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Thread: Evinrude 31.8ci Mod Advice

  1. #61
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    Have you tried to add a spacer between the reed plate and the inlet?
    I havent tried it but I would if I was experimenting with a 31ci

  2. #62
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    Yes we added a 3/8 spacer, to increase the intake side before the reeds and the results did not show any power gain.

  3. #63
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    Has anyone on here ever fabricated a V block reed set-up for the 31.8ci?

  4. #64
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    Quote Originally Posted by Emustard View Post
    Motor still runs great, donít use it as much, but works well during hunting season. As long as you can get the motor up, you should have no problem turning that 16pitch
    Did your cleaver slip like hell when trying to plane the boat off. All mine does is slip and the motor goes up to like 5000rpms and I've gotta practically run to the front of the boat to get it to finally bite. I've tried it with the motor jacked all the way up and down. Also tried different pin positions. Just wondering if this is normal

  5. #65
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    Has anyone used a factory test wheel to test their motors after modifying?
    I have wiseco 0.030” pistons in mine and a planed head. 30 deg btc and turn the test wheel 6650rpm.
    On my boat with a 10-3/8 x 18 mercury chopper, I run 44mph at 6000rpm. Similar results with a signature 10x18 cleaver.
    The 5 blade 10x18 cleaver turns 5500 rpm and runs 40mph

    Smitherik33, try making a larger cavitation plate like an oz plate.

  6. #66
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    The factory test wheel...

    I have an '81 35hp commercial engine that I recently picked up. I can tell the power head has been removed so I have no idea what all has been done to it.

    I do know it has the 1-3/8 dia carb that has the 1-1/8 venturi. It also has a power pack with no Rev limiter and very good compression.

    I ran it the other day in a 35 gallon barrel full of water with the factory test wheel.

    I let off the throttle at 7850 per the tiny tach in the interest of keeping the rods inside the block.

    Either I am using the wheel wrong, or it is not meant to hold the engine back much at WOT.

    Take that for what it is worth.

    Rob


    Sent from my SAMSUNG-SM-G900A using Tapatalk

  7. #67
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    Name:  652E3412-11AF-4C5B-97B0-569D40EF8BAE.jpg
Views: 332
Size:  195.9 KBHere is a pic of my test wheel. 6650rpm is at the ramp with the cab plate fully submerged.
    Running in a small drum may have a different effect. Or maybe you have a very powerful motor.
    Likes hydrospeed77 liked this post

  8. #68
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    Nothing special about my $100 motor other than cheap.

    The picture tells the tale. The wheel I have does not have that tube-like feature.
    Mine is what is called for in my service manual though.

    When I finish the deck on this boat I will run it on the ramp and see if more water volume helps.

    Thanks for the picture.

    Rob

    Sent from my SAMSUNG-SM-G900A using Tapatalk

  9. #69
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    Quote Originally Posted by smitherik33 View Post
    Has anyone on here ever fabricated a V block reed set-up for the 31.8ci?
    I have had a V block in one hand & a crank case in the other, looking at the possibilities & intend to do it. I know serious gain can be found there from a failed reed experiment. bending the dampers out too far will result in a broken reed however till then I had the best result I've seen since putting a gear case & prop on. It's there for the taking but the shapes don't cooperate. I think the answer lies in an extended intake but that will increase crank case volume which could result in loss velocity -defeating the purpose.

  10. #70
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    Here are a couple of heads I have. The top heade is from a 1982 30hp. The bottom is from a 1988 30hp.
    When I started with the 1982 head, I got 115psi compression. The other head had 120psi compression. After planing the 1982 head and making room for the piston, I got 165psi. I got 155psi out of the 1988 head. Another mod I did to the 1988 head was blocking the water off from going on to the exhaust. The epoxy can be seen in the lower water passage.
    I put 4 tubes in the water jacket cover to dump the water overboard.
    Name:  588CFB6F-F1D0-416B-A738-AA0AB1E2ECDF.jpg
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    Name:  F39359E4-E9B7-4232-ADB8-21B2B9D7F44E.jpg
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