Hello
I have some really strange problem, I can't figure out. I hope someone can help me again.
I transformed my 79 L48 with all edelbrock carb, cam, cylinder heads, and intake manifold. I set valves like in the book.
I have standard vacuum advance from L48 and advance is set to 16deg.
For some reason beyond me as soon as I connect non ported vacuum to the vacuum advance idle rpm go crazy up and down, at worst ( especially when cold) rpm can vary so much that engine stalls. Does anyone know this ?
when I drive everything is fine. but when I stop, the engine runs fine for about 1-2 sec. then the rpm bouncing starts. and gets worse. first just up and down 50rpm then 100, 200 enventually stabilizes. I have to turn idle screws way open 2-3turns so the engine stays running.
I put in a adjustable vacuum advance and tightened the spring to max, then the phenomenon is almost gone.
when I connect vacuum gauge the fluctuation is hardly visible. I suppose the vacuum advancing exaggerates the effect.
Sometimes it missfires well thats what I think it is. it sounds like that.
I tried about everything I can think of. any help please?

 

sounds like it's the incorrect vacuum advance for your motor.
you said you used the vac adv can from the stock L48 motor but you also went and changed out heads, cam, etc. This will change the vacuum level your motor puts out.
What it sounds like is happening is that the lower vacuum level the motor is producing is borderline for triggering the advance of the vac adv can and as the vacuum varies slightly it's letting the vac can cut in and out and therefore your timing advance is fluctuating causing your idle speed to jump up and down.

What i'd suggest is putting a vacuum gauge on a full manifold vacuum source and read what the steady vacuum level is at idle and purchase a vacuum cannister that is appropriate for that level of vacuum.
The rule of thumb is to use a vac adv can that is pegged out (brings the vacuum advance all in) at a point that is 2"HG BELOW the level of vacuum your motor produces.
If for example your motor is producing 15"HG of vacuum you want a vac adv can that is pegged out around 12-13"HG

BTW, the adjustable vac cans are notoriously inaccurate. you are better reading your vacuum level and getting a correct can rather than the adjustable ones.

 

I think you are running way lean at idle....and that vacuum advance is just enough to cause a huge upset in the engine, vacuum falls enough to cause the primary jets to enrich just enough to add more fuel....engine recovers, idle picks up, jets close down because vacuum increases, engine goes lean as hell, again engine stumbles loss of vacuum causes enrichment..and the cycle repeats itself....

I have had something like that happen to me on another car, years ago....that's best I remember it....methinks you need a LARS rebuild, he the resident carb expert here and everywhere, damn nearly....