My Ducati (a 1995 Supersport 900SS) had an awesome sound, but a number of problems. It ran horribly at low revolutions, jerked and started at low speeds, and ran out of puff quite abruptly when accelerating hard above 5000 RPM. It fouled spark plugs continually and also backfired, since it was running rich. Also the sound was so awesome that at times I found myself in physical pain while riding, earplugs or not.
After a long time fiddling with the carburetors, I finally decided that I’d gone as far as I could there and it was time to look elsewhere for the fault. The workshop manual had a single line in the section devoted to carburetion: modified exhaust systems may affect the running of the engine.
The previous owner had told me that he’d had the mufflers de-baffled. The other bikes I’d heard had sounded a lot quieter. Taking the mufflers apart was simple, just take them off the bike and then drill the rivets at the ends out. It was time to investigate, and what I found is pictured below:
This was quite a surprise. I’d had a mental picture of a welded can of plates, tubes and baffles, with metal baffle plates removed via angle grinder. Some reading and I found out that there are two main types of muffler – resonance box (with baffle plates, helmholz resonator boxes, and flow passages), and absorbers (with a simple perforated tube surrounded by packed sound-absorbing material). Absorber mufflers are associated with high-performance engines, due to their inherently free-breathing qualities. They put out a characteristic deep bass sound, and performance characteristics are largely dictated by the mass of packing inside the muffler casing. The usual story is that the more packing there is, the better they go. In my reading, I found that a standard trick in motorcycle racing is to go to track with up to four sets of mufflers and to weigh them after each race, changing or repacking as needed before the next race. This can make a difference of 1 – 2 %, in terms of performance and power output, and in racing that can be the difference between podium and pack.
I certainly hadn’t expected to see that the ‘de-baffling’ was just removing three quarters of the original high-quality packing material and then wiring the remainder on so it didn’t bounce around. Very much a standard Kiwi trade job.
Cheap. Nasty. Not impressed. I bought a box of generic fibreglass muffler packing, which looked like this:
The remaining original packing and the wiring was taken off. I spent a while scrubbing everything with degreaser to get most of the carbon buildup off, then I got geared up. Glasses, gloves, overalls (I should have had a respirator mask, too – particulates in this stuff are nasty), and got to stuffing the wooly, soft, pillowy fibreglass in. The technique which I found worked best was to get it most of the way there, nearly close the muffler up, and shove as much stuffing as possible in through the narrow gap remaining with a steel ruler or similar instrument. I managed in the end to get nearly 500g of packing into each muffler.
I re-riveted the ends to finish. I debated installing some kind of threaded fastener, but decided against it. Carbon, heat, and oxidation probably meant that rivets were used for a reason – removing them just meant drilling them. Screws would probably lock themselves into place permanently in a month or two of riding.
The repacking has transformed the bike. Acceleration is smooth and progressive to peak power at 6500 RPM, where it gracefully falls off again to redline at 9000 RPM. The bike is running smoothly at low revs and low speeds again too. The exhaust note has quietened to a level where riding in residential areas is actually possible without earplugs, and my mates are giving me some stick about that. One of the guys was nearly crying when I told him that I was going to re-baffle the mufflers, but I would never go back. It’s running that much better, and this is one of the best jobs I’ve ever done on the bike.
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