All exhaust popping or backfiring on a Harley-Davidson motorcycle is caused by fuel combustion in the exhaust manifold or the header pipes. Generally speaking, it may be caused by a an exhaust manifold leak. Check for that by spraying light, penetrating oil on the exhaust manifold while your bike is running in neutral and look for bubbles. Most exhaust popping on a Harley is actually deceleration backfire. Deceleration backfire results from a chain of events that begins with reduced pressure in the cylinders. Conventional wisdom says the noise can be ameliorated by fattening the tune of the engine.
Start the motorcycle in neutral. Close the petcock and run the Harley until the carburetor runs out of gas.
Remove the entire air cleaner assembly. Remove the cover with an Allen wrench. Use Torx drivers or sockets to remove the air cleaner.
Remove the breather tubes by hand. Remove the backing plate by unbolting the two retaining bolts with a socket wrench.
Unclip the choke assembly from its bracket under and on the left side of the gas tank.
Mark the throttle cables with masking tape and disconnect them. Loosen the hose clamp that connects the fuel line to the carburetor with a flat-head screwdriver and disconnect the fuel line.
Gently pull the carburetor from the seal ring that attaches the carburetor to the intake manifold. Set the carburetor upside down on a bed of clean rags on a flat, stable surface.
Drill a 1/16-inch hole in the factory sealing plug that covers the fuel-air, low speed adjusting screw. Pry the plug out of the low speed adjusting screw well with a machinist's awl (scratch awl). Note the location of the idle speed adjuster screw.
Reconnect the carburetor to the intake manifold by re-seating it in the rubber seal ring. Reinstall the air cleaner backing plate and air cleaner.
Reattach the fuel line and the two throttle cables. Snap the throttle to ensure the throttle cables are properly reconnected.
Open the petcock and start the Harley. Allow the motorcycle to run until the rocker head covers are warm to the touch before tuning the carburetor.
Insert a small, flat-head screwdriver into the low speed adjusting screw well. Turn the screw 1/8-turn counterclockwise.
Adjust the idle speed adjuster screw with the same tool until the engine begins to stumble. Turn the low speed adjusting screw another 1/8-turn clockwise.
Reattach the air cleaner. Test-drive the motorcycle.
