I installed V&H Longshots on my 2001 Roadstar 3 years ago. I also installed an airkit and rejetted the carb at the same time. For me, it was a very difficult process as I had never undetaken a mechanical project like this, and I had a tough time getting the upper and lower pipes to match up and be the same length.

In order to make the upper pipe/muffler the same length as the lower one, I had to pull the muffler off the pipe just a 1/4" or so and then tighten everyting down. They looked fine, but I was getting a little bit of soot coming out where the muffler joins the rear header pipe. So- it leaked. And- I had the "flutter" sound on decel that I'll bet is the same thing you have.

I lived with this until a week ago when I pulled the pipes off and did it all over again. This time I sealed the pipe/muffler joint with Permatex high-temp Ultra Copper gasket maker/sealant. The noise is now almost completely gone. I'm told that on decel, if you have any leak in the exhaust, air gets sucked into the pipes and it causes this flutter or popping. That's my experience.