Results 1 to 6 of 6
  1. #1

    1998 GP800 cavitation and porpoising.

    Hey, first post just after some help.
    i have a bone stock 98 GP800 except wave eater clips.
    it porpoises a bit with two people and gets worse to the point where the jet pump comes out of the water, I am going to put washers under the two back bolts on the ride plate and the sponsons are completely down, should this help?
    and also when I accelerate quite hard out of the hole it seems to cavitate above around 6000rpm and with two people on the ski and towing a tube it takes a while to get on a plane, not sure if this is normal, if not what could I do to help it, would a new impellor with a different pitch help?
    thanks heaps in advance!

  2. #2
    redneck71's Avatar
    Join Date
    Sep 2011
    middletown DE
    You can help the porposing by adding shims to the back of the ride plate and running the trim in the full nose down position this will scrub off some of your top speed.

    On my 2000 I went with a longer R&D ride plate with some shims and intake grate which helped out a lot.I was actually able to go one notch up toward neutral on the trim.

    Trim tabs-your hull being a 98 I believe is different in the resessed area at the back of the hull on either side of the ride plate.Some people have had custom trim tabs made and installed here to help out the porposing.I went with some tabs that protect-tec sells that helped out enough to be able to take a shim out of my ride plate and I can run with the trim in neutral.

    Cavitation-It might be due to a large clearance between the impeller and wear ring. The impellers seem to be a compromise on these and aren't the best form the factory my wife's XL800 was horrible and would hit the rev-limiter while trying to get up on plane. I went with a new impeller housing with the changeable plastic wear ring and the recommended solas impeller for that ski and it is night and day difference and there is almost no cavitation when get on a plane with full throttle.

    The GP's aren't the best 2 up ski's

    The impeller clearance is .013"-.016" with a max of .024" mines .026" ill get a new wear and impeller one day.

    Impros make some application specific impellers for this ski for not much more than the Solas might be worth checking out
    Attached Files Attached Files

  3. #3
    Thank you so much, it's good to have someone with your knowledge give me some info.
    thanks again!

  4. #4
    I have had a look at the impeller and the clearance seems to be good but looking at the impeller it is a bit rough the leading edges look to be eaten away, not sure if this is normal. I have attached photos if you could see what you think.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	image.jpg 
Views:	58 
Size:	21.5 KB 
ID:	349096   Click image for larger version. 

Name:	image.jpg 
Views:	63 
Size:	20.1 KB 
ID:	349095  

  5. #5
    Banned User
    Join Date
    Sep 2014
    Get rid of that factory intake grill, that's most of your problems

  6. #6
    Myself's Avatar
    Join Date
    May 2007
    North Central Arkansas
    Your cavitation and extra time to get on plane are definately from that impeller, that thing is beat! It looks too far gone to fix. Get a nice Skat or Solas. A scoop type intake grate will help keep it down in the water. Also add 2 washers under the rear of the ride plate to shim it down or get the extended one. Our '99 rides way better with the scoop grate and long ride plate.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Two GP800 Question: Oil block off and porpoising
    By the destroyer in forum Yamaha Old School Skis
    Replies: 18
    Last Post: 04-13-2013, 06:44 AM
  2. 1998 GP800 - Porpoising/Trim Tab Kit??? Which kit
    By jchale in forum Yamaha Old School Skis
    Replies: 38
    Last Post: 04-24-2009, 02:11 PM
  3. 1998 seadoo xp limited, cavitation and plaining out?
    By JBradshaw00 in forum 2-Stroke Performance
    Replies: 7
    Last Post: 07-06-2008, 09:54 PM
  4. 1998 GP800/GP1200 Carb Rebuild
    By DTV in forum Yamaha Old School Skis
    Replies: 7
    Last Post: 06-29-2007, 12:32 AM
  5. Cavitation and that dam seal..
    By Water4fire in forum 4-Tec Performance
    Replies: 6
    Last Post: 08-02-2006, 11:04 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts