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  1. #1
    Budman's Avatar
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    Top speed not where it should be

    Here is what has been done. Carbs rejetted by Bill, Cylinders ported by Tim Judge. R&D reed stuffers. Riva return flow restricter. Riva flame arrestors. Pro-x pistons and rings. Head milled by Riva. Riva primer with chokes removed. Premix with amsoil. Solas x prop. R&D intake grate, Riva ride plate. The hole shot is incredible. Everyone comments about the great power off the line. But the top speed is only 50.9 (GPS) What could be wrong? I don't have an aftermarket tach but the factory tach says it is topping out around 6700.


  2. #2
    Old Ski Insanity w/rpm500's Avatar
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    just info gathering..looking at the whole picture if we can..
    whats your compression at now?
    how is your pump in reguards to ring wear and hub condition..?
    impeller pitch factory or been tweaked for holeshot? did you check it??
    have you set the carbs up for your elevation?
    any modification to your exhaust??

  3. #3
    Flying Scotsman's Avatar
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    How many washers or how thick a spacer are you using on the ride plate.If you shim the front of the plate you are going to run a lot slower.Shim the front of the plate not the back. check the pitch of the impeller and as well you could run 50 mph on a ported motor on 2 cyl.Make sure nothing is draging under the hull like trim tabs intake grate etc.A ported motor should be high 60's easy.

  4. #4
    Yamaha artisan Cutlass's Avatar
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    The X prop is 13-16 pitch. that probably part of the problem. Do you have any other props you can try out?

  5. #5
    Budman's Avatar
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    Quote Originally Posted by w/rpm500 View Post
    just info gathering..looking at the whole picture if we can..
    whats your compression at now?
    how is your pump in reguards to ring wear and hub condition..?
    impeller pitch factory or been tweaked for holeshot? did you check it??
    have you set the carbs up for your elevation?
    any modification to your exhaust??
    The wear ring and hub look fine. The impeller is YD-SC-XO(14.5-17.5) The elevation thing could be something. I haven't had a chance to check the compression.

  6. #6
    Budman's Avatar
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    Quote Originally Posted by Flying Scotsman View Post
    How many washers or how thick a spacer are you using on the ride plate.If you shim the front of the plate you are going to run a lot slower.Shim the front of the plate not the back. check the pitch of the impeller and as well you could run 50 mph on a ported motor on 2 cyl.Make sure nothing is draging under the hull like trim tabs intake grate etc.A ported motor should be high 60's easy.
    I have one washer on the front to make it flush and two washers on the back to help with porpoising. Grate, plate and tabs look fine.

  7. #7
    Budman's Avatar
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    There is good power for the first 2/3 throttle. The last third gives me no extra speed. The elevation is 5200 feet. At that elevation should I lean the settings on the high speed needles.

  8. #8
    quadzilla's Avatar
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    ?

    Hey Budman..

    Somethings definitely Up with your setup..

    Some solid suggestions from the old schoolers in here..

    From what you've posted so far..
    I'd personally be suspecting..

    1/. Something hanging low or being pulled down at speed..?

    2/. A faulty Riva CDi breaking down under load..?

    3/. A spark plug going bad under load possibly a lead shorting to earth.?

    4/. The fact there is nothing more after 2/3rds throttle suggests the high speed circuits in the carbs to be very rich.?

    5/. Need confirmation on the prop an X @ 13-16 will be hard into the limiter.. (incidently what is your limiter set at.?)

    As Scotty says,
    A ported motor with a bump in compression should be running 65+ easy..??

    Can you do a compression test.? to confirm all is well in the motor. also examine the plugs while you're there.!
    Would also be interested what Bill has set the carbs at..?? (as my own jetting defies logic )

    Gerry

  9. #9
    Flying Scotsman's Avatar
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    Take the shims out of the rear ride plate bolt's.Your pushing the hull into the water and causing more drag.

  10. #10
    Budman's Avatar
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    I have the stock cdi. I bought a riva on ebay but it was bad so I sent it back to the seller. I will check the compression and get back to you. I was told by the judge that I may loose some compression with porting because of the over size exhaust port. He suggested an ADA head. The carbs have 105 low jets, 140 high jets, 115gr metering springs, 2.0 needles and seats. Highs and lows are 1.5 turns out.

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