Merc SportJet Question

[Crater]

1) I rebuilt the wear ring with JB Weld to tighen the tolance up a bit(very tight tolance in fact)
2) Sharpend the leading edges of the impeller
3) replace the thrust washer with a thinner one to move the impeller forward about a 1/16 of a inch (impeller is not hitting anything)

Now for the problem. I took my boat out this evening and it is worse than it has ever been. Boat feel like it is slipping from idle throughout WOT and at WOT and on plane is slips and makes RPMS race to 5600(too high)
It's the worst when getting up to plane (alot of thrust is needed)
So out of the things I have done which would cause this to be happening?

BTY I don't think it is the ride plate or the gasket , the're in good shape and I siliconed the **** out of the gasket before installing it.

 

[H20Rat]

Re: Merc SportJet Question

i'd move the impeller back with the original washer first of all, from what i've seen sportjets are very picky about geometry and don't respond well to big changes to the impeller or stator. (have owned both a SJ 95, SJ 120, and now a SJ 175)

then, even though you said you don't think it is, you described a bad seal perfectly. too much silicon is just as bad as not enough, and did you wait long enough before getting it back in the water? (jbweld needs at least 2 days to fully set up for full strength, longer if you used the regular formula.)

hmm, what else... you checked the obvious? there isn't seaweed or a rag wrapped around the shaft? You hooked up the reverse gate and verified it clears the outlet?

 

[Crater]

Re: Merc SportJet Question

Finally someone else that know the pain of a sportjet.
Yea that washer was my first thought.
and yea I may have got too much silicone in there and holding the ride plate down too much

Yes the reverse gate clears rudder, and yep I check for a rag or something else I may have left in there (couldn't be a surgeon I guess)
Whats the chance I could havce sharpened the impeller wrong? First time I've tried somthing like this. I just basically took the wired edge off, not like I ground on them, I used a straight file in fact.

 

[H20Rat]

Re: Merc SportJet Question

the 90/120 force based sportjets have a bad rep, but in reality they at least have well defined and common problems that are easy to fix. The two biggest things are the bad stators on early models, and the plastic drive gear on the oil mixer pump. Starters also seem to fail, I've replaced the starters on both the 95 and 120 within a year of each other. (you can get new ones for $85 if you search around) Really easy to do though, I did the 120 a couple weeks ago, took maybe 30 minutes from start to finish, while the boat was on the water yet.

i wouldn't think taking the bad edge off the front off would have caused much of a problem. i've never done an entire edge, but i'll knock down any nicks in the edge any time i have an impeller out and am not having it rebuilt.

 

[Crater]

Re: Merc SportJet Question

What could go wrong with the stator other than the seal? This is a 94', I would have thought that the pervious owner would have replace it by now if it was defective, or does it just wear out.
And is there a updated stator for it?
Bty, Yes the JB Weld had sufficient time to cure before reassembly.
Weird thing in my mind is that if these SJ are to tempermental about hydronics then why would the manufacture leave a rough casted surface in the pump housing and stator? By rough I mean from the casting not scratches or gouges, there's still paint in there so I know it not from abuse, plus you can just tell that it was from the casting.
Also is there any kind of break in period for the wear ring? I assume that if I keep running it after moving the impeller back, and fixing the ride plate gasket, that over time the JB Weld will start to wear down also just as the aluminum did in the first place. So the last resort would be to sand off some of the JB Weld to make a larger gap, is this correct?

BTY would you know if a SJ 175 would fit into the cutout of the boat? I mean is the SJ 90 and SJ 175 dimensions of the pump the same? I relize the power head will have different dimensions. Weird thing boat is not rated for hp just cap.

 

[Crater]

Re: Merc SportJet Question

Well I put the correct washer back in, and removed all the silicone I had in there. When I replaced the ride plate I noticed the plate was bent slightly so I straightend it out and behold the the plate fit up into the hull further, by taping on it with a rubber mallet evenly all the way around I think I got it to seal with no silicone.
Now there's very little cavitation and going 28mph up stream fully loaded.
I think the first thought that if a little silicone is good then alot is better was my problem.
Thanks for the help

 

[ProbeGT]

Re: Merc SportJet Question

it should go faster.

90HP SJ in a 14' boat should go around 35MPH with driver only.

I get 37MPH in my 16' sea-rayder with the 120HP. And my engine is not in very good condition.