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  1. #1

    testing waters 787 reed valve conversion post thoughts or finds

    testing waters on this subject. its been brought up before but no real pictures or info that i have found. Only thinking about doing this to trashed cases. and i have a few of them. ones that ate rotary valve and scored or corroded surfaces. But what size reed block/ valves would be sufficient for a 787? looking at the yamaha 800 and the seadoo 951 the reed blocks are huge. this certainly wouldn't fit in or on a 787 case without a lot of machining, welding and fabrication. If a 4 petal reed block can be found or machined would that be sufficient? and would putting it in the intake manifold/ modded rotary cover cause any negative problems like changing volume of the crank case because of the added length of the intake manifold to hold the reeds? Im looking at either dirtbike reedblocks. Its just different on how small the intake port is on a rotary valve motor only about 40mm but how big it is on a reed valve motor of the same displacement. In the readiong of when some race teams have converted rotary valve to reed valve that a rotary valve powered ski won the race that year. but other than that there is no backing up that the motor didn't make power and it was just a bad rider or everything wasn't just right to win.

    Im only wanting to do this to trashed cases so its worth a shot. please post some real finds on the subject, opinions welcome also. but hey its 2 stroke performance just trying something different. it may work or might be a complete failure. still worth trying.

  2. #2

  3. #3
    awesome, exactly what i was looking for. seems that rotax rotary valve motors cylinders , transfer ports, and intake ports were all developed together. not improving one or another but everything as a whole. seems like the reed valve conversion with a bolt on rotary valve cover that holds reeds and feeds stock round ports doesn't work too well. for reasons of intake port shape and cylinder sleeve height in the case. thats what i got out of it, BUT... if using say yamaha 800, 1200, or even 951 size reeds and blocks with the cases welded to accept them it works better and it was banned after that year by the rules.

    to me the port shape being open and free from restriction of the reed cage on a rotary valve motor is a little bit of a benefit, but what is the drawback of the rotary valve? from a different spectrum why is it a must for any seadoo rotax pipe oem or aftermarket to have some sort of water injection to have any bottom end power? when on a yamaha 701-1200 pv or kawasaki 650-750 the pipes can be run with or without water injection and not suffer like a sea doo pipe would without water injection?
    seems some of the only differences between these motors are the difference in intakes? rotary valve vs reed valve. I know ports heights, shapesm ect are different but basing it on engine cc's.

  4. #4
    I can see the difference in intakes, but I can also see SeaDoo gets WAY more HP per CC. The 787 is closer in power to the yamaha 1200, and kawi 1100. The 787 will take to mods well, too. I think there are easier ways to get more power out of it. Check GroupK's write up. My friend has a 787 96xp carbon fiber watercraft that will kill anything on the water.

  5. #5

    Join Date
    Apr 2006
    panama city beach 'fl
    have you tried a set of ported cases?

  6. #6
    my cases and cylinders are going to be sent to either perry performance, group k, or judge motorsports. no i have never personally ridden a modified seadoo with port work done, just customers stock ski's i fix and my own ive done simple prop, carbs, and pipe.
    the reed valve conversion was a thought for sets of cases we have that are either siezed because of flushing wrong, or cases gouged from blowing up motor and pieces going through the rotary valve gouging the rotary valve surface. Sure they can be welded and remachined which is what im doing on the set of rfi cases im building for my 787 hx.

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