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  1. #1
    Click avatar for tech links/info K447's Avatar
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    Arrow Virage TXi piston stuck, apparently running too lean at wide open throttle

    Quote Originally Posted by ripcuda View Post
    Let the stories, pictures and videos of this past weekends festivities commence...
    Finally had time to pull apart my unhappy Virage TXi engine. Turned out the repaired EMM was not yet quite right and was running the engine too lean at wide open. Or at least wide open sustained cruise was when the problem became apparent

    The engine would suddenly loose RPM and then shut down within a couple of seconds while the throttle was still held to the bar. Cranking immediately after was sluggish. Wait a minute and it would restart and run seemingly OK, until the next wide open run, then shut down again. Sounded 'funny 'when restarted. Rode it back to the dock at mid-range only, put in on the trailer.

    As best I can tell all three cylinders were running very lean (no piston wash, flat black across all three piston tops) and the PTO apparently piston expanded enough to stick in the cylinder at full throttle. I can see and feel the aluminum stuck to the iron cylinder walls. The piston sides are scored in multiple zones all around.

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    The funny marks in the PTO carbon are from the 'rope trick' used to loosen the drive coupler before pulling the head.

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    CEN and MAG pistons are OK, just no piston wash. Cylinder walls are smooth and unblemished.
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    PTO cylinder walls showing aluminum metal smear transferred from the piston sides.
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    This is an SBT engine with 1.00 marked oversize pistons. The now exposed PTO connecting rod and crank seem just fine.

    11 hours total running time on this engine, but it sat unused for three years prior to me installing it with zero hours.

    I decided to order another SBT piston to replace this one along with the rings, piston pin, etc.
    I think the cylinder will clean up without needing re-boring.

    I will also be switching to a different (2002 Genesis) EMM + injectors while the 'repaired' EMM gets sent back for a re-do. I did not have this alternate EMM on hand until after the piston stuck

    Before the piston sticking event the engine was running moderately rough in the mid-range so I knew something was not quite right, despite having smooth and abundant power above 4500. My error was not checking the piston wash after breaking in the engine. Until it stuck a piston it was running as well as could be expected (:

    No issues with engine cooling, as far as I know. Dual pissers were flowing well for the EMM and stator cover, engine never felt hotter than normal even after running it hard.
    Last edited by K447; 08-10-2015 at 04:46 PM.


  2. #2
    Click avatar for tech links/info K447's Avatar
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    Oh well, at least I am not pulling the engine for a complete rebuild (yet, anyways) ...

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  3. #3
    Dalts72's Avatar
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    You'll be back up and running in no time!

  4. #4
    ripcuda's Avatar
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    So it was just the one PTO piston that was seizing? The others were run lean also but didn't suffer any damage?

    I'm guessing you've checked WOT fuel pressure?

    Glad it wasn't a total loss or meltdown.

    Cheers!

  5. #5
    I like pipes. I love boost Mr. GP1800's Avatar
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    Did you ever leak test that motor?

  6. #6
    Click avatar for tech links/info K447's Avatar
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    Quote Originally Posted by Mr. Matrix View Post
    Did you ever leak test that motor?
    Nope. Air leaks are not a big concern with Ficht engines. Note the size of the air bypass holes in the throttle plates.

  7. #7
    Click avatar for tech links/info K447's Avatar
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    Quote Originally Posted by ripcuda View Post
    So it was just the one PTO piston that was seizing? The others were run lean also but didn't suffer any damage?

    I'm guessing you've checked WOT fuel pressure? ...
    Yes, just PTO piston and cylinder showing any damage.

    I shall recheck fuel pressure when the engine is operational again.

  8. #8
    I like pipes. I love boost Mr. GP1800's Avatar
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    Quote Originally Posted by K447 View Post
    Quote Originally Posted by Mr. Matrix View Post
    Did you ever leak test that motor?
    Nope. Air leaks are not a big concern with Ficht engines. Note the size of the air bypass holes in the throttle plates.

    I dont buy that one bit. Those holes are there for low speed operation. At higher throttle settings those holes are irrelevant. An already lean running engine sucking extra air that the EMM has no idea is there is a recipe for disaster. Any 2 stroke motor must be sealed from air leaks.

  9. #9
    Click avatar for tech links/info K447's Avatar
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    Quote Originally Posted by Mr. Matrix View Post
    Quote Originally Posted by K447 View Post
    Quote Originally Posted by Mr. Matrix View Post
    Did you ever leak test that motor?
    Nope. Air leaks are not a big concern with Ficht engines. Note the size of the air bypass holes in the throttle plates.
    I don't buy that one bit. Those holes are there for low speed operation. At higher throttle settings those holes are irrelevant. An already lean running engine sucking extra air that the EMM has no idea is there is a recipe for disaster. Any 2 stroke motor must be sealed from air leaks.
    Well, below 5500RPM the engine is running stratified charge combustion. The injected fuel plume is localized around the spark plug tip and spark firing is timed to intercept the fuel spray.

    Above (approx) 5500 RPM the Ficht EMM switches modes and fuel is injected well before the spark event and mixes with the air turbulence (homogeneous operation). This mode is analogous to how a carburetor 2-stroke engine works.

    Quote Originally Posted by Kawasaki STX 100 D.I. technical notes
    Stratified and Homogenous Combustion
    The engine uses a stratified combustion mode from idle up to approximately 5500 RPM. In this mode, the injector waits until the piston closes the exhaust port then fires a dense plume of fuel at the spark plug. Elsewhere in the combustion chamber the air / fuel mixture is extremely lean. The spark plug fires up to 15 times as the fuel passes by the electrode. This unique multi-strike ignition burns fuel thoroughly and keeps the spark plug insulator clean.

    The homogenous combustion mode starts above 5500 RPM. In this mode, injector timing advances and fuel is sprayed further into the cylinder where it bounces off the top of the piston and disperses throughout the combustion chamber in a uniform air/fuel mixture. The ignition timing also advances and the spark plug fires only once per cycle.
    In this particular engine all three pistons are showing very similar wash patterns (well, almost no pattern), which seems unlikely to be caused by an air leak through a crank seal (PTO or MAG).

    My understanding is that minor air leaks into a Ficht engine are generally not an issue. It is not so much that the EMM does not know about some 'extra' air, it is more that the engine does not care very much as it is already running with a very lean overall mixture, at least below 5500. The combustion zone within the stratified fuel plume is rich, the outer areas surrounding the combustion zone are lean.

    Above 5500 RPM in the water the throttle plates must be pretty far open so minor air leaks are a smaller percentage of the air flow.

    2-stroke engines have been associated with carburetors so completely that it can be weird to think about a 2-stroke which does not conform to the norms defined by a carburetor. In my view it is the carburetor that is affected by air leaks. The carburetor's function entirely depends upon all of the air flow into the engine going through the carb. Air leaks mess up this requirement, hence the desire to eliminate air infiltration into the engine from other routes.

    A Ficht engine cares much less about the air flow sources, or even (within reason) the air quantity. For example, if you unplug the TPS from a Ficht engine and start it, it will idle. Now squeeze the throttle to open the air flow into the engine. You can hear the air intake sound change as the engine draws more air but the idle RPM does not change much, if at all. That is stratified combustion not caring much about the exact air to fuel ratio.

    Think about how the throttle on a diesel engine works. Air flow is always high, fuel injection controls the engine power output.
    Last edited by K447; 08-11-2015 at 09:19 AM.

  10. #10
    ripcuda's Avatar
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    Interesting. The more I learn about the DI skis... the more I like them. Still miss my MSX 140.

    So if all your cylinders were running about the same leanness... why do you suppose the PTO became the sticking point?
    Is it the hotter cylinder?
    Leanest because last in fuel daisy-chain?

    Cheers!

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