I have a '79 FLH 80" stock with an S&S super E. Just put some slip on fishtails It seems to be back firing more. Do the fishtail tend to cause a lean or rich mixture?

 

torque cones might help - but any fuel that is off the negative wave from not having a baffle - will cause pops and going down hill it can be a thing after a while if you already don't care for it

but 1/2 bottle of jack using fixes just about anything - in chemistry alcohol is a solution

 

Muffled fishtails shouldn't make any difference. Put them on my '82 FLHC with an S&S super e. No difference in popping and plugs looked fine.
Now drag pipe fishtails are a different matter.

 

Ya your going to get some popping if its too much maybe re jet

 

Drag pipes sound wonderful, but they pop when down shifting. part of the scene.