Knocking at higher RPMs???

GCrews25

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Oct 1, 2014
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Location
St Louis, MO
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 :mad:. 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:
 
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)
 
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
 
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
 
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!
 
true knocking won't be happening if you still have standard low compression.
probably a clutch or chain problem.
 
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?
 
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!
 
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