Have spark, have gas, have zero compression!

Discussion in 'Tech Area' started by Dash, Jun 6, 2021.

  1. Dash

    Dash New Member

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    I have a K0 ct70. It ran fine last summer and suddenly died on a ride. I assumed it was electrical. Replaced points and condenser and questionable ignition winding. Spark was fine. Cleaned carb. Didn’t start even with a shot of starter fluid. Bought a harbor freight compression tester and it showed no - zero compression. Pulled head and cylinder. Intake valve wasn’t seated, piston, rings, and cylinder looked fine. Intake valve pulled and there was rust in the valve seat and valve stem is slightly bent. Can I have a new valve seat cut? Do I need to have the valve guide replaced?
     
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  3. Dash

    Dash New Member

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    Here are pictures of the valve issue.
     

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  4. ronzrx

    ronzrx New Member

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    I just had a valve job done on my 1970 ct70ko. My exhaust valve was not seating good. I bought new valves, guides, seals, springs machine shop did the rest was like $90.00 labor. Here is a pic of it when I got it back. And before
     

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  5. lukelaw1

    lukelaw1 Active Member

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    it looks like that intake valve has hit something from the photo, are you sure the piston didn't get tapped? how tight is/was the valve lash in the rockers?

    valve guides usually hold up, might as well do both exhaust and intake seats, both valves, along with new rockers and springs.
     
  6. Dash

    Dash New Member

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    I agree there is a small nib on the top of the valve. First thing I noticed. Not sure what caused that. Then I noticed it didn’t look like the valve was seated and I could see light looking through the valve adjustment hole. The valve came out fine. When I roll the stem on a granite countertop it is bent.
    In trying to get the bike running again I didn’t have spark. All of the electrical parts replacement solved that. I adjusted the valve clearance and they were both very tight and did not have the correct clearance.

    thoughts on what bent the valve stem? On what took the nib out of the top edge of the valve? You have me concerned about the piston now. How can I tell if it took a hit?
     
  7. Robert thran

    Robert thran Active Member

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    You should see a spot on the piston dome ( clean piston good and look for a witness mark) if it hit the piston..make sure your cam chain isn’t slap wore out..if it is it could jump time and possibly bend another valve.? Or weak valve springs and over reving? Both are long shots but I would still check.....
     
  8. Dash

    Dash New Member

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    Yes I looked this morning and there are scratches on the dome like you said. I should have noticed. All of my rebuilds in the past have been frozen top ends that were encouraged to release and not a diagnosis of what happened. I will take the cylinder to a machine shop along with new valves to get that resolved. I will check cam and rockers for integrity and tolerances. Can I use this piston and jug? Replace cam chain? What else to get this back together?
    70D6C610-CF96-4416-8698-ACA29D4FC359.png
     
  9. OLD CT

    OLD CT Well-Known Member

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    Looks like it may be the wrong piston. Need better pictures to be sure. Is there a tiny arrow on it pointing down/sparkplug clearance divot, present?
     
  10. Dash

    Dash New Member

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  11. Dash

    Dash New Member

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    Here are better pics. Looks to me like piston was printed correctly with arrow pointing down and dimple on the plug side. I noticed the scratches on skirt of both top and bottom. I do see very light scoring in the cylinder when looking at it in sunlight. I cannot feel it. what are thoughts - honing and a new piston and rings?
     
  12. Dash

    Dash New Member

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    Pic of cylinder FB9B3BDA-251D-43F5-83B8-FDF0FE2D9F03.jpeg 42209931-0C0F-46B0-A61E-E4F43EAB2B34.jpeg
     
  13. OLD CT

    OLD CT Well-Known Member

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    That is the right piston, The cheapest way out, I would transfer the dowels into a new standard bore cylinder from ebay and throw out the piston from the kit. You could harvest the gaskets and buy a ''new correct piston'' kit ''standard bore'', separate. Nothing wrong with the cylinders though. $29 plus shipping.. Or upgrade a little and buy a basic K0-81 88cc piston/jug. Wouldn't be a good idea if you are rebuilding the original head and cutting corners with a hopeful hone job.:)
     
    #12 OLD CT, Jun 7, 2021
    Last edited: Jun 7, 2021
  14. JHminitrails

    JHminitrails Member

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    One other item you should check is your connecting big end bearing. You want zero radial play, meaning up and down. Side play is normal, but if there's any up and down play, the crank will need to be rebuilt. This play coupled with the connecting rod expanding at operating temperature and a high RPM run, could spell disaster for piston to valve interference.
     
  15. Dash

    Dash New Member

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    Rod doesn’t seem to have any radial play. I found a machinist to cut new valve seats and place new guides on the cylinder head and bought an aftermarket jug and piston and rings. The cam lobes and rockers are within tolerances. Hopefully have it back together and running in a week or two. Thanks for all of the help!
     

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