SportJet cavitation problem

[Daley]

I've seen a few threads where the sportjets seem to have a cavitation problem after a backfire (which I've suffered - out of the water) and the problem is that the ride-plate could be damaged. Ok, so from what I've seen, I'm into it for around $200 - fine. Expensive oopsie, but after all it is a boat. What exactly happened here? What got damaged and what do I need to order/replace?

There's a foam gasket that seals the jet pump to the hull that appears to be sticking outward a little toward the bottom, is that where I'm getting air into the pump?

Foam Gasket

Also, my wear ring has a gouge running radially, along with some good-sized pitting - should I be off to the auto parts store for some JB Weld or buy a new wear ring?

Wear Ring

I didn't measure clearance, but considering the fact that a spark plug gap is way bigger than the gap between my wear ring and impeller, I think I'm doing alright.

My impeller blades look decent (not great - a couple knicks), and the stator is about the same. I can say that the impeller blades are really sharp on the outside edges, which I've read is a good thing.

So ultimately, my question is where is my cavitation problem - wear ring or ride plate, and what do I do about it?

Thanks for all the help!

P.S. My apologies if this is in the wrong place - these sprortjets seem to have a different home depending on whether the discussion is jet or motor.

 

[kend301]

Re: SportJet cavitation problem

Not sure about the gasket / drive plate stuff , will try to find out some info tomorrow from one of my techs who works on boats as a side job.
YOU NEED TO REPLACE the wear ring , clearance is critical at .008 to .010 you will never sand/ grind jb weld that close , Also if the jb weld was to come loose it would be like throwing rocks at the impeller . Spend some money and save alot !!

 

[ProbeGT]

Re: SportJet cavitation problem

I had .038 of clearance before i rebuilted my jet drive and it was still going 35MPH. Acceleration was slower but not THAT bad.

your jetdrive can cavitate because of the exausth gasket between the engine and jetpump IF i remember well.

according to your pictures, your rideplate seems perfectly seated.

for your foam gasket, mine is worse than that and I still achieve 37MPH no problem

 

[Crater]

Re: SportJet cavitation problem

Clymer manual says that @ .100 you will see performance issues, Merc did not put out any spec for this tolerance.

The gasket seal to hull in essential. Just remove the ride plate carefully as someone may have siliconed it in, inspect it for cracks or tears, replace it if it does have problems(like $35.00) inspect the ride plate also. I found mine was bent slightly (someone hit a rock). When installing the ride plate, take a rubber mallet and tap the ride plate in evenly and tighten bolts evenly also.
I tried the silicone trick, but way too much in and didn't get a seal. If ride plate seal isn't bad then re-install it without silicone.

BTY ride plate gasket stops exhaust gas's from entering the water before the pump causing cavation. So the seal is imperative.