In #3 above, I said to set the idle speed with timing and or idle air bypass.
But it seems Ineglected to mention vacuum leaks.
Idle air bypass is introducing a controlled air leak.Big cams want more air to achieve a decent idle speed, and the common fix is to increase the curb idle speed. But of course this upsets the sync you so carefully just set. So what to do? Well firstly;the PCV system needs to be functioning as it is the primary idle air bypass, as the carb is continuously pulling air thru that little valve. Then if the idle speed is not fast enough, you have to add air elsewhere until your engine is happy. The usual places are secondary cracking, or holes drilled thru the primary throttle valves. In my experience, introducing dry air past the secondary throttle valves, introduces its own problems, in that the rearmost cylinders run lean and the engine runs rough.
So I much prefer to introduce the bypass air near to where the low-speed circuit gets it's air.
If you've ever worked on small engines, and especially outboard engines, you may have seen small holes drilled thru the throttle valve(s). These set the minimum idle speed when the engine is fully idled down and fully retarded.Often there is no curb-idle screw.
Well the same thing can work for us hot-rodders. I drill my holes on the the T-port side, about half way between the shaft and the bore-wall. I usually figure out how much air the engine wants by introducing a leak into the PCV system first. Then recalculate that orifice size to the equivalent of two holes; one for each valve. I usually drill a little smaller than the equivalency, in case things change. On some bigger cammed engines, I drill two small holes in each valve cuz the smaller holes are easier to fine-tune.
Now, the above assumes that air is not getting into the engine from any other source besides the PCV and the primary throttle valves! It assumes that you have previously proved that there are no vacuum leaks of any kind, anywhere; including the power-brake booster,any vacuum device, and the various gaskets.
When the engine gets air from somewhere else, one of two things can happen; 1) the idle speed cannot be slowed down enough, or 2) it runs rough no matter what you do.
If the air leak is variable in nature, it can drive you crazy.
The brake booster is a special case. It may have just a tiny leak at idle when manifold vacuum might be 10 or 11 inches, and so you can easily compensate for it. But as the rpm increases, and manifold vacuum begins to increase to say 21 at 2100, at cruising speed, now the leak may be passing a lot more air, and so the engine goes lean, and you get a big problem.
Or ,again with the booster; perhaps the diaphragm is ok, but the differential valve leaks whenever you step on the brake. So now, say you are stopped, in N/P, and you step on the brake,and again since you are only pulling 10/11 inches, you notice the slight rpm drop, but maybe it doesn't raise a flag for you. So then, you put it into gear, the TC puts a load on the engine, and maybe the engine runs rough which you expect cuz well it has a cam. But maybe the engine stalls.So now the reaction is to crank up the curb idle until it stays running. Suddenly your idle speed is 800 in N/P, and 550 in gear and and it is idling very rough, you can't figure out what's going on. But you take your foot off the brake and motor away, with no further problems. Until you come to a stop again :(
For me, testing the booster has become routine, so I missed mentioning this in line #3.
