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  1. #1

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    What are these deposits in the combustion chamber?

    The compression in my modified 15F engine dropped suddenly. The symptoms were hard starting and dying at idle. Once on plane it still reached peak RPM. Upon removal of the head an abundance of this powdery gray residue was found in the cylinders. Cylinder #2 had the worst build-up and there was almost none in cylinders #3 and #4.

    What could be the cause of these deposits?
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  2. #2
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    Are you running leaded fuel in that? What did the head look like? The installation instructions for the endurance kit says to use ~ MS109 or c12

  3. #3

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    As a matter of fact, I was burning 110 octane leaded Sunoco Standard race fuel. What is the endurance kit you're referring to? The seller said it had 12F pistons, large cams, and a custom tune but I didn't know it was a "kit". Could you explain?

  4. #4
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    I would think purple would be enough but cant say for sure. I know the lead will make the grey powder in there but that is quite excessive. Maybe it was mixed with oil. Did you find out where you were losing compression? I thought the previous owner had installed the riva kit with a reflashed ecu.

  5. #5

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    The compression was lost because some of the exhaust valves were stuck. It appears that the powder migrated into the valve guides and bound up the stem. All of the intake valves moved freely. Once the machinist cleaned up the valves and head, all of the valves moved freely again. My local sources also theorize that the lead is causing the problem since the combustion temperatures in a Jet-Ski aren't hot enough to burn off the deposits. Presumably this is because there is no thermostat and an abundance of cooling water available from the open loop system.

    It will be easy enough to test the theory once the engine is put back together and an unleaded fuel is used. The seller said it has a custom ECU tune from Kawasaki and requires a fuel with a minimum 100 octane rating. I will be installing a new wideband system to monitor AFRs to help me determine what octane I can get away with.

    Thank you for your input!

  6. #6
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    Here is a pipe I just changed out that had 2-3 tanks of half purple + half pump gas ran through it. I had a drum of old sunoco standard that wanted to get rid of so mixed it in with 87 on my stock 15f, looks fine. I think you have something else going on there.
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  7. #7

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    Are you referring to VP110 when you say "purple"? If so, this is a leaded fuel (according to their web page). I agree that something else is going on but it is hard to come up with theories.

  8. #8
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    That doesn't look at all like lead deposits. I've worked on a lot of airplanes that use highly leaded fuel and most of the lead deposits are found on the spark plugs. Looks like bird shot inside the plug. Sometimes it shorts the electrodes.

  9. #9

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    That's what's weird-the spark plugs showed no signs of debris or damage in the combustion chamber. The top of piston #2 was covered in this powder-it was hard to believe that the combustion process wouldn't have blown it out of the exhaust ports. But I still can't understand what substance in the fuel could be precipitating out during the combustion process.

  10. #10
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    Quote Originally Posted by pro1200 View Post
    Are you referring to VP110 when you say "purple"? If so, this is a leaded fuel (according to their web page). I agree that something else is going on but it is hard to come up with theories.
    Sunoco standard 110 leaded is purple. Was there any fuel in the tank when you got the ski or did they drain it for shipping? Seen stuck valves from old fuel but usually intake. The valve guides were ok? Did you get any pictures of the valve stem or head?

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