If you own a CJ-5, CJ-7, or Wagoneer from the golden era of AMC (1970-1985), you know the drill. It runs great… until it idles like a tractor. Nine times out of ten, it’s not the carburetor itself—it’s the spiderweb of vacuum lines feeding it. The Symptom: Your Jeep starts fine but stumbles at idle. You adjust the idle mixture screws, and nothing changes. You spray carb cleaner around the base, and the RPM spikes. Congratulations, you have a vacuum leak. Step 1: The Usual Suspects Before you tear down the carb, check these three specific lines: 1. Distributor Advance: The line from the ported vacuum switch to the distributor. If this is cracked, your timing won’t advance properly under load. 2. PCV Valve: The thick hose from the valve cover. A leak here is massive because it pulls unmetered air directly into the intake manifold. 3. Brake Booster: If your brakes feel hard *and* the engine runs rough, check the check valve at the booster. Step 2: The “Cigar Test” Forget expensive smoke machines. Get a cheap cigar (or a vape if you’re modern). Disconnect the main vacuum line at the brake booster, blow smoke into it, and seal it with your thumb. Watch where the smoke escapes. * Intake Gasket: Smoke from the side of the engine block? Time for a new gasket. * EGR Valve: Smoke from the diaphragm? Replace the valve. Conclusion: Don’t fear the vacuum diagram. It looks like spaghetti, but it’s logical. Replace one line at a time, use silicone hoses for durability, and your AMC 258 or 360 will purr like it’s 1978 again.
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