I have two harleys:
1) is a 1995 FLSTN
2) is a 1984 FLHT

Both have the EVO in it and Both of them make a popping noise kind of like a back fire when you let off the throttle. The 95 has Vance & Hines big shot long shots and the 84 has a set of stock headers off a 01 FLH with a set of thunderheader slip on's Both bikes have recently had the little gaskets at the heads replace but still do this. Could it be because both sets of pipes have a cross over? My brother said when he had his FLH it did the same thing when he had the stock headers with aftermarket slipons but when he changed to a set of Vance & Hines long shots with out the cross over it stopped.. Any Idea's as to what would cause this?

 

On a carb setup when you get popping on deceleration or letting off the throttle its running lean. 1/4 -1/2 turn on the PMS (primary mixture screw) would normally straighten that right out. I guess the first question would be do both bikes have the original carbs (CV's if I'm not mistaken?). Second question would be has any jetting been done to either of them? If so, that's what you've got going on.

 

On the 95 we replaced the carb with a HSR42 and when I got the 84 as a basket case I put the CV carb off the 95 on it. The 95 also as an EV3 cam and the 84 has a EV27. The 95 is my dads and all he knows is that the CV carb was jetted at one point or another.