I’m getting my 1987 Jeep Wrangler YJ ready for desert service – two weeks of stop-and-start driving around a dusty alkali lake bed in temperatures raising ranging from freezing to 114° F (46° C).

To that end, the AMC 258 CID straight-6 4.2L engine and Weber 38 DGMS carburetor have been rebuilt. She now starts on the first key turn, idles so quietly that I think she’s stalled, isn’t leaking anything, and drives like an almost new Jeep.

Very occasionally, however, she doesn’t drive smoothly while I’m at a constant speed. There’s a slight repeated lurching. It’s not really “spill your coffee” lurch, but certainly it’s “make an engineer crazy” lurch. Of course, the worst thing is that I cannot reliably reproduce the problem.

It seems to happen more often in the morning with an engine warmed to normal temperature. It seems to happen between 1500-1700 RPM, and persists through 1st, 2nd, and 3rd gear. (I live on an island with a speed limit of 25 mph, so that’s what I have to work with.)

Here’s a video of the problem. This happens for a short while, and seems to go away, and when I least expect it, happens again.

I’m not a car person, so my description would be something like “it feels like there’s a kid on a pogo stick jumping on my garden hose. The water that lurches out of the end of the hose because of the pogo stick hitting it is how I imagine the engine feeling.”

Gas filter? Transmission something or other? I have no idea. The debugging continues.

Find out it the lurch is adding power, or causing a loss of power. he never says which one it is, and it’ important because two completely different things cause the different kind of lurch.

If it’s a loss of power surge, he’s sucking air somewhere between the fuel pump and the fuel tank. The bubbles are displacing just enough fuel to fuck with the level in the carbs at speeds over high idle.

If it’s a power surge, he’s probably got a vacuum leak in the line to the distributor vacuum advance.

Or, a tiny hole in that diaphragm. It’ll get steadily worse as the hole gets bigger and he’ll seem to just have less power accelerating, and a bit harder to start.

Mechanic friend of a faraway mechanic friend

Okay, time to take her out for a spin and see if I can figure this out.

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