Any vacuum-leak safari should start out with one important tool in hand: a vacuum-hose diagram. It might actually be placarded in the engine bay. If not, look one up in the service manual or consult an online service such as AllData. Armed with the right information, check that both ends of every vacuum line are accounted for. I photocopy the diagram and use a highlighter to verify that I've inspected each one. Is every line attached to the right fitting at both ends? Is it still supple enough to bend as needed to grip its attendant spud? Is the plastic fitting okay? When I get an older car that shows signs of vacuum leaks, sometimes I systematically replace all of the vacuum lines individually. Ten feet or so of rubber line in a couple of sizes can be replaced—one line at a time, so you don't lose your place—in a half-hour or so. A half-dozen leaks so small they wouldn't be traceable can add up to one major leak. Here's how we old-line mechanics learned to chase leaks: To keep the engine computer from richening up the mix, pull the wire on the throttle position sensor or some other handy sensor that will keep the engine in open-loop mode, where the computer just uses hard-coded default values for the amount of fuel instead of dithering the values around to stay near that golden 14.7:1 ratio. This is analogous to the situation in a carbureted car (or motorcycle, or any other IC engine that doesn't have a computer). Start the engine and let it idle. Now spray aerosol carb cleaner onto the suspected leak. Yes, this is dangerous, especially if you consider your eyebrows important. There's not supposed to be an ignition source on the engine anywhere, but once in a blue moon, a stray spark or an overheated exhaust manifold can make the carb cleaner flare up. You've been warned. The combustible carb cleaner will be sucked into the leak, and the engine will pick up its idle speed momentarily—and probably run on all cylinders if it's been misfiring. We quickly learned to use short, directed puffs of carb cleaner to localize the leak.
