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That sounds exactly like an intake leak. Maybe go back and recheck for leaks. How did you do it the first time? Pay close attention to the back of the carburetor where it mounts to the manifold.

Another possibility is the low speed / off idle circuit in the carburetor. Is it an S&S carburetor?

The third thing that comes to mind is an ignition issue of some kind. Like the advance isn't working right. Is it an older Dyna single fire unit or is it an all electronic unit?
 

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Try using the WD-40 again but also spraying it around the joint where the carburetor attaches to the manifold. Do the whole test after the bike has warmed up. In other words if it runs right when you first start it, and you do the test at that same time, it likely won't show the leak. It'll be easier to find if it's running rough. Hot temperatures can change the seals and cause a leak.

If it's a small leak that opens and closes you'll hear just a slight little change in the engine when the WD-40 hits it, and it won't last long. Listen for just any slight change.
 
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