Published In The April 2014 Issue Of Cycle Source
Article By: Vincent Stemp Photos By: Sean Fraser
Performance is important to all of us, in one way or another. Whether it means stripping fenders and swingarms to improve your power-to-weight ratio or just swapping mufflers to shape your torque curve, we all have our own ideas about making our rides go like stink. But sometimes other factors get in the way, and performance has to compromise with other less exciting factors.
This brings me to one of my toys that recently faced this sort of compromise: an early 1980’s Honda XR80 dirt bike. Now, don’t turn up your noses. I know I’m not the only person in the world who started riding on a bike just like this one when I was a kid, so I couldn’t help snatching it up for a song from a friend who tore the spark plug threads out of the cylinder head. After I re-tapped the threads and threw it back together, it was fun to mess around with, but felt a little slow. Signs pointed towards the rusted muffler, which had a big hole and no end cap. And it was loud! While my boss is pretty laid back, the sound of a tiny motor with a rotted muffler was driving him nuts. Especially when the rest of the guys around the shop realized how easily you can learn to do a clutchdrop burnout on an 80cc bike.
I diagnosed this performance situation as a case of “nobackpressure- itis.” A new exhaust wasn’t hard to find, but it was going to cost about as much as I paid for the whole dang project. Where I come from, that’s a problem that deserves at least one creative repair before I give up and buy my way out of it. So, with a crazy look in my eye, I rolled out the Lincoln Power MIG 216 and started rummaging through the scrap metal pile.
The muffler’s end cap had rusted away, and there was a hole in the body near the tip, but the baffle core was still there in solid shape. I figured it would be easy enough to make a little patch for the hole first, which I nipped out of mild steel and bent to the radius of the muffler body.
I pulled the paint off near my future with a Scotch-Brite disc, held the patch in place with a magnet… and learned a quick lesson.
Metal gets thin when it rusts! Even on the MIG’s lowest voltage setting, the rust-weakened edge was burning away almost as soon as I got my arc started. Big duh moment, but if you’re not screwing up you’re not learning anything.
I remade a larger patch so I could start my weld on good metal, and got the patch in place pretty easily after that. I’m still working on keeping my puddles consistent, but the pros say that practice is the best thing to do so I moved to the end of the muffler. I originally planned on just using a round steel disc with a hole cut in the middle for the new end cap, but then I found some 3/4” tubing and decided that I could get in more practice by tacking up a tip to the new end cap. Besides, I knew this was good for performance, because it looked more like a racing muffler I saw on a dirt bike one time.
Performance guesswork aside, I tacked the tip to the new cap, and went to pull a bead all the way around the circumference of the tip when more learning happened. The thin steel circle I had used for the end cap warped like a potato chip from the heat of the arc!
The weld was strong, but the different thickness between the tubing and the disc was the culprit. Lesson learned (again), I flattened the cap as best I could with a body hammer and ground the muffler until I had a nice, close fit. A quick bead with the Power MIG mated the end cap to the muffler, and I happily finished the job without any more “learning opportunities.” A quick coat of header paint later, and the little 80 was purring as quiet as a kitten. My welds held up nicely, with no perceivable exhaust leaks past my joints, and the end cap even looks good, from a few feet away. It even got some low-end grunt back, thanks to the extra backpressure! Now the boss has no clue when we’re killing time behind the shop, but the rear tire sure is looking bald.
Resource: Lincoln Electric Cleveland, OH 216-481-8100 www.lincolnelectric.com