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Pistons won’t turn over

Discussion in 'XJ Technical Chat' started by apato632, Apr 16, 2019.

  1. apato632

    apato632 Member

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    hi all,
    This is unrelated to my last thread on electrical gremlins.

    For an xj750rj Seca 1982 barn find

    This bike has been sitting for years. I put in new oil and turned the engine over by hand at the crank before I bought it. That worked so I brought it home and I’m trying a compression test.

    The starter turns over and when i put it in a high gear the back wheel will spin slowly. But the pistons aren’t turning over at all in any gear or in neutral when i hit the starter. They will still move by hand if I manually spin the crankshaft. I put a little oil down each cylinder to see if It did anything but no change.

    I think I already know the answer here but is my bike NFG?
     
  2. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    You should not be trying to turn the engine over with the transmission in gear, unless you are push-starting it.
    Never use the starter with the bike in gear unless you like rebuilding starters.

    If the starter is not working in neutral then you have a issue with the safety interlock system.

    Does the neutral light come on?
    Is the clutch switch hooked up and in working order?
    Do you hear a click from under the fuel tank when you turn the key on?
     
  3. apato632

    apato632 Member

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    Thanks K-Moe,

    So the thing is, I started it in fifth to see what would happen because the pistons weren’t moving at all. It’s not something I’d normally do but I hear you.

    The starter works in neutral. My issue is that when the starter turns over the pistons don’t move and the external portion I can see of the crank at the left hand side doesn’t rotate. They definitely aren’t moving.

    How could this be? Is something seized or broken internally? I have no history on this bike.

    On the bright side, it was $50 bucks and I have use for the parts if the engine is FUBAR. Still, I’d like to know what’s going on.
     
  4. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    OK. That narrows things down. What sounds do you hear when you thumb the starter?
    Coffe can with rocks in it rolling down a driveway, or just the sound of the starter?
     
  5. apato632

    apato632 Member

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    No coffee can. So not the starter chain guide?
     
  6. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    The alernator chain guide makes no moise when it fails.

    Do you hear a slipping/spinning sound?

    Can you make a video with decent sound so we can hear it?
     
  7. apato632

    apato632 Member

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    K-moe, I don’t know if I’m crazy but I went to take the movie, the pistons are moving now, and it sounds normal. I swear they weren’t moving yesterday but i haven’t changed anything.

    Could this have happened because I had oil down the cylinders? I poured about 1/2 an ounce down each one three days ago. I also put some onto each valve from under the valve cover. It was bone dry.

    I’ve got a new issue though. My 2/3 aren’t sparking but when I unplug that coil my gauge lights go out. Sound like a short?
     
  8. Timbox

    Timbox Active Member

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    I would check the ends of the coil wires to make sure they are making contact. Some cut a little off the ends of the coil wires to made a better contact. Just make sure they will still fit or reach the spark plugs. Testing the ohms on the coils will also give you a good read on the coils.

    When I get a new to me older bike, one of the first things I do it to take a look at the wiring and see how corroded the contacts are. If you start to work on the wires, make sure you disconnect the batter so nothing happens to short out or blow.

    Barn finds get hot, cold, dry and wet, the perfect setup for rust and corrosion of the wires. The ground wire should be looked at as well.
     
  9. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    Could be that one cylinder was hydrolocked. 1/2 ounce is quite a lot of oil to be putting down the top side of a bore. A tablespoon is typical.
    Next time use a 50/50 mix of ATF and acetone. It works better at freeing up rings than motor oil does, and the excess drains faster.
     
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  10. apato632

    apato632 Member

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    Thanks all,
    I guess I overdid it on the oil. Will dial it back next time. Thank you for your help. Onto the next problem.
     
  11. Timbox

    Timbox Active Member

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    Blasting each cylinder with compressed air to remove the excess is good too. Wrap a rag around your hand that is holding the air nozzle and use safety glasses. Works great if you are trying to clean up carbon off the top of the piston too. Just a suggestion if you have too much oil in there trying to free things up. Blowing the cylinder out before replacing the spark plug helps to ensure that it will not hydro lock.
     
  12. Ryengoth

    Ryengoth Active Member Premium Member

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    I use 2-stroke oil and gas sometimes too, depending on what's near me.
     
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