I need some general ideas about B18 issues I've been having on my 68 145.

I've given the engine a moderate once over since purchase. This includes oil/filter change, new plugs, wires, points, and a coil. Also adjusted the valves and timing, and finally, tuned the carbs (dual SU, running slightly lean right now, but pretty good).

Most of the above changes improved the engine performance drastically, save a few remaining issues. I think they may be related.

1. When cold, choke out, engine always starts great with a few cranks. However, after 10 seconds, idle drops, and it beings chugging like it's only running on 2 cylinders (vibrates, struggles, but never dies). During this time, it won't respond to gas petal advancements at all. Usually what I do to break it out of the funk is press the petal to the floor, and after 5 seconds or so of zero response, she roars to life, RPM's shoot up, things smooth out, and I get a massive cloud of black smoke out the tail pipe as I finally accelerate away. Once I've done this initial take off, things are better, but I can still get that same hesitating and black smoke at a stoplight until the engine is warm. Once warm, it can still occasionally happen, but to a much lesser extent, like when I take off from a stop light, and let the clutch engage too much before I've revved the engine. If the RPM's drop too low, it pushes her back into that chugging funk, ignoring petal pushes. I have to disengage the clutch, press the petal to the floor and wait maybe 1 second for her to roar back to life, and I can go. Only slight smoke when this happens when warm. I'm guessing its all the same issue.

2. I've getting horrible gas mileage. I'm driving conservatively, maybe 75% around town, and getting 16 mpg. What else can I check to find the culprit? I'm pretty sure I've got the mixture dialed in on my dual SU carbs, slightly to the lean side.

Any sage advice on babying this engine is welcome. Thanks!