Disable Reed Valves (easy) or
Remove The Smog Stuff Completely
So, you've replaced your stock exhaust system from something by an aftermarket manufacturer and now you have backfiring on deceleration. You have a couple of options. You can "marble" the reed valves system or you can remove the culprits along with all the rest of the hoses, valves and tubes cluttering the top of your engine. The "reed valves" are the boxes on front of the #1 cylinder visible from the right side of your Vulcan and the rear of #2 visible from the left side. They look like a box with a rubber hose attached to the top. On deceleration a valve sitting atop the engine is opened by vacuume allowing extra air into these valves and into the exhaust system. The extra air allows any unspent fuel to be burned reducing emissions but also causing the backfire you hear. The stock exhaust system covered all the pops so effectively you didn't even know it was happening.
On this page I'll just cover the simplest of the two methods. The more involved replacement of the reed valve assemblies is detailed on this Gadget Page.
Supplies needed: One or two 1/2" marbles, just regular old glass marbles available at most any toy store. If you know another Vulcan owner who's done this he or she will have about 98 marbles in a bag they have absolutely no use for. Ask for a couple.
This is the easy fix which will take maybe ten minutes of your time. On most 1500 fuel injected bikes simply remove the right "air cleaner" (misnomer) cover and look at the black mounting plate about the 2:00 position. You'll notice a fairly large piece of rubber hose routed into the cover. This is where the reed valves get their air. Pull the air injector solenoid away from the backing plate, put a marble into the hose and you have effectively disabled the system. Go ride.
On carbed Vulcans, FI 1600's and some California FI 1500's the hose is attached to the top of the crossover air tube between the air filter backing plate (left side) and air box backing plate ( right side). If you don't see the hose where it's shown in the diagram below on the air box backing plate then you'll have to do a little more work. You can lift (or remove) your gas tank to reach the hose or pull it off the air valve side. Insert the marble and plug the hose back in where you found it. If you disconnect the hose from the crossover tube and don't plug the hole in the tube you'll be drawing unfiltered air into your engine and that...would be a bad thing.
Once the hose is plugged you should notice a huge decrease in popping when you roll off the throttle. If you still have popping you may have an air leak in your exhaust. Any fresh air in the pipe will do exactly what the reed valves do, cause unburned hydrocarbons to ignite causing the poppping.
The prime suspect area is where the pipes are attached to the heads. If you re-used the original crush gaskets when installing new pipes go get some new gaskets and install them. If you used new gaskets check the torque on the cap nuts holding the pipes to the heads. Be sure the nuts didn't bottom out (often a problem with Vance & Hines pipes) giving you a false torque reading. A stainless steel washer under the nut usually fixes that. Check your exhaust with a stick of incense (or cigar, cigarette...anything that smokes). With the engine at high idle pass your smoking object around each joint. Any disturbance in the smoke at all indicates a leak that has to be closed off.