ORIGINAL: Gizmo-RCU
I think Switch and Mr. Cox hit it , tuned pipes require running on the rich side as more fuel is required when the pipe ''kicks'' in.
Sounds right AND this engine has a history of being lean in the mid range. I believe the heli version has a mid range needle.
They need to be set a bit rich with a standard muffler so set it as rich as it will run reliably for a pipe. Also some of the older 91 FXs had the needle behind the piston causing heat build up in the needle
assembly itself. OS fixed this with a bracket that moved the needle assembly out to the side and into the air stream. Mine likes 10% dosen't seem to like 15%. I have Macs muffler on it.