['68OrangeSunshine]

Originally Posted by M_R_Q

I didnt check it with a timing light (I can never make sense of them the few times Ive played with one)

[Find a buddy who has one and knows how to use it. It's easy, you point at the timing marks on the HB and Timing indicsator on the Gear Cover, pull the trigger and read the blinks.]

but I did try adjusting the timing. Usually I can get it pretty close just by feel. Not perfect, but certainly usable. I also cant get it to stay running well enough or long enough to bother with the timing light.

[Is your fuel pump working OK?]

When I do adjust the timing it doesnt seem to make any difference in the way it runs.

The problem originally popped up while still being in use, so sitting wont be a big factor. Ran it on fresh gas

[Have you changed the Oil yet?]
and replaced the carb with a known good one off the 250 with no change, so I put the old one back on. It could be a vacuum leak, I suppose, but I dont have a gauge to do that test.
[Get one, they're about $20. but you can check for manifold and carb base leaks with a spray can of carb cleaner. Spray around these places, and if the engine changes RPM then the carb spray is getting into the air/fuel mix.]

I doubt its vacuum though, based purely on my gut
[Gut is only reliable when shooting dice.]

Distributor cap has been changed. The whole distributor has been swapped, and the old one works perfectly fine.
[Have you run a resistance (continuity) check on the plug wires? Did you try swapping out the coil, too?]

Its my understanding that the 292/250 had a nylon/fiber timing gear that is fairly well known for losing teeth?
[Yes some L6s had a nylon/glas cam gear. Crank gear is steel. I think '68s had an alumin(i)um cam gear stock, but 47 years later who knows if some budget-minded person or shady rebuilder didn't slip in a plastic piece. Visual inspection is the only way to know, but that requires pulling the Radiator, Fan, Belts, Harmonic Balancer, etc. just to get at it.
The cam has to be out of the block to install a metal gear. The alum. gear is put in an old saucepan covered in light motor oil and superheated with a blowtorch while the whole cam has been chilled in a freezer. When the cam gear is cherry red it's slipped on over the chilled cam end and the assembly is allowed to cool.]

At least that is what Ive read. Perhaps thats just on the 250?

The problem happened while driving it, and not all of a sudden, so I doubt the distributor is off a tooth. Ive also pulled it and set it back in a few times with no change. [Fuel filter clogged ?]

[Without verifying that Cyl #1 is at TDC, we have no starting point.]

When I get around to it, I may buy some new plugs for it. I just wasnt sure if a bad plug could cause backfiring/coughing. I can see it running rough from that, though. I guess Ill just have to try it and see.

[Plugs are cheap. $2 to $6 @, depending on quality/hype. Copper plugs work well enough. Platinum tips can't be freshened up with a small file].

Hope this helps.
Good luck.