Thanks Frank. Sorry for A bit of a delayed response. Still have not fixed. Not sure on the hydra lock on the bent components. Guess it could have been but was very mild. Whomever rebuilt the engine initially did a butcher job. Will spare the details. That said I checked dwell and actually replaced points cond and coil so it was all new. The head did not need to be planed. Had my local race engine shop check. All good compression with in 5%. I guess I could try a leak down test but given the good compression 165-170 range thought we should be good. I did all the vacuum tests with no issue. The loadomatic dist is working as it should. The only thing I can think of is they replaced the carb with a aftermarket. It is a repo Autolite 1100 with scv. I pulled the carb apart adjusted float and the pumps. Just wondering if there is something not right with it. Still at a loss. Tough to find an Autolite 1100 FOMOCO with manual choke for rebuild.
Sounds like you're on top of the important details. . I watched your vacuum gauge video again. Usually on a carb mixture problem, the pattern of the vacuum change is different. Not sure I can explain- I look at a vacuum gauge constantly while driving, I can spot when the idle mixture needs tweaking instantly. Yours is in a "longer pattern" , that is, it's stable then drops several inches in something like a "surge", then returns to stable. The rpm drops when it drops too. .
What comes to mind: 1) the China carbs can be bad, few are correct out of box. You've checked the float and pumps, but there's been other issues posted. One was leaking fuel past a gasket, going rich. On my YF china carb, the vacuum advance port inside the throttle bore was machined too high, didn't advance soon enough. Stupid stuff, poor craftsmanship. Anyway, check that the carb is not somehow going rich momentarily. Look into the bore when it's idling and verify that no fuel is pulling out of the main circuit at the venturi, or from anywhere else. At night with flashlight or timing light shining into the carb will spot it best. If fuel is intermittently being drawn out of the main circuit, you will see it, and hear it when the fuel goes around the closed throttle. I've seen this cause similar idle symptoms, when the float is adjusted too high. Smell the exhaust while it's idling, see if it's rich. Excessive fuel pressure will cause this type of bowl flooding also.
All this said, it looks more like a valve problem to me. If a lifter stays pumped up intermittently it will cause that valve to stay just cracked open, and the symptoms are similar. I'm not making this up, I have this issue right now with an old 7.3L non-turbo diesel in a '89 F250. one exhaust lifter is stuck "pumped up", and it sounds exactly like a burnt valve at the tail pipe, and vibrates at idle but still has enough compression to fire. a "1/4 miss", if you will. When I loosen the rocker nut, it clears up instantly. I'm not familiar with the rocker arm design on the 200, but if the lifters have an adjustable pre-load, they may be too tight. A sticking or held open valve would align with the "1/4 miss" idle vibration also. An open valve may be discernable audibly at the exhaust as well. If the carb checks out and it sounds like a "partial miss" at the exhaust when it drops down, it's probably a valve.
Anyway, hope you find the source!
Peace
