Well after reading these posts and recalling how a friend used to set timing and curb idle mixture with a vacuum gage I decided to pull out mu old gage and give it a whirl. First I set timing without the vacuum to the distributor it ran great until plugging the vacuum back in. So the timing was adjusted again with the vacuum attached. After that was set to max vacuum I adjusted the idle screws. Idling vacuum went from roughly 16" to a little over 20". The drivability and throttle response is much better.
Try it guys it does work!
the correct method for setting the timing via a vacuum gauge. the curb idle mixture screws should be reset to 1.5 turns from bottom.
you need to make sure the idle is set as low as possible. the engine should almost stall out. this makes 100% sure the mechanical advance will not engage. vacuum canister should be unplugged, now if your running on ported vaccum the advance will not come on at idle but its a good way to make sure no vaccum advance is biased into the tune.
with the engine just running you take your vaccum readings, as you advance timing make sure the idle rpms are just above stall you will need to keep adjusting it, and playing with the distributor until you get the highest vacuum reading. now back off timing till the gauge falls about 1-2HG" this will give you leeway in the event of detonation because of too much timing.
next you need a helper.
Reset Idle Rpms to factory spec between 625-675 with the car in Gear and the headlights on. (if you have A/C you add like 50 -75 rpms)
with the car in gear, engine warmed up, idle speed in range, and buddy standing on the brakes, you now adjust the air idle mixture screws, your looking for MAX vacuum, without raising Idle RPMS. and without stalling the engine. it takes forever since you have to work back and forth between 2 or 4 idle mixture screws and the idle speed screw on the carb.
once you have the best HG reading and the idle is in spec, with the vacuum advance caped go for a ride, good chance you will need to richen up the idle air mixture screws about 1/8 of a turn. watch the tach if you have one, if it starts bouncing at idle with the engine warmed up you know the mixture screws will need to open a little until the idle rpms are steady.
all this gets you in the ballpark of a good tune, it will change with weather, moisture in the air,temp etc.
next you go back home and complete any base line adjustments you didn't like on a test drive.
now you plug in the vacuum advance (hopefully its adjustable)
so you go for a drive and try to find a very long straight steep hill, you keep adjusting more advance in until at a steady speed going up the hill you start hearing marbles dancing around the mufflers.(the engine is pinging) now you back off until the sound goes away(this may need to back off 1-4 turns depending on the Quality of fuel on each fill up.
now you will have a car setup with a good tune, but not by the book perfect, every car will need a tweak coming off optimal numbers to fix drivablility issues.
now getting back to what you most likely did. you just advanced the timing a bit, advanced timing will make the throttle more responsive on its own, it will increase idle rpms which will increase vacuum. it may even be closer to an optimal tune then you were before but the method of arriving at that tune i not the right way.
the biggest mistake people make on a vacuum tune is not adjusting the vaccum advance canister, and not readjusting the idle rpms everytime you make a change to timing or the air mixtures.
on my car 'going by the book' on a vacuum tune, i gained about 7HG"
after driving the car for a while i had to richen the tune to get the engine to run better, so i gained about 5HG" Its not max vacuum but its where the car is happiest. Its a very hard thing tuning an engine for max performance when cold and when hot, you will find the perfect tune when the engine is hot will not work when the engine is cold, so you have to come off perfect to make things work right.
there is other issues with advancing the timing to get a good idle vacuum, you need to adjust the total timing to make sure its not too much.
as 71, 72 , 73 passed on ford reduced the initial timing to pass idle emissions standards and increased total timing to gain performance back. Say in 71 Ford used a L13 setting on the mechanical and ran 10 degrees initial by 73 this was a L16 and 4-6 degrees initial timing. so the 71 had less total advance then a 73 because it ran more initial timing.
71: 10+ 26 = 36 total mechanical.
by
73: 4 + 32 = 36 total mechanical.
this can be a problem if you take a later car and vacuum tune it to 12 degrees initial without limiting total or changing the distributor plate. you have to be careful. too much mechanical timing can kaboom the engine.
in my car i went out of my way to find a L13 plate for the distributor because i'm running 10 degrees initial. to get to the sweet spot total timing 32-38. i found i still needed to adjust the plate with a stopper since the tolerance was bad.
if you want to be sure of really what the car should tolerance you pull the distributor cap and the vacuum advance plate and look at the mechanical advance plate there will be 2 L numbers on the plates read off the one from the side that has the stopper under it. take that number and double it, then add in your initial timing to get total mechanical. it should be less then 38, depending on your engine build this might be less, Hypereutectic pistons Scream at you with the specs sheet not to exceed 34 degrees of total mechanical advance.
you can see things can get dicey and every motor is different.
When we rebuilt the engine, I put brand new Edelbrock heads on it and it has only about 600 miles on them. It has a performer Plus cam. When I was tuning the carb with the vacuum gauge hooked up, I was able to raise the vacuum at idle and steady it a bit at idle when adjusting the idle screws. Not perfectly steady, but a definite improvement. I want to play with it some more and look at the timing when I have time.
did you check the rods to make sure they are the correct length, its possible the valves are slightly hanging open
and can hang with the best of em.
