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  1. #1
    JFizzleJR's Avatar
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    Observations after GPRXP seat time (input welcomed)

    After a few hours of riding on my completed conversion I thought I'd share a few things I've noticed. The chine walking on the GPRXP is very noticeable around speeds from plane to approx. 35 mph. Around 40-60 mph there is a horrible porpoise. However, at higher speeds it glides through the water great. After a good 5 minute warm-up the engine temperature is relatively easy to lock in around 160 degrees. After 20-30 minutes of riding though and some no-wake zones the temperature likes to jump around to 190-200. I don't like that one bit, but there's not much you can do about it if the idle zones aren't constant. Getting back on the throttle the temperature goes right back to 160-170.

    Overall I'd like to throw it out there that these skis aren't very rec. friendly. Some people assumed this, some people didn't. I guess my point is that these skis are not for everyone. Those of you with a lot of seat time please feel free to chime in with cures and procedures for solving any of the hiccups that I described on the GPRXP.


  2. #2
    EZ Dock of Long Island Shibby1485's Avatar
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    fizz... head bolts are IN the mail already

  3. #3
    Moderator RX951's Avatar
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    Good information Fizz.
    Question about the operating temp: what is the normal operating temp of the 4-tec stock with closed loop?

  4. #4
    JFizzleJR's Avatar
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    Thanks Steve! Let me know if you want the set of new exhaust valves. You've done more than enough. Thanks again. And your ski is pretty damn clean. I can't wait to hear some results from it.

  5. #5
    JFizzleJR's Avatar
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    Quote Originally Posted by RX951 View Post
    Good information Fizz.
    Question about the operating temp: what is the normal operating temp of the 4-tec stock with closed loop?
    I believe the thermostat operates so that the ski usually runs around 180 degrees. Riva's current and Rotax's old thermostat were 160/158 degrees respectively. The more current Rotax thermostat is around 130 degrees I believe, but is not recommended for rec. riding or the average user because it will shorten engine life.

  6. #6
    Moderator Insanediego Joe's Avatar
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    The spitfire has been very rec rider friendly....When we first built her it was quirky...after nearly 100 hrs the quirks have been worked out. We have found the XS intercooler is the best intercooler for the gprxps. It flows water through it like a garden hose.

    I installed my battery in the very back behind the waterbox with the intercooler on the otherside. I have not noticed any chine walk.

    Porpoising is directly related to ride plate angles and trim tabs shimmed or not. The looser your set up the more porpoise at low speeds will occur.

    I send my girlfriend out on the spitfire by herself. She prefers the gprxp over my huge 3 seater. FYI she is a huge baby!

  7. #7
    mrbtd's Avatar
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    That chine walk has nothing to do with the fact it is a GPRXP. Push your tabs down and it will help.

    Not sure what plate you have but the more power you are making the less angle you need to get the hull out of the water (If it is a Jims plate take a few minutes to call him).

    If you are running a 800 grate simply take it off and put on a 1200 grate.

    I am very curious/nervous about the cooling thing. I dont think its unique to the GPRXP platform though. What about the guys running open loop on RXP's ? From reading all of JD1's comments on the open loop he has ridden it everywhere without issue.

    You may be pleasently surprised to see what a few washers would do for your ski.

  8. #8
    Moderator The Bandit's Avatar
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    You have to dial it in Fizzy. Mine has the bouncies as well, but I have not had a chance to dial it in. Not sure if I should adjust the plate (Jims) or lower the tabs(no washers).
    When mine starts getting hot, I just open the valve a tad for more water.
    Having said all this, I have not ridden in 2 months. I have put on a total of 4 hrs on mine this year..
    I have not noticed any chine walking either BTW. My battery is in the front, and it is a tad nose heavy at idle with a full tank. A little throttle and the nose is in the sky FAST.
    Maybe this weekend....

  9. #9
    JFizzleJR's Avatar
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    Cool. Thanks guys! I know mine isn't dialed in, but speaking to another member of the GPRXP club that has his dialed in, he has the same porpoising at lower speeds. His top speed is good though. I'm looking for the culprit(s) behind missing RPMs so the hull will be last to tune in.
    Last edited by JFizzleJR; 08-01-2008 at 09:55 AM.

  10. #10
    mrbtd's Avatar
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    I definetely know what you mean on the bouncing in the 30-40 range but it can be stopped.

    Another thought is Nate's keel mod since this may be the speed it transitions off of the keel and onto the rideplate are.

    From all accounts - The Tampa crew down there knows how to get a GPR to handle.

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