Knocking at higher RPMs???

Discussion in 'General Questions' started by GCrews25, Oct 1, 2014.

  1. GCrews25

    GCrews25 New Member

    Not exactly sure what is going on. At mid to higher RPMs I'm getting a loud knocking sound that doesn't sound great. As soon as it starts making this noise I have to grab the clutch and slow down a bit and it goes away until I start moving as fast again. If I keep going at the same speed once the knocking starts, then she slows way down and will keep slowing down until I grab the clutch again :veryangry:. I'm running my last tank of break in mixture through now, and I was told to use 16:1. I am pretty sure the chain isn't hitting anything, so I have no idea what it could be. First build, so I am also pretty new at this... Any help would be great!! :confused:
     

  2. crassius

    crassius Well-Known Member

    if it sounds like a high-pitched popcorn popping sound, that may be normal until motor runs in - if a low-pitched clunking sound, that may be backlash in the output shaft (a little can be normal, but a lot can mean a loose chain that needs adjusting)
     
  3. GCrews25

    GCrews25 New Member

    It's more of the low clanking sound. My chain is tight though, so if you could please elaborate on that a little more maybe?? I'm totally stumped
     
  4. crassius

    crassius Well-Known Member

    not sure then, wish I were there to see/hear it - if you could get the back wheel in the air & watch & listen to the chain run, you might find something there
     
  5. GCrews25

    GCrews25 New Member

    Yea, that's my next step. Hopefully I'll find something there. Thanks anyways!
     
  6. GCrews25

    GCrews25 New Member

    Update:
    The chain seems to move freely, and doesn't hit anything. Same reoccurring problem though. Maybe the piston is traveling too far and hitting the spark plug, but it only occurs at higher speeds and runs just fine at slower speeds? I'm really out of ideas; any help is greatly appreciated!
     
  7. jaguar

    jaguar Well-Known Member

    true knocking won't be happening if you still have standard low compression.
    probably a clutch or chain problem.
     
  8. GCrews25

    GCrews25 New Member

    What could be going on with the clutch? I was also reading that the piston may be loose and have to much side-to-side play in the cylinder?
     
  9. Purple Haze

    Purple Haze Active Member

    Check the small obvious things first. I've always found that the chain hits the cover, a little dremel work will fix. The best way to trouble-shoot these engines is to remove the small drive gear on the clutch side, eliminating the clutch from the equation. Then remove the spark plug to release compression. Now you can spin the engine over by hand, that will help pinpoint any internal problems. On a side note, I once got an engine from BGF on ebay that shipped without a small end rod bearing. Needless to say, it had quite a knocking noise!
     
    GCrews25 likes this.
  10. GCrews25

    GCrews25 New Member

    Awesome, I will check that this week! Thanks for the help
     
  11. jaguar

    jaguar Well-Known Member

    the piston gets too much clearance after the cylinder wears down but your ride is new so that is not the problem
     
  12. GCrews25

    GCrews25 New Member

    Gotcha. Well that eliminates that!
     
  13. jaguar

    jaguar Well-Known Member

    too much chain slack can cause the chain to hit the crankcase at high speed
     
  14. GCrews25

    GCrews25 New Member

    The chain is pretty tight, so I don't believe that it is that.
     
  15. GCrews25

    GCrews25 New Member

    Watching on YouTube and the guy said that the arrow on the top of the piston needs to be pointing towards the exhaust port. Is this true? I have never moved mine, yet it is pointing towards the carb port?
     
  16. Fly1

    Fly1 Member

    Well these are china engines so go figure. They may have stamped it on the wrong side. Pull the cylinder & make
    sure the ring end gaps are toward the intake side. The back of the rings always face exhaust. I don't think that's
    the problem, but to be sure check. I would have thought you would have already hung a ring by now.

    Fly
     
  17. GCrews25

    GCrews25 New Member

    the top ring end gap was facing the left, and the bottom ring was facing the right... Should they be aligned with each other?
     
  18. Fly1

    Fly1 Member

    No there staggered, but should be more facing the carb an intake side the cylinder.

    Fly
     
  19. GCrews25

    GCrews25 New Member

    Ok, I got that figured out. They must've stamped the arrow facing the wrong way then. Still not sure what's causing the knocking sound though.
     
  20. jaguar

    jaguar Well-Known Member

    exhaust can pop out from the cylinder head mating surface if its not perfect.
    all these engines have warped planes there. You have to plane them flat (both cylinder head and cylinder top) with sandpaper on glass.
     
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