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  1. #1

    Is carb fuel level too high? Worn out carb parts?

    1994 Kawasaki 750 SS - how do you know/check if the fuel level in the carburetor(s) is too high, possibly from the needle/seat being worn out?
    I realize I probably made mistakes here when I thought it would be ok to run this jet ski for a few seconds with the gas petcock off, after running it without the hose attached (under 30 seconds), so I could keep the float bowl somewhat drained to help prevent any buildup in the float bowl and on the jets. I did this a few times over the last few months with no problems. From my experience this is fine on dirt bikes and ATVs, but I know it's pointless if I'm going to start up the ski every 1-2 weeks anyways. This ski also needs the gas drained and new fresh fuel which I will do ASAP.
    I have good experience with 2 and 4 stroke dirt bike carbs, but have never touched a PWC carb, so I dont know if there are any major differences. I'm thinking the float needle/seat in this 750, which as far as I know are the original factory parts (if that's possible?), finally wore out/got dried out from the old fuel being "drained" in the float bowl. So now all of a sudden, the float level is too high and the engine is getting too much fuel, also fouling the spark plugs. The inside of the hull has smelled like gas for some time though, so that makes me think the float/fuel level has already been too high and this problem was about to happen anyways.

    What happened was after I started it up (after about 2 weeks of sitting, it immediately fired up) and it ran good for 15-20 seconds, the engine died. The cylinders and upper header were already pretty hot. Started it again and it died right away. Then within the next few times, it ran poorly and wanted to die unless I held the throttle open. It backfired a couple times (a light "coughing" backfire, not a loud bang) while coughing out oil sludge from the exhaust. Never seen that before. I started to worry if it was overheating. I saw some gas residue on 1 or 2 of the hoses connecting to the carb. The next day a little oil sludge was dripping from the exhaust exit on the hull.
    I started thinking about the float needle/seat getting worn and dried out, so the float level would be too high in the carb, and all this made sense to me and I hope I'm right lol...

    If my thinking is correct on what is going on with my 750, should I order just the carb needle/seat, OR the float bowl gasket(s) as well, OR the whole carb rebuild kit? Seen here: https://www.shopsbt.com/mm5/merchant...t=jet-ski-fuel
    I am about to check the plugs which I expect to be covered in oil. I think I already have the correct size replacement poly fuel line(s), and I know that should be replaced too. Anything I'm missing, please let me know.


  2. #2
    steve45's Avatar
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    Whoa! Settle down!

    First, do not run this thing more than about 15 seconds without water. You can actually run it a little bit longer, but only at idle. Running it with the throttle wide open and no water, you may have done some serious damage. I would recommend doing a compression test before going any farther.

    Your carburetors are diaphragm carburetors. They do NOT have floats. A little fuel smell is pretty normal for PWCs. The tank is sealed so that it doesn't leak gas into the water if you flip it upside down. It can pressure up and leak a little fuel into the carburetor throat and into the engine. It's also possible that the diaphragm may have a pinhole that leaks fuel into the hull. Inspect the system carefully for leaks.

    Poly fuel hose is NOT approved for use in marine applications! You need USCG approved hose.

    Do NOT use a carburetor kit from SBT. Use ONLY genuine Keihin parts. We've had several guys recently on another site that used aftermarket kits and learned that the metal 'button' on the diaphragm was not the correct height, which totally screwed up the mixture. Rather than setting the float level, the opening pressure, or pop-off pressure, is what is used. There are lots of videos on YouTube showing how to do it.

    How old it the fuel? For storage, you want to treat the fuel with a stabilizing product, then run it long enough on the hose to circulate it through the system. Top off the tank to keep air out and prevent condensation. Ethanol-free fuel is definitely better than fuel with ethanol.

  3. #3
    Rodsatheart's Avatar
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    10-15 seconds MAX run time without water. This isn't a bike, don't treat it like one. Not even the carb is the same. The only similarities is it's a 2 stroke, reed valve engine. The sludge is from unburnt oil. The engine is not staying at running temp long enough to burn it efficiently.
    Your thinking is not correct.
    You need to read this. The fundamentals are the same across the board.
    You need to get a Mikuni pop off gauge to set the needle pop off. Should be 25psi. Your mixture screws should be set at 1 turn out for both low and high speed. Do not use the mixture screws to adjust idle speed. That what the idle screw is for.
    As Steve said, use a quality carb kit, or your going to be chasing your tail trying to tune it.
    http://www.mikuni.com/pdf/sbn_manual.pdf
    All that said, sounds like you might have already fried the engine. You need to do a compression check. Should be 140ish. If it's still good, you need to replace the oil injection lines and fuel lines (and don't use standard clear poly line for fuel, it MUST be fuel line), pull the supply assy off the tank and clean the vent. it corrodes and plugs the tank vent so it will be unable to pull in air as fuel is used.

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