not getting full power from 120 hp sport jet

[grizzly1733]

I have a 1996 Rinker Rocket Jet Boat. With 120 hp sport jet in it. The boat starts fine, engine appears to be running fine. Sounds normal. But I am loosing as much as 20% of its power. The boat did 40-43 mph last season. This season it won't run over 34 mph. It also won't pull me up on one ski anymore and it even has a hard time with 2 skis. I recently replaced the rev limiter because it had went bad, now the boat is having power problems. It sounds like the impeller is spinning faster than it can intake the water. Then I have to cut back on the throttle to allow more water to pass through the jet. Once I plane the boat off and punch it to full throttle it again sounds like the impeller is spinning too fast for the water intake and I get a skipping sensation. Does anyone have any idea what is going on with the boat. Its no fun not being able to ski<br /><br />Thanks

 

[amtsst]

Re: not getting full power from 120 hp sport jet

Hey Griz,<br /><br />I'm a fairly new boater but have a little experience with pumps. I apologize but have to ask you a dumb question.<br /><br />1. Have you removed the intake grate and checked to see if there is anything restricting flow into your impeller?<br /><br />It sounds like your impeller is drawing a vacuum over a certain RPM level. This would be consistent with a flow restriction. Where do you store the boat? Could a critter have built a nest up in there?<br /><br />Also, with the power off, can you see up into the back end of the unit to confirm that there is no restriction on the pressure side of the pump?<br /><br />Some other questions:<br /><br />Is the problem consistent both when the engine is hot and cold? (ie. do we have a engine problem). Can you turn it over by hand and confirm that everything sounds right?<br /><br />What service has been performed on the jet unit this season? (grease, impeller change)

 

[grizzly1733]

Re: not getting full power from 120 hp sport jet

BackWater<br /><br />I did remove the intake grate and it appears to be clear of any debris.<br /><br />Your decription of a drawing a vacuum is exactly what appears to be doing on. It sounds like the impeller is spinning but water is not being drawn and forced out. <br /><br />Do you own a jet boat? If you do or even have driven one the problem I am having is similar to when you put the boat in a spin or 180 degree turn the impeller spins wildly untill you cut back on the throttle and allow it to catch up.<br /><br />The problem with this issue is it is happening when you initially punch the boat to go fast or pull a skier. If I cut back on the throttle it draws water better but you don't go as fast. Once I plane the boat off I try punching it full throttle and it sounds like it is skipping or spinning but no water is going out the back. <br /><br />I replaced spark plugs last year and replaced drive housing fluid the year before that. Besides that I have not done any other maintenance. The manual I have calls for replacing the stator housing fluid. I have not done this. <br /><br />My first attempts at solving the problem will be replacing the stator fluid and drive housing fluid<br /><br />Thanks for you interest.

 

[Capt Ken]

Re: not getting full power from 120 hp sport jet

More than likely your impeller is getting worn. The gap between the impeller and the wear ring is suppose to be real close. After a season or two, the gap grows and allows the pressure to drop thus loosing speed. The only way to get them back to normal is to replace the impeller and wear tube.

 

[Capt Ken]

Re: not getting full power from 120 hp sport jet

More than likely the tolerance between the stator and the wear ring is getting bigger. This is normal and can't be prevented. This just comes from use. To get the old speed back, you'll need to replace the stator and wear ring.

 

[clanton]

Re: not getting full power from 120 hp sport jet

Hello again Capt Ken, was by your place yesterday. Trent sent me.

 

[amtsst]

Re: not getting full power from 120 hp sport jet

I have another thought. The more I think about this, (after re-reading your description), the more I think its a pump caavitation problem.<br /><br />Basically, above certain RPM's, your impeller can't draw enough water and so the pump draws a vacuum. No correct me if I'm wrong but once you plane off and get going, if you rev too high, the jet alternates between lugging and screaming. (too much water and then not enough). Almost like the flow isn't consistent.<br /><br />I'm not going to disagree with Capt. Ken because I bet he knows a lot more than me but it *really* sounds like you have a flow problem into the intake of the impeller.<br /><br />Another question...since I"m not familar with sportjets. Is there a little scoop on the grate that helps collect water at high speed?? If not, could you fashion one just to see what happens? I don't want to give you a bum steer but give this some thought: If you could increase the pressure and volume going into the intake side of the impeller, that would prove or disprove our 'starving pump' theory.<br /><br />If the boat is supposed to have one and you knocked it off at the end of the season or something, that would help explain why the pump can't get enough water at higher travel speeds.

 

[damage inc]

Re: not getting full power from 120 hp sport jet

iv got an 90hp sportsjet my warering got a rock stuck in the impeller and wore a big hole around it,no much loss or power if any to nodice but i did use some liquid medal sanded it down and let the impeller cut it clean to fit,low costs & a good fix.***

 

[kev_alaska]

Re: not getting full power from 120 hp sport jet

Couple ideas, we run a lot of jets here...On an outboard jet, you can change the distance from the impeller to the wear ring by changing a shim from the top of the impeller (between the shaft and impeller) to the bottom of the impeller (between the impeller and the locknut) to compensate for wear. I don't know if inboards have the same setup, would make sense. <br /><br />As for on throttle cavitation, there are fins that can be attached that direct flow into the pickup grate. Cavitation from an turbulent keel flow is a major issue. We have had several instances of hitting a gravel bar at speed causing damage to the bottom of the boat. This would not show up as very much visual damage, but it interrupted the water flow enough to make a difference in cavitation..??

 

[wakataka]

Re: not getting full power from 120 hp sport jet

These drive units are also prone to leakage of exhaust gas around the ride plate, which can cause the symptoms you describe. The ride plate is gasketed around it's outside circumference, but there is no gasket on the inner circumference, where the ride plate is screwed to the inlet screen. The plate can appear to be securely attached to the bottom of the boat and still leak enough exhaust around this ungasketed area to cause cavitation. <br /><br />I have fixed my Bayliner Jazz (at least temporarily!) by putting blue silicone sealer around this ungasketed joint and gently pushing the plate up against the bottom of the boat with a hydraulic jack while tighetening the screws.<br /><br />You'll probably need an impact driver to get the screws out, as they are put in with thread locking compound. Be sure the put the locking compound back on them when you replace them. If you were just interested in diagnostics, you might be able to work enough silicone sealer into the joint to get it to seal for awhile without removing the plate. This could tell you if exhaust leakage is you problem.

 

[grizzly1733]

Re: not getting full power from 120 hp sport jet

I think it was the exaust manifold problem. When I looked under the boat I noticed that the ride plate was pushed out a little on one side inaddition to 2 screws missing. So I pushed the plate back up and found some new screws. I had a heck of a time finding 6mm screws. I could not get stainless just zinc coated. This seemed to solve the problem the boat is running well again. The power appears to be back and it doesn't spin at high rpms. Thanks for all your help and suggestions. This is the best place in the world to get help on your boat. You guys help me save tons of money. Now I can spend it on gas. Because this boat sucks up a ton. see ya grizzly

 

[DwightM]

I had the same issue. Ride plate was not flush around the front and sides. I replaced all (10) screws with allen head type. Phillips head gets destroyed over time. Made sure the ride plate was all the way in. You may have to tap in with a small hammer. Full power was restored as soon as I hit the water.

 

[GA_Boater]

I had the same issue. Ride plate was not flush around the front and sides. I replaced all (10) screws with allen head type. Phillips head gets destroyed over time. Made sure the ride plate was all the way in. You may have to tap in with a small hammer. Full power was restored as soon as I hit the water.

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