Sudden loss of power and grinding noise

Toby woodman

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I have a 66cc motor. It starts on the first drop of the clutch and idles and performs well. After about five minutes of running even just moderately fast alternating between 1/4 and 3/4 throttle the bike doesn’t have enough power to sustain an idle and loses power overall. It also makes a grinding/clanking noise. Does anyone have an idea about what’s going on?
 
I have a 66cc motor. It starts on the first drop of the clutch and idles and performs well. After about five minutes of running even just moderately fast alternating between 1/4 and 3/4 throttle the bike doesn’t have enough power to sustain an idle and loses power overall. It also makes a grinding/clanking noise. Does anyone have an idea about what’s going on?
Short of guessing, there isn't much we can tell you that you won't discover by taking things apart and inspecting it to find the potential culprit. May be on the clutch side, could be in the bottom end. Just gotta poke around a bit.
 
Why would the engine lose power though and why would the noise only occur when the engine has been running for a few minutes
 
Why would the engine lose power though and why would the noise only occur when the engine has been running for a few minutes
Something that changes with temperature tends to come back to either a leak or clearance problem. As the engine heats up and the metal expands things that are ok cold can open up or rub when hot.

As for the power loss, it would be expected if something is grinding or locking up.
 
I’m running 28:1 and I am building up to 32:1. The engine has been broken in. I’ve just taken the head and jug off and there is no damage. I am running a pre painted engine and I know that the paint over the bearing surfaces. When the case expands maybe the bearings are squished and start to rub?
 
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I’ve checked all that. The small bevel gear seems to be quite far out away from the engine but it wouldn’t explain why it only makes noise when it’s warmed up
 
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