Catalytic cutting catastrophe

But first, Previously On Bike-in-a-box-blog.

Measure twice, cut once, goes the mantra. Well, in an uncharacteristic display of common sense, I only drilled once before measuring the choke assembly.



The small metal widget in the top right is the actual choke actuator, which rotates to operate the choke inside the carb. Note the shiny new hole drilled in it, where I was going to attach a choke cable. Note also that with the plastic choke lever down (i.e. choke off), the actuator is raised. Annoyingly, it's sprung so that without being held up there in the off position, it would spring down and turn the choke being fully on. This is pretty shoddy design to begin with - if the choke assembly fails for any reason, the choke will come on full. Not good. Note that this is on a Japanese Mikuni carb emulating the behaviour of the Japanese designed CG125: I don't think a Chinese design would be as fail-fragile.

The show stopper is that the actuator needs to be pulled up in order to turn the choke off. However, all sane choke knobs are pulled up to turn the choke on. I'd have to reverse the direction, which could be done by attaching the cable to the plastic choke lever instead. However, the lever isn't sprung, and would require the choke cable to not only pull it up, but to positively push it back down again, which wasn't going to happen.

I tried fettling it various ways but couldn't find an arrangement that would operate the actuator correctly and reliably (i.e. stop the choke from springing on), so I declared it a draw and shelved the idea.

Decatalysing, Round One

Guru Forchetto from defunct TheChineseBikeForum always held that the cheap basic catalytic converters fitted to our bikes were one of the primary causes of the drop in torque and power from their original configuration. Arguments about exhaust baffles and back pressure and two exhausts versus one aside, it's undeniable that these engines were not designed with cats in mind, so it's worth a try. The polar bears can just deal with it. What have they done for me lately?

So I've had a plan for some time to de-cat my bike, and this weekend saw it swing, saw and weld into action. This may turn out to be a mini series in its own right.

First, where and how to cut? From doing a bit of wikiing I'd hoped that the cat would be a relatively fragile ceramic, that I'd be able to drill, chisel or just smash to pieces by going in at one end.

So I decided to make a straight cut just under the cat bulge. First, I drew a couple of lines across the cut so that I could match up the orientation again later - this turned out to be pointless because the amount of handling required quickly erased them, but not a disaster as it's easy enough to offer the pieces back up again and re-mark them prior to welding.

A hacksaw went through quite easily, and I was pleased to find that the mild steel is a good 2mm thick, and thus OK to arc-weld back together again.

It turns out that inside the pipe there's an inner pipe of slightly smaller diameter. I'm not sure what the purpose of this is; probably some sort of circulatory magic gubbins. In the event, I inadvertently lopped off an inch or so of it, which I just discarded rather than trying to weld it back on.

This exposed the end of the cat bulge and the cat lurking inside. It starts 3/4" or so inside the bulge, and looks like... well, I'll take a picture first next time. It's a honeycomb structure that looks to be stainless steel, but very fragile, so in I went with a 20mm wood-hole cutting drill bit, expecting it to shred fairly easily.

Hmm. Er. Mmmm. Nope.

The honeycomb immediately flattened and toughened up and the bit was just polishing it. OK, switch to an 8mm HSS metal bit. That went in 1" or so, and that's all she wrote. I drilled a few such holes, but they weren't anywhere near close to going right through the cat. Pressing hard resulted in another 1/8" penetration and a glowing red bit tip. Oh my.

Next up was a curved chisel, which I whacked into the cat around the edges to see if I could detach it from the pipe walls and just pull it out. It went in 2" or so, and I managed to deform the cat a fair bit and pull some small lumps out with pliers before the chisel, well, snapped clean off while I was wiggling it around.

OK, rethink. If I couldn't get it out from the end, what if I cut the cat bulge in half in the middle? I might be able to chisel the half-cat out from either end and pull them out. So in with the hacksaw, which went through the pipe wall, hit the cat and... that's all she wrote (again). I just couldn't get through it.

After some more bashing, whacking, stabbing and hammering, I had to acknowledge that all I was doing was choking up the cat, and called it a night before I made things worse. These things are tough:



Note the snapped off chisel at the bottom!

That left me with a butchered cat, and a half-cut pipe. I wanted to get the bike back the road, so decided to just weld it all back up, and source some pipe to replace the whole cat bulge with, rather than trying to remove the cat and retain the pipe.

My welding is... well, weld is as weld does. If it holds, it's a good 'un.



Not the best weld in the world, but it's been a while since I did one. That's using a 1.6mm rod and about 50 amps; I didn't want to blow right through the steel. Actually though, I reckon it could stand a 2mm rod and 60+ amps, which I'll use when I replace the pipe.

Getting the main weld right was a bit of a fuss. Putting the pieces back on the bike and marking the join was simple enough, but I'd cut too close to the bulge, and hadn't enough straight pipe on the cat side to get a clamp on the pieces. In the event, I had to hold the pieces together while tacking them, sans face shield, and just wearing clear goggles to protect from weld splatter.



I got it eventually though, and butchered my way around the join, brushing and grinding it down and going around a few times to try and get it solid - I should have switched to 2mm sticks, really.

Well, good enough. It'll only have to hold until I get the pipe.



I'm not even going to paint over it, since I'll just be cutting and re-doing it soon anyway.

Despite the choked up cat, the bike started and ran fine. The weld is gas tight, and the exhaust didn't drop off, so job done.

I'll get some pipe with an ID that matches the OD of the main pipe, so that I can sleeve it over it (much like the cat bulge) rather than trying to weld a piece inline; that's unnecessarily tricky. Having it sleeved will also make it easier to keep it together while I tack it and offer it up.

Well, not a success, but not a disaster either, and I did achieve my real objective: Welding Stuff.

Welding Stuff is all that separates us from the animals. That, and blogging.