I've been Whupped

Discussion in '2-Stroke Engines' started by dougsr.874, May 8, 2013.

  1. dougsr.874

    dougsr.874 Active Member

    Have a Skyhawk 49cc GT2 engine that was made in 2010...Won't start, won't run.... I haved replaced with new NT carb. verified that gas in the carb bowl...installed new mag-coil, new plug wire, new plug.....used CDI, put in fresh gas, compression checked at 90psi........when I removed the mag cover there was a small amount of oil in the cover...which I assume can only come from the crank seal...plug is producing a bright blue spark....If the crank seal was bad wouldn't the compression ck. show it...no kill switch used..
     

  2. jaguar

    jaguar Well-Known Member

    crank compression and cylinder compression are separate.
    an engine wont run without crankcase compression (caused by leaky seal).
    you can replace the two crank seals w/o splitting the cases. Just use a jewelers screwdriver to pry them out. use a file on the right side shaft because there is a sharp lip there that the seal sometimes wont go over. Best seals to buy are those from Rock Solid Engines but any of the others will get you by.
     
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  3. Fabian

    Fabian Well-Known Member

    If the case seals are damaged they will allow air to enter the crank case which will make the engine not only hard to start but will cause all sorts of problematic idling issues. A great many of the the carburation problems that people describe on this and other forums have a lot to do with crank case seal issues, but in many situations, the seal (externally) looks like it is in good condition.

    Pressuring the crankcase (for leaky seals) is a must-do when evaluating poor carburation characteristics.

    The photos are an example from one of my engines. In the older engines a very thin (2mm) crank case seal was used on the magneto side and it doesn't take much for it to leak. The newer engines use the larger clutch side seal on the magneto side, which solves most of the poor sealing issues on the magneto side.
    It's also difficult to see if the magneto side, crank case seal is leaking because it's covered by the magnet, but typically if you have a damaged seal, it will leave an oil trace in the direction of gravity.

    If you have a sudden and dramatic loss of power and the engine starts to backfire, a blown crank case seal (that has catastrophically failed) is most likely going to be the problem, which is displayed in the photos.



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    Last edited: May 10, 2013
  4. dougsr.874

    dougsr.874 Active Member

    How do you pressurize the crankcase to check the seals,,,,,,,,other than soapy water & bubbles...while spinning the engine.????
     
  5. Fabian

    Fabian Well-Known Member

    That's an easy answer: make sure the piston is somewhere near top dead centre and then simply remove the carburettor from the intake tube. Go and get yourself a 5mm thick piece of rubber that's about 20mm in diameter and drill a 4mm hole through the centre.
    Grab yourself an air gun and jamb the nozzle end into the 4mm hole, then force the rubber against the front face of the intake tube. Gently hit the air gun trigger and watch the left hand and right hand side crank case seal to see if the soap spray you applied earlier shows any rapid exit of air from the seals.

    You need to be very careful with this technique so as not to blow out the seals and also not to blow out the case half centre gasket. Ideally you don't want to be placing any more than 7 psi into the crankcase.
    Worst case scenario if you (physically) blow out the case seals like a champagne cork, it's an easy job to reinstall new seals. However if you blow out the case centre gasket, you are completely screwed because you'll have to totally strip down the engine and do a full rebuild as the centre case gasket is a vitally important part of the engine.
     
  6. Fabian

    Fabian Well-Known Member

    May i also add that the air gun technique is not a the job for a beginner; it's almost a certainty that he will over pressurise the crankcase and bad things could happen.
     
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