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Knocking sound behind front sprocket cover?

rafalc

CB750 Enthusiast
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Title pretty much says it. While idling I hear a light metal on metal knocking. I hear it the most clearly when my head is down by the front sprocket cover. This knocking is consistent and follows a timed pattern. Any ideas?
 
What bike? year? does it go away when you pull the clutch in? get better with a higher idle?
 
So I did more looking around and its not coming from behind the sprocket cover, it comes from the carbs. Yes,carbs, this leads me to believe its probably not metal on metal knocking but rather air being sucked in louder than I would expect therefore making that sound resembling a constant popping/knocking sound? I used a stethoscope and I can def hear the same patter when I press it against the outside of the 3 & 4 carb bowl.

But to answer the question anyway - The sound does not go away when clutch is pulled in, it speeds up with the revs although hard to hear since bike runs louder as revs go up. This is a 1973 cb750K
 
Are the slides worn bad in the carbs? You could also have maybe a valve issue on the intake side and it is resonating back into the carbs. Either tight or loose or maybe even a burnt valve blowing compression back into the carb. To the simple stuff first, adjust valves, compression check, sync the carbs. You shouldn't have a noise in the carbs. Very late model carbs have been know to make a noise as they have a phenolic slide that rattles in the carb body from the intake pulses. The CBR900 were especially bad for this.
 
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