You have driven an hour to look at a motorcycle described as "clean, solid, just needs a little attention". You knew before you left that something was wrong. The listing said "ran when parked," and you went anyway because the price was right and you had the afternoon free. We have all gone anyway.
So there is a paper called Ran When Parked: A Bayesian Analysis of Truthfulness, Incompetence, and Motivated Reasoning in Private Motorcycle Sales. It comes from the Department of Adversarial Epistemology at Craigslist University. This is not a real university, but it’s an accurate description of where this knowledge is built—lying on a cold garage floor at ten o'clock on a Tuesday.
The researchers looked at 47 Honda CM400 listings. Ninety-four percent said "ran when parked," and eighty-nine percent said a carb clean would sort it out. Every single one described tires as having plenty of life—including bikes with tires manufactured before the owner was born.
"The posterior probability of an accurate listing claim converges to a value we decline to specify, on the grounds that it is depressing."
He's not lying, exactly
The seller is usually not lying. The situation is more human than that. There's the guy who doesn't know enough to know; he's heard "carb clean" is the answer to every problem, and it seems reasonable to him. Then there's the guy who knew the bike was fine when he parked it three years ago and hasn't updated his beliefs. In his mind, the bike is still fine, frozen at its last good moment. The paper calls this "motivated reasoning".
"Ran when parked": Started once, at some point in the past. He hasn't checked; he prefers not to.
"Just needs a carb clean": This is what he read online. The carb needs cleaning, and so does everything else; the clean will open a door you didn't know was there.
"Lots of tread left": The rubber is fifteen years old with the flexibility of shoe leather. He looked at tread, not the sidewall date code.
"Chain has plenty of life": The adjuster is at the end of its slot; there is no more life.
The tires and the chain
The tire section introduces Goodhart’s Law: when you start using a measurement as your target, it stops being useful. Sellers check tread depth because it's visible, but they ignore the four-digit date code on the sidewall. A tire from 2004 with 60% tread is just waiting for the rubber to decide it's had enough. For chains, researchers propose "chain carbon dating"—estimating life from sprocket wear and link stretch. For chains described as having "a lot of life left" with maxed-out adjusters, the remaining life is measured in weeks, with a lower bound of zero.
The carb clean
When buyers actually cleaned the carb, it fixed the problem only 5% of the time. In 61% of cases, the bike still didn't run right, and in 34%, the clean revealed something worse. Researchers call the 5% "the lucky five percent" and note they have nothing interesting to teach us. The useful information is in the other 95% where you have always lived.
"The carb clean both will and will not fix the problem until it is performed. In a substantial fraction of cases, performing the carb clean resolves the superposition in the unfavorable direction."
On the Schrödinger Diagnosis
Phrases like "ran when parked" have stopped meaning anything—they are signals, not claims. They mean: I have a motorcycle and I would like you to look at it. Experienced buyers decode these as "seller is hopeful" or "has good feelings about a memory". You bring a compression tester and check date codes because you have learned the listing is not the bike. The paper concludes that data collection was stopped after the third bike turned out to need an engine rebuild. That bike is running now; it just needed a carb clean.