26° BTDC for idling?

Hi GUys,
it's been a while since I was here last, but now I have finally got my 350 SBC to run. Everything stock, except for an Edelbrock 600cfm carb and Performer intake, double exhaust.
I have three questions:

1.) When an engine is fully assembled, everything connected, in the car and in good running condition, like if you just had everything checked. What should the timing be at idle?

2.) My engine idles very rough. At higher engine speeds it sounds better, but never really right. The ignition is almost new, plugs also, valves just newly set. What can that be?

3.) I can turn the idle screws on the carb any way I want, with no effect on the idle speed. Of course, if I set them under half a turn out, the engine will stop, but from about one turn out I can change as much as I like and nothing changes. I have no air leaks. left and right screw both have no effect.

And a few more questions:
My situation is this: About 300 miles ago, I had everything set and it seemed to be running fine, the engine has just been professionally rebuilt. I had some trouble with flooded lifters and wrong valve settings, but that is now over. But I still have to set it to about 26° BTDC to run nicely, it still has a bit of a rough idle, but if I step on the accelerator the timing goes past the 40° mark. Without a knock, even under high load. If I set it to about 10° it will idle extremely roughly, and has lower power. Can it be that different engines have different timing marks? I've seen some with the marks welded onto the timing chain cover, others have them screwed on by a bolt at the side of that cover. Can the marks then be in different places? (I.E. am I just checking my engine against the wrong timing mark?
Can you set the timing just by turning the dizzy til it runs nicest (highest revs at same throttle position) and leaving it there? That way it seems to run nicely but I get very low mileage, about 9-10 mpg in a Chevy G20 Van at 5.000 lbs , I was expecting around 13. But as I said, I can change the carb settings as i like, with no effect, the carb is new and cleaned (no stuck needles).
Slowly vut surely I think I have a problem ;-)
Thanx a lot for your help,
MadMax

Hi Tech!
Thanks a lot, those threads are really interesting, never thought of the damper of course. What I'm wondering is, would an engine with a slipped damper (should be slipped about 20 degrees) wobble like that? At idle it sounds as if a cylinder wasn't firing, every turn it bucks to one side and revs like: BRRWWBRRWWBRRWW if you know what I mean. Ignition system is OK, valves also. But I think I might try running manifold vac advance, now I'm running ported, maybe thats why I need so much initial static timing (thats at about 25° but as I'm running ported I might get down to 10° without the vac advance) And Mabe then the damper isn't off mark ??? I'll just have to play around, the real concerning thing is the unevenness , if I set the idle throttle real low so that it just about runs it really turns two tunrs, almost stops, just gets onto hte next cylinder, turns twice, just reaches the next cylinder, turns twice and so on, as if it wasn't getting enough fuel most of the time, but I have just adjusted my floats. Funns thing, I guess I'll have to play around for some time. Just another word: In a 350 SBC built in 83, would you run ported or full vac?
Thanks and I hope to hear from you soon, CU,
MadMax

many times a customer brings in his toy with no clue as to what cam or history of engine. Just wants it to run better. I follow my dads lead here and just set by ear and performance, then go backwards to find what settings are out of normal windows.
Start with a reading of manifold vac. , with near stock cam should be at least 16 inches and closer to 18. disconnect distrib. vac and plug, set timing by ear to best or highest vac reading, set idle screws on carb if possible ( only affects idle anyway and after about 1400 rpm not part of running function anyway. Start at 2 turns for grins. If not adjustable come back and fix after looking at big pic. Check color of spark at any plug wire end for a neon blue color. If yellow or orange you have a fault.
Now that its good and warm shut her off and restart. if bumps against starter you may have too much timing but skip for now and take her for a drive. Keep advancing timing until it pings when loading or lugging in say a 2nd gear leave from light. Now check for pinging on WOT or at higher rpm's , increase timing until pings on low rpm load and then back down 2 degrees at a time until it stops. initial timing on stock motor about 12 degrees with about 20 to 25 more mechanical lead totaling say 34 to 39 . regaurdless of readings make 39 degrees total timing your MAX when numbers are verified correct. When running good go back and look for settings out of normal window and find out why..