SX/SXi/SXi Pro Old guy, new ski, constant porpoising/cavitating?

[Speargun]

Back in the early 90's, I used to ride a JS (300 then a 550) every day and was a pretty proficient. Fast forward 35 years and an extra 35 pounds and I decided to try a stand up again.

I just picked up a couple of '95 750sxi's and, while I am definitely out of practice, I was able to get on my feet on the first try and handle some basic turns & wakes without getting thrown off. The problem that I'm running into is that any time I try anything more than that, the pump cavitates/comes out of the water and I'll lose all momentum.
For example: Takeoff from a dead stop - Squeeze throttle, pump cavitates, feather throttle, pull myself onto ski, pump cavitates, feather throttle, stand up, pump cavitates, etc... It kinda feels like the hull is too short for my 6'0" 225 and I keep leveraging the bow up, uncovering the intake.
I remember riding a "new" ski back in the 90's and not really liking the way it handled compared to the JS although I don't remember if uncovering the intake was an issue or not.

Do those of you with more recent experience on a JS & SX have any suggestions?
Am I doing something wrong?
Am I too big for the ski?

- Both skis have aftermarket poles, Jet Dynamics intakes, one has an Ocean Pro ride plate, the other has a Jet Dynamics ride plate as well as a few engine upgrades. (Single carb, Factory pipe)

Me back in 1992ish

 

[Storbeck]

Can't really speak for the kawi 750 hull other than saying it is notorious for porpoising according to reading on the internet, but I've never ridden one so I don't really know.

But as far as the cavitation I think you have some other problem than the hull, likely pump seal. I'm pretty much the same height and weight as you and have ridden many sxr's, x2's, and sx650's all of which were fine with no cavitation with the same pump. I've also ridden an sx300 that struggles a bit to get a guy our size on plane, but can do it and with a much shorter hull and smaller pump.

 

[kevbo]

You are too big for the old 750 hull. A shorter pole will help a lot. How bad is the cavitation, does the pump completely blow out? Does it just vibrate more than expected? Either way the pump is sucking air or has a lot of damage. Look at the driveshaft carrier, the rubber pump seal, and the grate. A mangled impeller will cause the same issues.

 

[WFO Speedracer]

225 shouldn't be too big for a 750 hull try different riding positions in the tray, most of the 650 -750SX hulls liked for you to be farther forward in the tray.

 

[Speargun]

225 shouldn't be too big for a 750 hull try different riding positions in the tray, most of the 650 -750SX hulls liked for you to be farther forward in the tray.

I'll have to try this.
IIRC, when riding the JS, my right heal would be right at the back of the tray and my left foot would be just in front of the halfway point with my weight centered just aft of center. If the SX likes more weight to the front, I'm definitely too far back which would explain why the front keeps wanting to come up and blow out the pump.
I'll have to look at a shorter pole. (didn't know there was such a thing until today)

I don't think there is anything mechanically wrong with the skis. Everything looks good and the 120lb boys riding them didn't have the same problem.

 

[E350]

You could also consider a longer ride plate.

 

[WFO Speedracer]

What I have seen some guys do is remove the steering plate, cut it off, then use the old clamps and redrill the plate and tap it for straight handlebars , it puts you about three inches farther forward without messing with the handlepole or the steering cable , if you don't like it you will have to buy a new plate to reverse it though .

I looked back at my 300SX build, they porpoise way worse than anything else , I am at -4 inches on that handlepole , with a stock rideplate it doesn't porpoise at all now.

One more thing you might could possibly look into is an angled pump spacer to change the trim angle of the exit nozzle angling the exit nozzle down a few degrees might help your situation and keep the nose of the ski down.

 

[Speargun]

Apparently I already have a shortened pole on one of the skis. The pole on the blue ski is about 37" long (pivot to pivot) and the white ski is about 40". I could easily shorten the tubular pole on the white ski though. I have a TIG & know how to use it. LOL

 

[Myself]

Shouldn't be doing what you describe. I'm 6'8" and currently 235# and didn't have those issues on a few 750's I've ridden. I'd be looking at pump shoe, pump seal, impeller condition, etc.

 

[WFO Speedracer]

Those rubber trim seals for the pumps get compressed over the years and they don't seal up well anymore , especially if the pump has been off a few times , I have had to back them up with Right stuff sealant before to get them to seal up properly .

Impeller damage should be obvious if it's there , all it takes is one bent back blade , inspect the bearing housing for cracks and maybe reseal it to the hull as it can pull air there as well , I agree with Myself if it's doing as you describe there are issues in the pump or bearing housing somewhere.

I have ridden skis that the pump wasn't sealed to the hull at all , eventually it will catch up and go but getting there is rather difficult.

 

[john zigler]

A rider of your size/weight should be able to ride a 750, with some skill. The white ski appears to have a longer than stock pole which will definitely NOT help.

I am going to guess the hulls are wet as well, and heavy. This is a common problem. You can drill a couple of holes in the rear, down low and see if any water come out. If so leave the hull tipped up for a while and try to get as much water out as possible. You can then seal the up the holes with Marine tex/JB Weld, etc.

I am 6+ ft at over 200 lbs, and have ridden a customers total soaked (Hardly would float) 750sxi pro without troubles. It can be done. As said above you really need to get your weight forward.

 

[JC-SuperJet]

Assuming the Impeller Edges are in good condition, Pump-to-Impeller "daylight" Clearance is less than 0.6 mm, the Pump Seal is good and the Stator Vanes are not chipped, jagged or broken, then it is likely that you have a Lightweight Flywheel along with those other Bolt-on Mods you listed.

The Lightweight Flywheel will let the Engine Rev too fast before the Ski has a chance to Plane, so it will do a a Wheelie or Pogo instead.

This effect can be decreased SOME with a longer Ride Plate + Hull Extensions, Ride Plate Trim, Nozzle Trim Angle, Rear Sponsons, Pilot CG forward and shorter Handlepole.

But, at the end of the day, that Lightweight Flywheel makes for a ride that is hard to control from dead starts, pulling out of turns and cutting thru chop.

My first Ski was a 1995 SXi and rode like that after I put in a Jet Dynamics Flywheel. The Ski rode horrible around the Buoy Course and thru the Chop. Took 3 times the effort to pilot the Ski around the Course. Total disappointment.

I went back to the OEM Flywheel and the Ski rode a lot smoother and predictable. Took a lot less energy to ride. Cut thru the Chop well because the Engine retained Momentum from the heavier OEM Flywheel.

Alternatively, you could install a Heavier PTO Coupler instead of a heavier Flywheel.

 

[bird]

Assuming the Impeller Edges are in good condition, Pump-to-Impeller "daylight" Clearance is less than 0.6 mm, the Pump Seal is good and the Stator Vanes are not chipped, jagged or broken, then it is likely that you have a Lightweight Flywheel along with those other Bolt-on Mods you listed.

The Lightweight Flywheel will let the Engine Rev too fast before the Ski has a chance to Plane, so it will do a a Wheelie or Pogo instead.

This effect can be decreased SOME with a longer Ride Plate + Hull Extensions, Ride Plate Trim, Nozzle Trim Angle, Rear Sponsons, Pilot CG forward and shorter Handlepole.

But, at the end of the day, that Lightweight Flywheel makes for a ride that is hard to control from dead starts, pulling out of turns and cutting thru chop.

My first Ski was a 1995 SXi and rode like that after I put in a Jet Dynamics Flywheel. The Ski rode horrible around the Buoy Course and thru the Chop. Took 3 times the effort to pilot the Ski around the Course. Total disappointment.

I went back to the OEM Flywheel and the Ski rode a lot smoother and predictable. Took a lot less energy to ride. Cut thru the Chop well because the Engine retained Momentum from the heavier OEM Flywheel.

Alternatively, you could install a Heavier PTO Coupler instead of a heavier Flywheel.

Lighter flywheel just requires more throttle input from loads. I remember that feeling...

Check your impeller clearance.

Can't really speak for the kawi 750 hull other than saying it is notorious for porpoising according to reading on the internet, but I've never ridden one so I don't really know.

But as far as the cavitation I think you have some other problem than the hull, likely pump seal. I'm pretty much the same height and weight as you and have ridden many sxr's, x2's, and sx650's all of which were fine with no cavitation with the same pump. I've also ridden an sx300 that struggles a bit to get a guy our size on plane, but can do it and with a much shorter hull and smaller pump.

The 750 pumps have a rubber seal. Check it!

650 pumps get sealed in place with silicone. Comparing an SXR 800 to 90s Kawis is apple to oranges. They fixed all the stupid $hit and put stuff back where it should be like a JS550 setup.