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hi I am helping a good neighbor with his Bmw R100 it works but becomes weak at higher speeds. so after a short drive it becomes weaker. the carburetors are cleaned. it knocks the valves on the right cylinder even though the valves are shuttered. or it may have to do with the carburetors not being calibrated. or it could be something else
 

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Does it feel like its dropping a cylinder?

Heat can cause things to expand... that can include wiring sheathing. I've run into instances where a hot motor caused a plug wire to lose connection. It would "reconnect" when the motor cooled down
 

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Lots of things it could be.
Ignition timing?
Faster you go the more the ignition timing needs to advance. If the ignition timing is retarded it will work reasonable at low revs and fall on it's face at higher revs.

The valve noise on one side is not good, you need to explore that. Check your plugs and see if they look the same, if the carbs are off badly enough, that should reveal on inspecting the spark plugs. Compression test should be close enough to same on both cylinders or balancing the carbs will never be possible.
 
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