Sometimes it really *is* zebras

Posted: Wed, 7 September 2005 | permalink | No comments

You know the old saying, "when you see hoofprints, think horses, not zebras"? It's a nice reminder for all computer folk -- look for the = where there should be a == before blaming the compiler, that sort of thing.

I was helping my wife troubleshoot her bike the other day. Although she's more than capable, skills-wise, of doing most service work herself, she gets nervous when pulling it all to pieces, so I tend to do most of the more grizzly work. She uses her bike to commute to work most days, so it was a bit troublesome when it more-or-less failed to start one morning recently. Luckily, she just jumped onto mine and kept going, but it was still painful (primarily because she'd only bought the thing about three weeks previously).

The symptoms where that the bike would start, run for a few seconds, then die. It would then be keen to turn over and over and over, but not actually fire. Giving it a few minutes of peace would restore order, and it would run for 5 seconds again before dying.

[Caution: bike geek ramble ahead]

Hmm, fuel system, says I. Definite blockage, which is only letting a poofteenth trickle of fuel into the jets, so a little bit of rest lets the bowls fill up just enough to make it run again. No problems -- pull the tank, drain fuel, clean all the filters (which looked quite shiny already), clean the carbs, bugger it, may as well change the spark plugs and clean the air filter while I've got all it's clothes off. That'll sort it.

Nope. Identical symptoms. Maybe it's running for an extra second or two, but that's it. Certainly not enough for useful bike territory.

OK, maybe it's a battery problem. By now, the ol' chemical capacitor is starting to look a bit shagged, so I break out the porta-jump (an SLA battery with a switch and car-grade alligator leads, basically) and attach it. This thing packs one hell of a whallop. Does it help? Not a bit.

By now, my Bob The Builder ("Can we fix it? Yes we can!") spirit is starting to waver. We're down to the CDI units, more or less. But they appear to be giving quite a healthy spark on the new plugs, so that doesn't seem the likely cause.

Now, it's down to the Final Count -- wield the multimeter of doom over pretty much anything that deals with agitated electrons in this bike and give it a going over. Thankfully, I've got the factory service manual, which gives multimeter-compatible test regimes for everything.

Long (very long -- finding the connectors is a bit of a battle, let alone getting multimeter probes into everything) story short, it was a CDI computer -- a small black box which watches the position of the engine in it's rotations and puts out nicely timed pulses of "you go bang now". I have never heard of one of those going faff before. The final check was swapping the computer out of my bike (yay for having twins) and watching the previously dead bike come to life, and mine doing the regular death routine.

To top it all off, when I pulled the computer out of it's little rubber bra, it had stripes on it.

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