2.5 liter six with Webers and standard ignition. Runs fine, then, at speed, it starts running rough on what sounds like 5 cylinders. Then, sometimes, there is a click/pop, as if clearing itself, and it starts running normally again. Cruising on five cylinders ends when accelerating hard but returns when accelerating is done. Has anyone else seen this. Your thoughts on how to troubleshoot? Seems like it is fuel-related given that it ends during acceleration.

Clogged jets. Fouling plugs.

plugged idle jet circuit for 1 hole. . the idle jet controls mixture up to 1/8 throttle. . .
with it running, crack each idle jet lose one at a time enough to unseat it. . the good ones will make a difference in idle. . when you hit the bad one there will be no change in idle speed. . . .
then change your fuel filters. .

thanks for responses. of course, at the moment, it is running fine. if I find the culprit cylinder, what do I do?

Clean the idle jet then put it back and drive.

With carbs, caring for your idle jets is pretty much a way of life. Get to know where they car, how to check them, and how to clean them. Keep the tools necessary to do this in the car. They clog with some regularity.

DUH. I don't know where the idle jet is. Is there a site that helps an amateur learn a Weber?

Along the top. Pink IJ in this pic.

"Most of the time" you'll see a speck of dirt in one of the tiny holes in the jet. the holes in the jet are the smallest passage in the entire idle circuit so that is where dirt is most often trapped. .

. when you remove the screw cap from the carb, it is just that, a cap that is the jet holder. . . gently pull the jet from the holder, hold it up to the light and look through the holes. . most often you can just blow it out with your mouth. . or PB Blaster or carb cleaner. . if this is a repetitive problem, you'll want to pull the tops off the float bowl and see if dirt has accumulated in the float chambers. Drain and clean as needed. .
Dirt can get into the carb in 2 ways, with the incoming fuel, or through the air filters and down into the fuel chambers through the top passages of the carbs. .
If you pull your air filter elements and the top of the metal filter base on the carb is covered in dirt / dust, then the air cleaners are not sealing well or doing their job somehow. .

Setting up triple Webbers is somewhat of a science, especially when you are having issues. . It is something you can do yourself with a little knowledge and tools, but I'd be careful about turning the balance and mixture screws without a flow guage

Don't lose the Rubber Grommet when you take the Idle Jets out!

you guys are the best. can't thank you enough. i want to be driving this car over the 4th weekend.

In an effort to educate myself, I have collected a Weber manual, parts list, troubleshooting guide and information provided by members here. It is all in a 22-page .pdf. I don't know how to make the pdf available here, but if anyone wants it, let me know.

however, some do and some don't. See Paul Abbott's sit for info on which idle carriers use them.