Super Jet '96 SJ700 Porpoising

S

segmentlime

Location
near beautiful bobcaygeon
The following text belongs with the pics above:

I have a 1996 SJ700. This was my first stand up jet ski and I have had it for a few years - i traded a motorcycle for it. It had been modified for freestyle. I can get up and ride it around but i find that i can never go very fast as it seems that whenever it hooks up and i apply throttle it begins a cycle of porpoising with increasing magnitude. i have tried to apply more weight forward and while this may delay the onset of the porpoising action a little bit, it seems to begin no matter what i do (other than reduce throttle) and then just gets worse and worse. I am older and am not interested (or going to be ever able) to be taking much air. I have no desire to even attempt to try to flip it. What i mostly want to do is ride it around faster and then be able to take faster and harder corners. I wonder if i should be looking at changing either the ride plate (pump cover) or the grate. I am attaching pictures of these. In terms of the grate i wonder from the behavior and some other threads i read if it may be that the grate is too aggressive and pulling the front down until the buoyancy of the front pushes it up and then this builds in a cycle? So, i wonder about getting a milder grate? Also, I wonder about getting a more extended ride plate?

btw - the jetski had a trim kit installed which i will never make use of, so wondering if this would make any difference as regards the porpoising i have effectively disabled this by bolting a small piece of wood to the top of the nozzle frame which prevents the nozzle from being pulled up - this and tension in the cable hold the nozzle fixed in terms of up and down motion so only the side to side of steering is now allowed. This actually did not seem to make any difference in terms of the porpoising.

Please see the pics.



Any help / suggestions would be appreciated
 
nucleus_coolins

nucleus_coolins

Does things to makes the do goods
Location
Bay Area, California
Best answer is more ride time. If youre a heavy guy, a longer ride plate should help but its not necessary. When u shift your weight to counter the bouncing, dont just shift all your weight forward, try to constantly adjust your weight. Eventually youll get it to plane out real smooth and you wont need to shift weight as much.
The trim should not be an issue at all, remove that block of wood u put there. When the pressure of the jetstream flows thru your steerinh nozzle it will automatically straighten it out for you, think path of least resistance.
 
TaintlessEd

TaintlessEd

Location
Lake Elsinore
Depending on the style of trim I'll buy it from you and send you everything you'd need to make it like it was from the factory.
If you're interested in getting rid of it since it doesn't get used.


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S

segmentlime

Location
near beautiful bobcaygeon
Rushford_Ripper said:
You mean cavitation? Bad pump seal, worn wear ring, damaged impeller condition would all cause the cavitation issue.
Click to expand...
rushford_ripper - no, i do not mean cavitation. In fact pump and impeller etc. seem to be working great.

I mean a porpoising action whereby the ski, particularly the front part first moves lower in the water then moves or pops back up. The down / up motions occur with quite rapid frequency. This occurs in an occilating fashion with increasing magnitudes, and if i hold or increase throttle the motions continue increasing in size until the upward motions cause the ski to be so high that it breaks loose from the suction and takes some air into the impeller. I find it becomes increasingly hard to hang on. Unlike some situations where a bit more throttle helps ride it out, more throttle just seems to put more energy into the oscillations and increase size of bouncing

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S

segmentlime

Location
near beautiful bobcaygeon
TaintlessEd said:
Depending on the style of trim I'll buy it from you and send you everything you'd need to make it like it was from the factory.
If you're interested in getting rid of it since it doesn't get used.


Sent from my iPhone using Tapatalk
Click to expand...
I tried to pm you but didnt seem to be allowed to start convo. Maybe pm me and i can reply with pics of what i have?

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mthomps

mthomps

Location
Cleveland
segmentlime said:
rushford_ripper - no, i do not mean cavitation. In fact pump and impeller etc. seem to be working great.

I mean a porpoising action whereby the ski, particularly the front part first moves lower in the water then moves or pops back up. The down / up motions occur with quite rapid frequency. This occurs in an occilating fashion with increasing magnitudes, and if i hold or increase throttle the motions continue increasing in size until the upward motions cause the ski to be so high that it breaks loose from the suction and takes some air into the impeller. I find it becomes increasingly hard to hang on. Unlike some situations where a bit more throttle helps ride it out, more throttle just seems to put more energy into the oscillations and increase size of bouncing

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Click to expand...

In order to break that cycle where throttle increases the intensity of the hull bouncing you have to stop thinking of you're throttle as speed control and as your main source of control over your ski.

Your hull will follow your body language plus acceleration. So once you feel that bounce come on maybe you chop off the throttle, lean forward, and gas it hard. And then an instant later you might have to cut it quick again. Your finger might be moving rapidly but your ski might be cutting through the water and you're still gaining speed.
 
surfjunkie

surfjunkie

Location
deland fl
this and all the other threads regarding this are the same answer. more ride time is really all you need. use the money from chasing parts to buy gas and ride it and it will go away. you will learn how to control that with body english and moving your weight around.

i know i hopped all over the lake for a while. it just takes time.
 
Pro-pulsion

Pro-pulsion

Location
Ontario, Canada
It does appear to have a rather short ride plate, there is a very easy and simple way to help the situation quite a lot. I have the stock ride plate on my SuperJet and it is short enough to make the ski porpoise. I just removed the 2 rear plate bolts, loosened the front two to allow the plate to hang momentarily, add 2, 3, or 4 (however many you need to suit your preference) stainless washers in between the hull and ride plate at the two rear bolts and re-install the bolts (4 should be the maximum, you don't want to over-stress the front mounting holes with the new angle on the plate). This little bit of added angle will help quite a bit by pushing the nose down just enough. If you want a plate that will help keep the nose really planted and aid in aggressive cornering, consider the Worx Dominator extended plate. It has a little "D" cut out of the center but not at the end of the plate, this helps to act as a brake for cornering. It's not a great high speed straight line plate though, the drag from the D cutout makes the ski chine walk on a bit of an unpredictable nature. Mine made my ski also seek after a stretch and threw me off every time lol. I also shortened my Worx plate to get more into freeride applications but I still like the shimmed with washers stock plate more. I will use it until it breaks before I fall back to the Worx.
 
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