Help me understand this vacuum/idle setting thing. If I use the adjusters to change the slide position and gain vacuum, that will only lower the idle. I use the idle knob to gain back the idle, but then I lose the vacuum. Isn't it six one way/half a dozen the other?
OK, I'll give it a shot.
Let's begin with all slides being equal and all runners having the same vacuum.
If you raise one slide, the vacuum on that runner moves closer to atmospheric (less vacuum) The rpm rises (because one slide is open more than the others, it behaves as if the operator asked for more power and that cylinder provides more energy to turn the crankshaft) Further, because of this, the vacuum on the remaining three improves (more vacuum). This is because the increase in idle speed allows the cylinders to increase the frequency of the their vacuum pulses (they also provide less power for the effort of spinning the crank, but not much is required to do so at idle.
If you move the idle knob to restore the same RPM as before, three cylinders return to their previous reading, and the mal-adjusted one still shows a lower vacuum reading than the other three as the more open slide position allows more air in to equalized the pressure with outside atmospheric.
Now let's do the opposite condition starting with all runners having the same vacuum.
If you lower one slide, the vacuum on that runner moves away from atmospheric (more vacuum) as the slide provides a better seal against incoming air for pressure equalization. The rpm falls (because one slide is closed more than the others, that cylinder provides less energy to turn the crankshaft) Further, because of this, the vacuum on the remaining three also falls (less vacuum). This is because the decrease in idle speed makes the cylinders decrease the frequency of the their vacuum pulses (they also provide the dominant power for the effort of spinning the crank).
If you move the idle knob to restore the same RPM as before, three cylinders return to their previous vacuum reading, and the mal-adjusted one will show a higher vacuum reading than the other three as the more closed slide position allows less air in to equalized the pressure with outside atmospheric.
I've seen this condition first hand. My 78 550 came to me with badly imbalanced carbs. I can only guess that someone prior had tried to "tune-by ear". This model has a non-adjustable #2 carb position. The dials revealed this cylinder had the highest vacuum and the #2 header pipe was of a lower temp than the others. If the others are a tweaked for idle speed improvement, each slide is opened farther than the others in turn until the idle seems "smooth".
The problem was that return to idle was hit or miss. Mostly miss and the idle speed changed the longer the the throttle remained at idle. So, it was a battle among cylinders for crankshaft speed of rotation. You had to set the idle high, for the event that it really wanted to idle low.
Perhaps a note on vacuum sync. There is a lot of instability while tightening down the lock nuts which you must anticipate during lock down. It is one thing to adjust for equal vacuum. it is another to maintain that after lock down. The goal is to have all the ports show equal vacuum AFTER the lock nuts are tight. The tie plates also enter into the picture as they can place a torque on the linkage causing mis-alignment and side pressures on the actuators/ball links.
You never did answer the question about return springs pressure changes.
And, at this point, I guess I would query about whether the linkage for each carb was assembled with the parts in the correct orientation. I would remove the return spring and the cables and feel the movement throughout slide travel to ensure no binding or change movement force is required at any point in the travel range.
You still haven't eliminated the mechanical timing advance mechanism, have you?
Cheers,
