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  1. #1

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    1996 ZXI 900 engine problems

    Hello all

    Iím just getting onto this forum to get some help with my ZXI 900 I bought a couple months ago. Iíve used the forum to get familiar with the ski, and fix problems such as the frozen trim motor, clogged temp sensor, homemade sponsons, and switching over to pre-mix. I have been building 2-stroke dirt bike motors for 20 years, so I am very knowledgeable about motors.

    For the first two weekend rides, the bike ran great. Lots of power, revved out to 7000 rpm. On the third weekend, it tended to be hard to start after a stall/crash off the ski, and some novice rider friends ended up killing the battery after not being able to get it started. We towed it to shore, and after about 15 minutes, the battery regained some juice, I changed plugs for good measure, put it on reserve (for the first time ever), and I was able to get it started. It again ran fine, revved out great, got about 20-30 min of riding on it, but the battery appeared not to be charging, and by the end of the day, battery was toast. It also seemed to be bogging a little more with fast throttle action.

    At home, I determined it wasnít charging while running, so I popped open the e-box, and found the problem; one of the two stator wires backed out of the connector, and wasnít making contact. I fixed this, tested all the stator coils, and everything specíd out fine. Gave it a quick start and the voltage was 12.6+ running, so some charge happening now.

    Took it out to ride yesterday, and on the first run out, it would bog hard down low, and only rev out to 6000 rpm. If you feather the throttle, it will get up on plane to 6000rpm, but if you advance the throttle quickly, it would bog out and die. Playing with the choke only made it worse. At WOT, 6000 rpm, it would surge a little bit.

    I changed out plugs, and found the plugs to be a little leaner than the last set that came out. The last set was a little lean, but some color on them. That was at sea level. Yesterday the lake I was at was 5500 ft, so I would have expected a darker plug, but they were lighterÖalmost white, but not quite white. But, they only had about 20-30 min of running from the last trip out.

    I noticed very little fuel in the inline filter, and removed it, which didnít help the problem. I have had one of the plug wires come off the boot in the past (not while running), but I didnít remember to check this. I was focused on the white plugs, and not generally thinking that this may be a spark issue. I also tried the reserve, but no change. I know that two strokes tend to race when they are leaned out too much, and this isnít happening, so Iím not sure if the actual problem is a lean condition, or a spark condition, or worse.

    BTW, when the ski hasnít run in 1-2 weeks, it takes about 10-15 seconds of cranking with the choke on to fire up. Not sure if thatís normal. Otherwise, it starts instantly.

    So at this point, Iím thinking either its running on two cylinders (though its still smooth), or I have a lean condition, or the cylinders are fried, and am going to try things in this order:

    1. Spark: Try to start the motor with only one plug wire attached to see if thereís a spark issue on one cylinder. Test all plug boot resistances. Trim plug wires and reattach to boots.
    2. Compression test: When I bought the ski, it hadnít run in 1 year, and a dry compression test revealed 90-100-105, which isnít great. I never retested after using the ski.
    3. Fuel Supply: Disconnect the fuel return line, and run the motor to look for adequate fuel flow. How much should come out of the return line when the motor is running at the lower RPMs?
    4. Dirty Carbs: Pull carbs apart and inspect for any obvious crud, see if the float bowls are full.
    5. Test ride the ski
    6. Still not running right: Leak down test

    Any thoughts or suggestions to guide me along would be great, especially if you are familiar with the symptoms it is having.


  2. #2
    DavidR's Avatar
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    There are no float bowls, they're diaphragm carbs. You should pull the fuel pickup out if the tank and clean or remove the strainers on the ends of the tubes.

    Do another compression test, it was ready for a top end when you first got it.

  3. #3
    steve45's Avatar
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    So, do you normally boat at 5500 feet? That will definitely affect performance. Don't know why the plugs are white, but it takes a while for them to show some color.

    I'd do a compression test.

  4. #4

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    Hmmm, in regards to the elevation question....

    We have many lakes in California, and unfortunately, the ones close to the bay area ban the older two strokes. So with the current drought, we have few lakes to choose from that have good water capacity. All the other places I have ridden the ski were about sea level. I just happened to be up in the mountains for dirt biking, and saw that we would be near some lakes, so dragged the ski along with us. Not to mention how beautiful those lakes are in the deep evergreen forest. We were at Ice House Reservoir near Lake Tahoe, and near the venerable Rubicon trail.

    Do you think the elevation could have just made the boat have that much less power? I would have expected the plugs to look richer, though maybe they would have gotten there if we had ridden longer.

    I know my cars have a lot less juice up in the Tahoe area (6500 feet), but my two stroke bikes seems to always have an endless amount of powerband. Maybe because they are overpowered to begin with!

    Although the compression was getting low, it wouldn't explain the difference in performance from the last ride to this one. Though, the elevation might! So, thanks for the reminder of the elevation issue.

    I can actually launch the ski at the bay near my house. After a couple quick diagnostic checks, I'll take it out on Wednesday and see if the power is back up. I'll report back the results. Thanks for taking the time.

  5. #5

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    Ok so I did some preliminary diagnostics:

    Compression was 102/105/108, so looking ok at this point.

    I pulled the tank pickups, and they were clean as a whistle.

    I pulled the carbs though I havn't gone through them. I'm assuming that they are clean given the cleanliness of the inline filter and the tank pickups, so I'm guessing that the problem was the altitude, and perhaps the motor could use a little richening up, so I may bump up the mixture screws a bit since the carbs are off.

    At this point, I need to test it down here at altitude to confirm that the glaring problem was the elevation. I'll post back after a test ride. Many thanks!

  6. #6

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    Oh, one strange thing I found....when I took off the fuel lines from the carbs, the return line had a bunch of pressure in it.

    I always thought that the tank vents would vent out any pressure. Sound like a problem there, or just my lack of knowledge on how these work?

  7. #7
    DavidR's Avatar
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    You should probably look into that. Blow out the vent hose and the return line.

  8. #8

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    Well after poking around, I came across a "Group K" carb tuning guide, which specifically says that the tank should be pressurized, allowing air in, but not out. So it sounds like whatever I noticed on my ski is correct. Here is the excerpt:

    "Pressure test fuel system - All pwc utilize a sealed fuel system that has a check valve on the gas tank vent. This check valve (which permits pressure in the gas tank but not out) causes pressure in the gas tank that helps deliver fuel to the carb(s). Any air leak in the fuel system that permits the leaking off of this pressure, will also affect fuel delivery to the carb(s) at low speeds. To test for leaks, follow this procedure. 1) remove the return line from the fitting on the carb. 2) Blow into the return line while sealing off the return fitting on the carb with your finger. This will pressurize the entire fuel system. In a quiet room you'll be able to hear any remaining leak in the fuel system. When you remove your finger from the fitting on the carb, fuel will eventually drip out indicating that the float chamber is primed full of fuel."

    Any thoughts?

  9. #9
    DavidR's Avatar
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    Good find.

  10. #10

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    Went out and rode the ski in the bay at sea level, its runs perfect, as usual. A little embarrassed to say I never thought of the elevation as a cause of the low power....but there you have it. And I flushed it afterwards, as it was saltwater.

    Thanks for the replies!

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