Ultra Newbie Buys 1st Ever Jet Ski - Problems on First Day (Cavitation?)

[coneten]

Hi Folks!
What have I got myself into?

I purchased my neighbor's 1997 GTX (86 hours on it) for $1050 with trailer. My kids were begging me to do it. I've never owned a boat or jet ski before nor ever driven with a trailer. Anyhow, I got a hitch installed on the family van and went jet skiing today. Figured out how to launch it OK. After a slow speed spin to figure out the controls I came back for my kids, loaded them up and headed into the harbor. The GTX would barely move even though motor was running well over 5000 rpm.

I unloaded the kids and went out alone. The jet ski seems to work fine. I was able to run at about 35 to 40 mph (water was choppy). It seemed to accelerate ok. Could this simply be a weight issue?

I weigh about 210 and each kid is about 80 lbs. I went back out with only 1 kid and still had the same problem - jet ski would hardly move even at high rpm's. Now this is a 3 seater with the 787 engine is it not?

After we got home, I took a look at the impeller with a flashlight. It looks clean and undamaged. I did look through the intake grate and could see that there were no foreign objects, etc in there. What I did see was a badly rusted drive shaft with one deeply pitted area. Definitely a replacement candidate. The wet area in the picture appears to have been almost half rusted through. Could this cause flex in the shaft?

Am I off base in thinking this is a cavitation problem? Anyone care to comment or make a suggestion of what to look for / repair to get this ski running right before I strangle my neighbor?

Your input appreciated! Thanks in advance.

Picture of rusted drive shaft:

 

[SabrToothSqrl]

Wow, i've never seen a shaft that rusted... was this run in the ocean? Mine are spotless... and from 1998!

the 97 GTX was 110 HP if I remember correctly... the WOT of a GTX should be closer to 7,000 RPM... the wear ring and impeller look ok...

simple stuff: put new plugs in it. about $4.00 each...

How long did it sit? bad fuel?

what about the fuel lines, and did you check the fuel filter?

 

[Dr Honda]

Yep... you have cavitation. It's common for it to get worse with more load.

Cavitation can come from a bunch of places.

1) Too much clearance in the pump. (warn impeller and ring)
2) Nicks and dents in the impeller. (a source of nucleation when under vacuum)
3) A bad carbon seal (sucks air past it)
4) a leak in the pump (normally between the pump and the shoe)
5) a leak around the pump shoe.


Since the ski seems neglected... I would take the pump out, and remove the driveshaft... then replace the carbon seal, and seal everything up as you put it back together.

 

[coneten]

Thanks for your input folks.

I took a better look at the drive shaft. It has to go. It's ready to snap at the pitted area.

Thanks Dr. Honda - I'll read up what I can on your recommendations.

 

[LouDoo]

I'm not sure, but it looks to me like one the right side of the picture, one of the impeller blade is broken. You'll be able to tell when you remove the pump.

Lou

 

[ski-d00]

That looks pretty normal for an older saltwater ski. Most of the guys on here aren't used to seeing saltwater machines. The saltwater is really hard on skis.

I see some weeds on your impeller as well. This is another big cause for cavitation, just a few weeds stuck on the intake grate will cause it. When in water, shut engine off and reach underneath to clear the weeds. I'll bet you just had some weeds stuck on it, sometimes it will clear itself just by shutting engine off but not always.

 

[coneten]

Hi Folks,
I ordered a replacement drive shaft off of eBay. There is a very deep pit (1/4" deep) in the existing drive shaft - could that cause the cavitation?

Anyone care to elaborate on the possible "leak between the pump and shoe" as mentioned by Dr. Honda? Is this some sort of seal or gasket that fails here? Anyone have a link to any pictures or other threads showing what to look for regarding pump/shoe issues?

SeaDooLou - I took another look, the impeller is not cracked.

Ski-d00 - We checked for weeds while it was in the water. Not the issue. The ski also ran fine with reduced load.

 

[crisso]

Wow.That is one corroded shaft.I am a 100% saltwater user.If you spin that shaft in any chamber,it will create turbulance to the prop.As to how much?Thats up to the gods.Get some wd40 etc now onto all your moving parts to help you get them apart when the time comes.Everybit will help.Keep reading all that the Doc and others write because they have probably forgotten more than most will ever know.

 

[Dr Honda]

Yes, there is a Neoprene gasket between the the pump, and the shoe. When it get's old... it gets hard, and can leak. Also, sometimes people take it out, and use RTV to glue the pump on, and that doesn't always seal.

As above... the drive shaft can cause turbulence... so the new shaft should help things out.

When you take things apart... buy a new carbon seal, boot, stainless ring, (and o-rings for it) and a new clip. I find most "Bad" cavitation comes from the carbon seal.