On the first couple of test rides, The Bride’s exhaust was clearly too loud—even at low speeds—and had a very sharp, unpleasant bark. The muffler was from an old Bultaco roadracer I had in the mid-80s, and the straight glass-pack design was simply not the right choice for a neighborhood runabout. So, I clipped the safety wire that I’d put on in 1985, and pulled it apart.
I removed the fiberglass batting and made up an alloy stopper plug to fit inside the perforated metal core. With this, the exhaust gases now had to travel through the holes in the mesh tube, around the plug, and then back into the tube to exit.
The aluminum plug didn’t block the tube completely. It was made out of a leftover core from using a hole saw, so there was a small hole in the center.
I re-mounted the muffler and started the bike. It was definitely quieter than the original straight-through design, but still objectionable.
I spent some time reading up on muffler theory and talking with an engineer at work. I don’t have the engineering know-how to attempt a tuned system, so I decided to make a simple labyrinth baffle. It’s not the most efficient way to cut the decibels, creating more back-pressure than a reverse-flow chamber, but it would be easy to adapt to what I had, and not prone to creating unwanted resonant waves at the wrong engine speed (aka “megaphonitis”).
I designed an alloy spool that would fit around the muffler core with a series of offset 7/16″ holes, six per side. Given the close spacing of the holes, I was pleased that I managed not to siamese any of them.
I then drilled a hole down through the center to screw the spool and center plug together.
I took the bike for a quick spin, and the exhaust isn’t whisper-quiet, but the overall volume is certainly acceptable now. Just as importantly, the tone is deeper and mellower. In fact, it was quiet enough that I could hear for the first time how much mechanical noise was coming from the engine and torque converter. Also—now, I might be imagining things—the bike seemed to come off the choke quicker and the off-idle stumble wasn’t there. Perhaps a bit of extra back-pressure was all it needed?
