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Thread: 20,25,30 and 35hp mods

  1. #1
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    Default 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.
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    e-tec?

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    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.
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    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.
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    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.
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  6. #6
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    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. Name:  19A9A3E7-95C3-4208-B85F-2C81ACAE0C91.jpg
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  7. #7
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    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. Name:  C6D03178-B25A-4A6B-9DA7-DD6C24025CC9.jpg
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    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. Name:  9D1C01D0-0B8F-4985-8D4E-CEE7C548633F.jpg
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