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  1. #1

    STX 900 Top Speed Issues

    I'm reaching out to the community for help with my ski.

    Jet Ski Model: 2005 Kawasaki STX 900. All stock. Triple CV carbs. Original owner

    Where I ride: I am on Long Island, NY. Air temps are generally in the mid 80's on avg. Water temps in 60's in beginning of season and can get close to 80 late in the season. I ride mostly on the Long IslandSound. Very little sea grass if any. More of the cruising/sight seeing type. In other words, I don't beat on it.

    Maintenance: Always have used 87 Regular gas. Always flushed with fresh water after use. Always lubricated. The ski still looks new. Every winter, the ski is winterized. Stabil in the fuel. Antifreeze in the lines. Motor fogged. In the summer, fresh gas, fresh battery, fresh plugs, fuel filter every other season.

    Prior Symptoms: The ski turned 9 years old this summer. Years 1 through 7, never had a single issue. Did the routine maintenance as mentioned above and it was flawless. Started up on the first try every summer. Ran great. Even made the 90 mile round trip to NYC and Statue of Liberty without a hiccup.

    Last summer, year 8, was when I noticed a mild issue on occasion. What started happening, was whenever I was cruising at more than half throttle, it would once and a while bogg down for a split second, clear up, and then run fine. It would do this about 5 or 6 times in a couple of hours. Say I was cruising at 40 mph, steady throttle, going straight, it would drop off dramatically. Just as quick as it happened, it would rev back up again and take off. If I was at a lower speed, it wouldn't happen. I noticed I had to keep it under 25mph to keep it from bogging. I changed the fuel filter, plugs, sprayed the carbs on the top end, and put a tank of high test in. Ran like brand new. Nice and smooth. Next trip out, I filled up with mid grade fuel, to get it back down to what it was used to the last 7 years of its life. Ran great. No issues. Next trip, I filled up with 87 regular. Ran a little rough and started to do the hesitation/split second bogg as described above. Next trip, I filled up with high test and ran perfect. The rest of the season I consistently used high test and it ran great. Didn't have any issues.

    Current Symptoms: Now. While summerizing, I noticed it was a very rough start. I installed a fresh battery and spark plugs. But it just wouldn't turn over. I ended up spraying carb cleaner in the tops of the carbs and it fired up, but ran pretty rough. Put in some fresh gas and went out for first ride. Where I launch, it's a huge peninsula. So, it's 5mph no wake zone for about 3 miles from where you drop in the water to where you can open it up. I get to the ramp, dry start it for a second before I launch, no problem. Started right up. Get in the water and head out. Idled great, ran decent 5-10 mph. I came around the end of the peninsula and out into the open waters of the LI Sound. Worked my way up to top speed and no problems. I opened it up and ran 3/4 to full throttle for about 5 miles or so. I wanted to open it up and burn out the old gas and clear things up. Cruised around for about 20 minutes without an issue. I got to Port Jeff Harbor where there are speed restrictions. I drop down to about 10 mph through the inlet and into the harbor I go. I cruise around, sight seeing and checking out the massive multi million dollar yachts. I was cruising on that side for about 20 minutes around 20-25 mph and then back down to 5mph on my way back out to the sound. The ferry was coming out and it makes some pretty sick 6-8 foot swells off the back end of the ship. Tons of skis hang out on the other side of the inlet and catch the rollers coming in to shore when it passes through. Anyway, I get out and go to hit the throttle to hit the waves, and bam, it Boggs down. No matter what I did, I couldn't go more than say 25 mph. It never stalled, never backfired, never ran rough or like ****, it just simply won't go. You hit the gas and it tops off at like 25mph. Checked the intake grate for obstructions-clear. Always plenty of water out if the pisser valve. I cruised back pissed off.

    2nd day out, I spray the top end of the carbs with carb cleaner again, change the fuel filter, new plugs, dry gas, fresh oil, high test fuel and out I go. Ran great. Got up to speed. Ran consistent. No bogging. I just figured it was clogged fuel filter or bad fuel. Problem solved.

    3rd day, out I go. Starts up fine, idles fine, runs smooth in the no wake zone. I round the corner, and bam...just doesn't get up to speed. Same thing as day one, except this time it happened immediately when I opened it up. It just went slow, like 25 mph. No rough running, starts and stops fine, doesn't backfire. Doesn't stall. Doesn't sputter. It just won't go fast. It starts, runs good, doesn't stall, yet it Boggs down and won't go fast. I pulled it up on the beach, checked the plugs. They were chocolate brown, with tiny bit of oil on the ring where the plug seats. It wasn't yellow or burnt up and it wasn't black or covered in oil, so I knew it wasn't too lean or rich. I put in new plugs anyway. Double check the intake grate. Plenty of water out of the pisser. I fire it up, runs good. It takes off and I get up to decent speed. After about 5 minutes, it happened again. It just got real slow. Not more than 25 mph. Again, no sputtering or stalling. No backfires or rough idles. Even let it sit for a bit and cool off. Fired right back up. But ran slow. So, I just went out to see if maybe it would clear up. It was a little wavey out there. It was like perfectly spaced out small 2-3 foot waves. What I noticed was this. I would cruise along full throttle at 25 mph. I roll over a wave and when cleared the wave and kinda came out of the water slightly and then bounced down into the water, it would accelerate. It did this with me not changing the throttle position. It was full throttle already. Something, whatever it was, would clear up when I bounced over a wave and you would feel it accelerate. 2 seconds later, it would drop back down to 20-25mph. All with keeping constant pressure on the throttle. I don't know if it was reduced pressure when coming out of the water a little or if it was just coincidence that it was shaking a little extra fuel loose which gave it a little boost of power. This led me into thinking something about the jet pump or impeller. Maybe the wave was shaking something loose in the impeller that was stuck or jammed up??

    So I went under the ski when I put it on the trailer. I looked up into the intake and into the impeller with a flashlight. I noticed it had one or two very small dings in it, but nothing that was drastic. No chunks of metal missing. There was some evidence of slight abrasions and scorching on the housing/metal ring surrounding the impeller. I was barely able to spin the impeller by hand. I used a long screwdriver and it took a decent amount of strength to turn it even slightly. In other words, it didn't spin freely. Could this be the issue? I started leaning towards this because the ski runs fine. Starts fine. Doesn't stall or show classic signs of a fuel issue. It just simply won't go above 20-25 mph with throttle fully pulled. And when I bounced in the waves, whatever it is, seemed to open up and accelerate for a split second. Would kind make sense that bouncing slowly on the water on the back end, would put more pressure on the intake/impeller and cause it to kick in and take off right?

    But then why would the ski run good at full speed on my 2nd day out when I changed the fuel filter, plugs, and sprayed the carbs? And why would it run good for the 5 minutes it did when I changed the plugs again on day 3? That led me to abort the impeller theory and to actually break down and rebuild the carbs for the first time in 9 years.

    So, I rebuilt the carbs, pulled them apart, soaked in carb cleaner, new gaskets, float, diaphragm, base gaskets, etc. Cleaned out fuel pump, checked oil lines and fuel lines. Everything looked good. Put it all back together. Fresh plugs. Started right up. You could tell it was running better than ever. Nice and smooth. Previously I would have to feather the throttle a little when first starting to clear it up. Now, it starts up without touching the throttle and it maintains steady idle without breakup or hesitation. I take it out on the water and I could feel the difference. It was much smoother and ran great in the idle zone. I brought the speed up and the throttle responded crisp and smooth. As I get out to open water, I hit the throttle, and nothing. It levels off at like 20 mph. Full throttle and goes nowhere. Again, no sputtering, no breakup, no stalling. It just won't go above 20 mph at full throttle. It almost feels like the throttle cable isn't pulling enough to get top speed. So I beach the ski. I inspect everything. I pull the plugs and they look good. Not burnt dry and not black and wet. I inspect the throttle cable. It pulls all the way as far as it possible can. Just for trial runs, I adjust the idle screw, high speed adjustments, in both directions. I lean it, richen it and nothing made it better or worse. I must have did about a dozen trial runs and adjustments. It all responded the same way. Aside from the idle adjustment changing the idle speed, nothing helped the 20 mph restriction. No matter what I did, the ski idled perfect, ran smooth, didn't stall, yet when I pulled the throttle fully, it just wouldn't go faster than 20mph. I then pulled the temperature sensor to see if maybe it was faulty and sending a constant false over-heating signal to the motor, causing it to be consistently in "limp mode". No change.

    Any ideas? Suggestions? Someone in another forum said it could be the cooling temp sensor?

  2. #2
    steve45's Avatar
    Join Date
    Feb 2008
    I'd guess water in the fuel.

  3. #3
    I like pipes. I love boost Mr. GP1800's Avatar
    Join Date
    Jun 2010
    Milwaukee WI
    Does the ski have a fuel shut off valve? If it does, check that for free flow. Are the carbs internal filters clean? Does the fuel system have a water separator? I don't know the Kawi machines very well, just throwing some ideas out there.

    Steve may be correct with water in the fuel. Maybe drain the tank completely and start with fresh 91 octane fuel.

  4. #4
    Myself's Avatar
    Join Date
    May 2007
    North Central Arkansas
    I'm betting you have a bad coil or plug wire. You can ohm out the secondaries while wiggling the wire to find it, or run it a bit then feel for the "cold" cylinder. I've replaced coil wires and coils in many Kawi triples.

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