How does vacuum advance work on an HEI distributor?
How does vacuum advance work on an HEI distributor?
This is how vacuum advance works. At part throttle, high manifold vacuum moves the diaphragm in the vacuum advance canister on the distributor to add more timing. But at WOT, the vacuum drops to near zero and vacuum advance is removed and the total timing then is established by the initial plus the mechanical advance.
How do you adjust the vacuum advance on a HEI distributor?
You can easily adjust the vacuum advance of the Street Fire Distributor. Insert a 3/32″ Allen wrench in the canister inlet and turn the adjustment screw clockwise until it is all the way in (Figure 2). Counter clockwise reduces the advance, clockwise increases.
Where does the vacuum pickup go on a corvette?
One of these band-aids was “ported spark”, which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle.
What does vacuum advance have to do with timing?
Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the “total timing” equation.
What was the problem with the vacuum advance?
The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems – “ported vacuum” was strictly an early, pre-converter crude emissions strategy, and nothing more.
Why does my vacuum advance control unit fluctuate?
If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures.
One of these band-aids was “ported spark”, which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle.
Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the “total timing” equation.
How to replace the vacuum advance unit of a Chevy?
Remove the distributor cap by unscrewing the two mounting screws using a Phillips screwdriver. On some Chevy models, you only need to rotate the screws about 1/2 turn to release the cap. Pull the rotor off the distributor shaft. On some Chevy models, remove two mounting screws using a Phillips screwdriver.
If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures.
