Problems with a 120hp sport jet

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ranman6

Seaman Apprentice
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Aug 22, 2010
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I have a 120 hp Merc. Sport Jet, the problem is the engine is running fine and seems to get WOT, but I have trouble planing the dang thing!

It seems to get worse everytime I take it out , when I open it up it takes for ever to plane it out and I have to get all the weight up front.

Its like when a clutch is slipping in a car, when I give it the throttle I have to back it off a little and you can feel it start to get a little bit better than about halfway across the lake it will start to plane.

I don't understand it, it use to take off like a bat out of hell and now I have sail boats pass me (not really but it feels like it)


Please help!:(
 

bigdaddypt

Petty Officer 1st Class
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Aug 10, 2009
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233
Re: Problems with a 120hp sport jet

It sounds like the issue is with your jet pump, specifically the wear ring. It could also be a dinged up impeller, or both. Have you sucked anything up recently that you're aware of? Stick, rope, etc?

Shine a flashlight in the jet pump and take a look. If there is damage, or that it's just worn, you'll need to pull the drive, and replace the ring and/or impeller.
 

ranman6

Seaman Apprentice
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Aug 22, 2010
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Re: Problems with a 120hp sport jet

Yeah I have looked at that and I do have a few tiny dings in the impeller, do you think that would make the difference?

What about the wear ring how do I check that and how do I make sure of these before I spend the money and time and find out that it might be something else?



Thanks for the advice.
 

Anders67

Petty Officer 2nd Class
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May 23, 2012
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Re: Problems with a 120hp sport jet

Check for grooves ore unsmooth areas in wear ring.

If the dings in the propeller is to big it can make trouble. It makes it cavitate.



Nothing other thing on the boat changed. Like something hanging down in the whater?.
 

H20Rat

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5,199
Re: Problems with a 120hp sport jet

Most likely your wear ring tolerances are too high. Search around for the jbweld fix, it works amazingly well.
 

ranman6

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Re: Problems with a 120hp sport jet

I did look at the impeller and it looks pretty wore on the edges and the ring looks like it has some pretty deep grooves.

So you can use JB weld to repair the wear ring? How do you do that, do you have to pull it out? Would it be better to replace it and how hard would that be, I do have a manual and have rebuilt this engine from the ground up, but have not ever did any thing with the jet side of it.

Thanks again for all the advise.
 

hypersniper

Banned
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Jun 18, 2011
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Re: Problems with a 120hp sport jet

seriously, try the jb weld fix. it works and is much much less expensive then a new wear ring. i think a new wear ring was like 400.00+ for the jetdrive.
 

boatparty

Cadet
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Sep 3, 2012
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Re: Problems with a 120hp sport jet

Solution: I own a sportjet 120. Had same problem. 3 things make the boat fast.
1-clean propeller with no dings/work edges on blades make a MAJOR difference. Trust it. Either repair blade at a shop or get a new one...$150-400. cast iron or stainless works.
2-Wear ring tollerance between impeller and "ring" the blade sits in. I found that it needs a micro tollerance of IDEAL 0.030"!!! dissassemble lower unit nossle, blade stator, then wear ring (4 bolts). This will expose the impeller. Lots of oil will pour out so be carefull. The oil is for end of the shaft lube, remember to refill thru the hole on the stator ring. Measure by getting a spark plug feeler guage that you can slide in the gap to figure where your at now. JB weld worked for me. It is a 2 part liquid that hardens to steel. Simply expose wear ring and clean real good with sandpaper scuffing to roughen surface. mix JB weld. USE a 2inch putty blade (cut off handle so it easily fits in ring)or use a popsickle stick. Tape off areas you dont want JB weld on cuz it can get sloppy quick. Spread jb weld like butter evenly. Let dry. Machine shop cut back to right tollerance at a shop for $50-100 or sand down by hand (less accurate). I sanded mine and it worked fine. Improved plane by alot.
3-Ride plate HAS TO HAVE 100% seal between metal and fiberglass!!!! The exhaust runs in that chamber and it the seal breaks its game over. If a screw comes loose or falls out it is game over. MAKE sure it is sealed perfectly. I had a busted up old seal and I caulked the hell out of it and it worked until the exhaust heat did it in. I bought a new one from Merc for $74 and replaced all of the bolts on the bottom(8qty). Remember to use marine grade ATB Sillicone around inner metal to metal seal. ($6-8) hardware store or auto store.

I had to do all three to get back on plane. Lucky you have only one issue.....for now! lol.
 

emoney

Commander
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Jul 19, 2010
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2,551
Re: Problems with a 120hp sport jet

You should post up pics of your ride plate, making sure you get the entire seal in the photo(s) AND your impeller/wear ring. Check on Ebay because occassionaly you'll find a take-off. It's not that common, but worth searching for. If the gap between your impeller and wear ring is too much, the JB Weld fix will definitely require a machine shop for it to work. That's got to be a perfect circle in there, so keep that in mind. If you're planning to keep the boat, buying a new impeller & ring is a worthy investment. If you do go that route, think long and hard about replacing that ride plate seal while you're at it. And, in case no one has ever told you, DO NOT start these motors out of the water. It's not one of those "wive's tales" when it comes to the SportJet. The only way to start it on the trailer is to plumb a water fitting on the bottom of the motor by disconnecting the line that is already there. It takes a bit to get it all in place, but it's not expensive and it's the ONLY WAY to start these motors (sorry, I get a little redundant, but I've known guys that didn't believe that piece of advice and paid dearly for their decision). GOod luck and keep us posted.
 

Capt Ken

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Re: Problems with a 120hp sport jet

I've owned about a half dozen sportjets and experimented with a few. On one 120 model, my son had access to a machine shop at a local shipyard. He took a wear ring and cut the inside bigger. Then took a piece of stainless steel pipe and turned it down to fit the wear ring. Had to heat the ring to expand it enough to press the pipe in. He then turned the inside diameter of the pipe to where he had less than .005 clearance with the impeller. Dangest running jet I ever saw. One problem it had was it was sucking more water than it could suck thru the grate at low speeds. If you took off at a normal rate, it would jump up and go. If you slammed it down, the negative pressure on the intake side would cause the water to breakdown into tiny bubbles and cavitate. Once you got some speed up, the speed would help force water through the grate and it would go faster and faster. We once checked the speed with a gps and was showing 56 mph with a light chop.
 

ranman6

Seaman Apprentice
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Aug 22, 2010
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Re: Problems with a 120hp sport jet

First off I want to thank you all for your input, it all helps.

Looks like I need to concentrate on the wear ring and the impeller, I did look at the seal around the ride plate and it looks like there is small pieces missing.

What can I use to seal this that is waterproof or is it better to replace the whole thing?


Thanks again
 

boatparty

Cadet
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Sep 3, 2012
Messages
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Re: Problems with a 120hp sport jet

I pulled the ride plate, (tricky on some trailers, you may have to lift the boat by hydraulic jack) and placed ATB silicone there for a trial run the next day to ensure it wasa the issue before spending $75. It allowed me to actually get on plane in 5 seconds. Then I had to refinish the impeller, plane in 3 seconds. Now were working on the wear ring tolerance. We got it down to 0.050" and it made a huge difference but still at 25-30 MPH tops. The closer to 0.01" the better (50mph I guess). I recomend the machine shop to finish the wear ring. Even a small bend in the ride plate can cause air leak. The new seal was so strong, I had to place the new seal and plate in with hydraulic pressure. it is a very tight seal. The exhaust has massive heat and pressure under load at high RPM so it must be solid!
 

emoney

Commander
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Jul 19, 2010
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2,551
Re: Problems with a 120hp sport jet

If you have a piece missing, you really need to just order a new one. Any "temporary fix" may not isolate your issue. They're not that expensive and it's going to HAVE to be done at some point.
 

ranman6

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Aug 22, 2010
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Re: Problems with a 120hp sport jet

Where is th best place to buy the seal, and how would a machine shop be able to finish the wear ring do you have to remove it?
And I was going to replace the impeller so would I take that in to the shop so they would be able to fit it?

So many question, but I do appreciate all the help, hopefully i can get this to plane like it use to.
 

emoney

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2,551
Re: Problems with a 120hp sport jet

Yes, you'll have to remove the wear ring in order to try and have it "fixed". You really should post some pictures, however, because if the damage is that bad, you might be spinning your wheels. Most machine shops will do what you ask them to do, but that doesn't always mean it's going to fix your problem. If you take them a wear ring that's grooved, their natural response will be to machine the sides down to the bottom of the grove. If that's the case, you will still have the exact same problem as your tolerances will be out of spec. The "fix" that was mentioned above usually applies to minor groves, not major ones. Pictures tell a 1000 words.
 

ranman6

Seaman Apprentice
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Aug 22, 2010
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Its been a few years after this post but I fixed everything and got it going good.

Didn't take it out for a couple of years but tried to start it this year and I have water in #4 cylinder?

I'm going to pull the engine and look at it but where should I start to look for a problem..exaust gasket, or head?


Thanks for any advice.
 

glutendokter

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Sep 24, 2019
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And again couple of years later
I have the same problems and would lik to try the jb weld fix witch jb weld is the right one?
Gr Martijn
 

GA_Boater

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Please start a new thread. It's been 4 years since this one has been active or the OP visited.
 
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