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  1. #1
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    31.8 Cross Flow Info Needed!

    Hey guys, hoping some of you have some good info on the old OMC cross flow 2 cylinder! I'm putting together what will hopefully be a "fast" jon boat...

    Any input on what parts are best? I'm going to be running the late model "split" housing lower unit. Best props available? Any info on interchange on these is appreciated as well as part numbers.

    I have built a few of these, for the next one I'm looking for a late model "bridgeport" case!

  2. #2
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    Well, I got my first test motor broken in on my 16' console steer MonArk. Stock '79 35hp did a best of 24 mph with me only, 21 with passenger. New motor does 30 with me/26 with passenger and same prop as before. 34 mph with me and a different prop. I guess I figured out what to do with it...

  3. #3
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    Any details on the build? I've got a couple of mid-90's "20hp" Johnsons. Both with late model 30hp carbs + intakes for now. I haven't gotten around to having the heads shaved (by .040-.060, haven't decided yet) or CCMS reeds installed but they're already substantially faster than my "benchmark", a stock, 120/120psi '78 Evinrude 35 I've had for a long time.

    Aside from similar interests as OP like prop choice, I've thought about experimenting with a V4 crossflow dual barrel carb, on a flange welded on a cut intake manifold.

  4. #4
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    Get your hands on a large bore 35hp carb and intake. Pick up a 35hp Advance cam for the mag plate, and check your timing. CCMS sport reeds, and check your heads. Some heads had 2 "fillers" in the Chambers while others only had it on one side of the combustion chamber. Run good premium non-ethanol fuel. Block off the top 1/3 to 1/2 of the water intake, and mount it as high as possible without loosin water pressure. But first and foremost, get in touch with Ron Hill Propellers for a large diameter prop. He makes a killer over the hub cleaver for these engines. The standard diameter props start loosing bite as you start elevating the engine height. These rigs are VERY sensitive to weight and weight distribution. Work on your setup/prop selection, as this is where your biggest gain will be.

  5. #5
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    Quote Originally Posted by cdubya View Post
    Get your hands on a large bore 35hp carb and intake. Pick up a 35hp Advance cam for the mag plate, and check your timing.
    As a sidenote and a personal observation, propshaft rated '84+ 30hp carbs are slightly larger than 70's 35hp ones. The transition may have happened in early 80's before rating system changed from crank to prop hp so some 35 carbs may well be identical to 30:s.

    Intakes are the same. Larger volume, intake hole and surface area against the reed plate than on 20/25hp. Using a 20/25hp intake w/ enlarged hole works in a pinch, I haven't noticed a difference but on the other hand I don't believe OMC would have used two different designs without a reason.

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