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Thread: Can a reed surface be too large?

  1. #1
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    Default Can a reed surface be too large?

    Dear 2stroke enthusiasts,

    I am tunig a small 70ccm 1 cyl. outboard just for fun. I did change and test a lot of different things using my DIY hydraulic dyno. I'm happy with my progress till now. Certainly it becomes more difficult to raise the power any further without having the top peak at a to high rpm range.

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    One of my last tests was about using different reed cages.

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    Left is stock with 3 pcs steel reeds and a small surface. Middle is from a scooter with 2 glass fiber reeds and a larger surface. Right is for tuning cylinders for small semi road racers with 8 carbon fiber reeds and a huge surface.

    I tried to keep the crank volume for the right reed as small as i could but the throttle reponds compared between left and right reed cases was huge. The small one works agressive direct and the right one was more week and the peak power less. Due to that i had smaller ignition problems (as i found out later) i have to repeat the test runs to confirm.

    But my question in general: Can a reed surface be too large? What do you think?

    Oliver

  2. #2
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    YES.

    Years ago I was racing Kawasaki 125 based engines. I had the brilliant idea to use their 250 sized reed cage in my next motor. (Six pedal verses 8 pedal) Built a new billet crankcase and some real trick welded up aluminum intake manifolds over the winter. The motor was a dog. Tried all kinds of stuff and it was several mph off my good motor. Ended up filling in the crankcase with Marinetex epoxy and put a set of 125 reed cages in it with real short intake manifolds and it ran real well.

    Looking at your three reed cages, the stock one and the scooter one have way too much reed stop area. The one on the right looks like a 125 motocross sized cage. Huge jump from the two on the left. My vote would be to find a normal pyramid reed cage out of a 70 or 80 cc motorcycle, not a V Force and test it. Other option is to close off some of the reeds in your existing V Force cage.

    Years ago we did a lot of dyno work on 100cc Yamaha kart motors. We found that you need to get the carburetor as close to the reed cage as possible. It improves the "signal" to the carburetor.

    Good luck

    Michael
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  3. #3
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    Thank you for sharing your experience!

    There is a 10mm adapter only between carb and reed.

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    A very good idea to double check the surface influence by blocking reed by reed. I'll put this on my to do list.

    Oliver

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    Reed casing made from nylon platic for a quick test.

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  5. #5
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    Quote Originally Posted by Mike Schmidt View Post
    YES.

    Years ago I was racing Kawasaki 125 based engines. I had the brilliant idea to use their 250 sized reed cage in my next motor. (Six pedal verses 8 pedal) Built a new billet crankcase and some real trick welded up aluminum intake manifolds over the winter. The motor was a dog. Tried all kinds of stuff and it was several mph off my good motor. Ended up filling in the crankcase with Marinetex epoxy and put a set of 125 reed cages in it with real short intake manifolds and it ran real well.

    Looking at your three reed cages, the stock one and the scooter one have way too much reed stop area. The one on the right looks like a 125 motocross sized cage. Huge jump from the two on the left. My vote would be to find a normal pyramid reed cage out of a 70 or 80 cc motorcycle, not a V Force and test it. Other option is to close off some of the reeds in your existing V Force cage.

    Years ago we did a lot of dyno work on 100cc Yamaha kart motors. We found that you need to get the carburetor as close to the reed cage as possible. It improves the "signal" to the carburetor.

    Good luck

    Michael
    was the engine a dog due to the larger crankcase volume?

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