90hp mercury sportjet

[RICHRICH26]

Hopefully someone can help me. I have a 90hp merc sport jet that will not go past trolling speed. I have rebuilt the carbs, got 120-122-120 compression, new fuel pump, new plugs, engine turning 4750 rpm's, .032 clearance between impeller and housing, new ride plate gasket. I have about 500.00 invested in time and repairs and still no luck. If someone has any info as little as it seems I would love to hear it. Thank you

 

[Trent]

Re: 90hp mercury sportjet

Check to make sure the impeller is not slipping on the driveshaft??

 

[RICHRICH26]

Re: 90hp mercury sportjet

it is a threaded drive shaft so I dont beleive there is any possibility of slippage.

 

[Terry H]

Re: 90hp mercury sportjet

Rich...I saw your last posting and my thinking was that the impeller was slipping...well if that's not the case than there's only 2 more possibilities....either there's no water getting in to the impeller or there's no water getting out...I would take out the impeller and make sure the water passages are clear. There's several hundred pounds of water pressure coming from the impeller that should clear the passage but you never know. When you did have the impeller out I assume you made sure the leading edges were properly sharpened, and it was shimmed properly. There's not much mystery to these pumps but in your case there's something basic missing...just a Thought

 

[RICHRICH26]

Re: 90hp mercury sportjet

that is the big problem I have. I know it is something simple and that is what really baffles me. The intake is perfectly clear and the output as well. I was thinking there was air getting in the intake but that is why I replaced the new ride plate gasket. The impeller had a nice sharp leading edge but there was rust spots which lead me to beleive cavitation. I am open for anything right now.

 

[DJ]

Re: 90hp mercury sportjet

RICHRICH,<br /><br />I can read the frustration. I'm NO jet expert. Let's back up here.<br /><br />Why did you replace that gasket?<br /><br />Did your problems start after that?<br /><br />If not, what was going on that made you replace that gasket?

 

[RICHRICH26]

Re: 90hp mercury sportjet

I replaced the ride plate gasket because I had a feeling that there was air getting in there. Boat ran fine last year, after sitting up a year it will not move very fast at all. The engine is great so i can rule that out its in the jet. There was no difference in performance after the new gasket.

 

[DJ]

Re: 90hp mercury sportjet

Let me see if I have this straight.<br /><br />1. You thought air was getting in, so you replaced the gasket.<br /><br />2. The boat worked fine after the gasket was replaced.<br /><br />3. It all of a sudden stopped moving? The engine turns up ok, but no go? Or, did it stop moving right after you took something apart? Can the impeller (jet drive) be mounted backwards or upside down?

 

[RICHRICH26]

Re: 90hp mercury sportjet

kinda but in this order: took boat out wasnt acting right so i replaced the gasket no luck still the same as before. impeller was put back on exactly as it come off.

 

[Terry H]

Re: 90hp mercury sportjet

Rich...I thought those sportjets were direct drive...I just looked at a new one and it has a gear reduction unit between the powerhead and the pump impeller...I am not sure that you don't have a problem there. You might try to turn the flywheel with a breaker bar, another wrench on the impeller nut to see if you can turn one without the other turning(long sentence...I'm outa breath)...just a Thought

 

[RICHRICH26]

Re: 90hp mercury sportjet

I have also tried that, I can turn the impeller and feel the compression strokes of the engine.

 

[Thena]

Re: 90hp mercury sportjet

Hi, I am also facing the same problem with a force 90Hp sportjet. I got the engine busted and I rebuilt it from scratch. New pistons e.t.c.<br /><br />Now my mechanic says the engine works fine but the speed is 20mph top in a donzi 152 medallion. (It should be around 35mph)<br /><br />Everybody tells me it must be in the jet but nobody can figure it out. If you have any suggestions please tell me cause I've run out of any....<br /><br />Thanks<br />Lucas

 

[RICHRICH26]

Re: 90hp mercury sportjet

Lucas, I am in the same position as you. I have had my boat to 3 different dealers and still no luck. Have you checked the ride plate gasket? what is your impeller to housing clearance?

 

[xdr]

Re: 90hp mercury sportjet

I have zhe exactly same problem with my mercury sport jet 120. To me it happend when i went off with my friends on wakeboarding. The boat went great (about 40 - 45 knots).<br />So we stopped, I prepared on the wakeboard, and it just couldn't pull me out of the water. Since then the boat won't plane, and when I throttle up realy fast it "slipps".<br />I pulled it out of the water and saw that the drive plate was bent (but it was bent before and it ran just fine), I checked everything: gears, proppeler, housing (the housing looks a bit worn, but not that much)! Then I took the engine apart, and it's all OK (It's a direct drive, no clutch)!?<br />Did you solve your problem?<br />Just rememberd: My proppeler wasn't thightly screwed on, Could it be that it vibratat and coused turbulance?

 

[xdr]

Re: 90hp mercury sportjet

I meant the RIDE plate, sorry for tipos but I'm stressed out and from Croatia

 

[funpilot]

Re: 90hp mercury sportjet

xdr,<br /><br />Sometimes something gets caught in the blades of the impeller, and is hidden by the overlap of the blades. Check to make sure your impeller is completely clear. Also, your bent ride plate may have been making a good seal, but now it doesn't (gasket shifted, or bent more, something like that). Try taking off the rideplate and smoothing it out, and replace the rideplate gasket.. If the impeller was loose, it could vibrate. On the 120 it takes a special offset wrench to tighten that nut properly.<br /><br />good luck<br />fp

 

[mfhlmr0]

Re: 90hp mercury sportjet

Originally posted by xdr:<br /> I have zhe exactly same problem with my mercury sport jet 120. To me it happend when i went off with my friends on wakeboarding. The boat went great (about 40 - 45 knots).<br />So we stopped, I prepared on the wakeboard, and it just couldn't pull me out of the water. Since then the boat won't plane, and when I throttle up realy fast it "slipps".<br />I pulled it out of the water and saw that the drive plate was bent (but it was bent before and it ran just fine), I checked everything: gears, proppeler, housing (the housing looks a bit worn, but not that much)! Then I took the engine apart, and it's all OK (It's a direct drive, no clutch)!?<br />Did you solve your problem?<br />Just rememberd: My proppeler wasn't thightly screwed on, Could it be that it vibratat and coused turbulance?

 

[mfhlmr0]

Re: 90hp mercury sportjet

Sounds exactly like my problem. It was working fine, I stopped the engine to change riders and when I re-started it I had these symptoms.<br />Cavitates during acceleration out of the hole <br />Cavitates while on plane in flat water with frequent jumps in the RPM as if it is ingesting air. <br />These symptoms have gotten steadily worse. <br />I think that this all began after the engine backfired while starting it (in the water). Is there an exhaust gasket that could be blown (leaking) exhaust into the impeller area? <br />Thanks for any help.

 

[ezryder]

Re: 90hp mercury sportjet

have same problem but a little different.. i sucked up a rock with mine and then there was alot of cavitation and would not go. so i replaced the wear ring and still had the same problem. so i asked tghe dealer what they thought and they said the was some sort of a coupler on the crank shaft that attaches the crank to the axel comming from the gear box.. any input would be great

 

[LargoDon]

Re: 90hp mercury sportjet

mfhlmr0,
If this happened after a backfire, I would really look at the ride plate. It needs to sit perfectly flush with the hull or else you will get a water flow problem. I know a long time ago I had a ride plate problem that caused the jet to feel like it was slipping, My boat was only a few years old and the dealer that sold me the boat was able to get Mercury to replace the ride plate since it was bent and that fixed all of my issues. Also make sure you locktite the ride plate screws- with all the vibration they will back out unless lock-tited.