There is always a risk of introducing new complications when working on decrepit machines (kind of like gerontology, I suppose), and this enters the diagnostic logic. Measured in likelihood of screwups, the cost is not identical for all avenues of inquiry when deciding which hypothesis to pursue—for example, when trying to figure out why a bike won’t start. The fasteners holding the engine covers on 1970s-era Hondas are Phillips head, and they are usually rounded out and corroded. Do you really want to check the condition of the starter clutch, if each of eight screws will need to be drilled out and extracted, risking damage to the engine case? Such impediments can cloud your thinking. Put more neutrally, the attractiveness of any hypothesis is determined in part by physical circumstances that have no logical connection to the diagnostic problem at hand, but a strong pragmatic bearing on it (kind of like origami). The factory service manuals tell you to be systematic in eliminating variables, but they never take into account the risks of working on old machines. So you have to develop your own decision tree for the particular circumstances. The problem is that at each node of this new tree, your own unquantifiable risk aversion introduces ambiguity. There comes a point where you have to step back and get a larger gestalt. Have a cigarette and walk around the lift. Any mechanic will tell you that it is invaluable to have other mechanics around to test your reasoning against, especially if they have a different intellectual disposition.