Backfire

[Straightpipes]

27 T backfires out the exhaust when I shut it off. Otherwise it runs fine. I must be getting unburnt fuel in the exhaust but I don't have any indication of running rich. Needle valve is set right in the "sweet spot". Any ideas on this? Thanks

 

[PaulWI]

Are you shutting it off with the timing fully advanced. You should ahve the lever all the way up when you shut it down.