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Hey everyone!

I bought this old 1988 Yamaha Waverunner 500 just for the kids to boot around the lake. Runs great but not really familiar with pwc's in general ( more an atv guy ) So thing runs great when the lake is smooth but any sort of wave you hit ( even small ) the engine just revs up and you lose momentum. You just have to let off the throttle for a second and then go again. To me, it seems like the system is losing pressure but it's not like it's being jumped in the air or over wakes or anything....doesn't seem like the prop is coming out of the water at all. We have a 1993 650 as well and it doesn't do this. Any idea on what could be causing this ? Or if anyone has had one of these machines is that just the way they are ?

Thanks for any help/info!
 

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From your description, it sounds like your experiencing cavitation. This could be caused by air getting into the jet pump. Or excess impeller to wear ring gap. From idle, if you go full throttle, does the rpm go up but not speed? Like as if a clutch is slipping. If so this is cavitation.
 

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Thanks for the reply and help Dave! If the lake is calm I can go from stand still to full throttle/full speed no problem. It's as soon as you hit any sort of bump or wave it's like it goes into neutral ( obviously there is no neutral on these things ) and the engine over revs but you don't go anywhere. Soon as you let off the throttle and go again your fine, until the next wave. Was reading into cavitation and I suppose might be worth looking into the wear ring and propellor, the back end of it was full of rocks and sand when I got it so maybe that damaged some of those components. Thanks again for the help and advice, much appreciated!
 

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It may still be cavitation caused by air infiltration from the front of the intake area. It looks to have a very long intake grate where air could infiltrate, based on what I see in the parts schematic for that model. See OEM parts @ babbittsonline or partzilla.
 

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Definitly cavitation, these machines have impellers not propellers and the wear around the outer edges of the impeller to pump housing is very important as they are a pump.
Try pumping up a tyre with a worn plunger and you will get the idea, most of the air will go around the plunger rather than get pushed into the tyre, well the water in the Jet pump works the same if there is to much wear the water won't flow correctly and just gets beaten up like mixing cream :) the more you allow it to cavitate the worse the wear will get as the heat caused by the cavitation will do more damage.
I had the same problem on my first 440 Kawasaki Pole Ski, was fine on the flat but when you tried to do anything that needed pump action it just went like my old Datsun 1600 up a hill with a slipping clutch, sounded like it was doing something but barely moving :), becaue I was young I thought the wheels were spinning and so no traction and all I was doing was making it worse :).
The 440 was when I learnt about Jet Pumps as I knew about propellers from the Ski boat.
One of the best ways to bugger up a Jet Pump is coming into shore full tilt as the sand going through a jet pump is just like sand blasting it, I won't let anyone riding my Sho come into shore under power, as the sand and shells or rocks not only can destroy a pump the sand can fill the water jacket of the engine.
 
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