I have non-ethanol 93 octane pump gas available where I live.