hi, so i'm working on a 1957 235. it has just been rebuilt using a mild comp cam. i'm also running dual zenith 28 carbs and fenton dual exhaust.
this is a new area for me so i'm learning as i go and thanks to this forum, i'm finding my way so please keep that in mind. if i seem like i don't know what i,m talking about, well i really don't!
anyway, i'm trying to hook up the vacuum advance from the distrubutor. in doing some checking on the internet, i see that i need to have a ported vacumm sorce or some say, none is needed.
so the questions are is it needed and if so, where can i get it from?
in looking at the carbs, there dosen't seem to be a port. thanks
You are going so far away from stock that I doubt that the original vacuum advance will do what it was design to do. I have heard that the big trucks did not use vacuum advance. Many of them came with a governor. I don't know if that was why they did not come with vacuum advance.
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission
You only want to use ported vacuum from one of the carbs. I'm not familiar with the Zenith carb and it's entirely possible they don't have a ported vacuum port. With that said, a ported vacuum port will be located just below the throttle plate. 1st locate where the throttle plate cross shaft is where it's ends are on the carb housing then look just below all the way around the carbs housing for the ported vacuum port. You might find one thats plugged
Optimally you want ported vacuum, for a 57 that is a vacuum source under the throttle blade when the throttle blade is closed. When the blade begins to open, the vacuum port sees the vacuum as the air goes past the blade, and then it diminishes as the blade comes to wot. Not all carbs have a ported vacuum source, and while some that have a source, the vacuum may not be enough To actually operate the advance.
I ran a pair of carter yf carbs for a good many years. Those carbs had a ported vacuum source that was sufficient to operate the advance. Sometimes you can use the port on each carb to get the vacuum up, sometimes not.
When Tom Langdon convinced me to try a pair of pinto Holley Weber 5200 carbs (has to be twenty years ago now, maybe a bit longer) I discovered that the port for the vacuum was not providing sufficient vacuum to operate the advance. Tom had recommended full manifold vacuum which is what I used and it worked fine. Same thing on the 38/38 Webers I currently run so I still use full manifold vacuum from the intake manifold, even with a vertex magneto.
With a stock set up, you don’t need to disconnect the vacuum advance to set the timing because there is no vacuum acting on the advance diaphram at idle. When using full manifold vacuum, you now have to disconnect and plug the advance while you set the timing so that no vacuum is acting on the diaphram at idle while you check or make adjustments to the timing.
Hook up a vacuum gauge to the carb port once you identify if the zeniths have one, and watch the rpm and vacuum at different throttle blade openings. Here is a chart that has the specifications. I posted the sun specification chart for 56 and 57.
"Ported" vacuum is accessed just above (not below) the leading edge of the throttle plate. If the port was below, it would be exposed to constant manifold vacuum. If the idle speed screw is adjusted properly, there should be little or no vacuum on that port until the engine speed is increased, then it will build gradually as speed increases. At light throttle cruise speed, it will be very close to full manifold vacuum and the vacuum advance will provide an extra 5-10 degrees of timing advance over what is provided by the centrifugal advance weights in the distributor. If the throttle is opened fully, the vacuum will drop and the spring in the vacuum chamber will retard the spark to avoid detonation during acceleration. As soon as the throttle returns to cruise position and the vacuum rises, the advance cones back in. A very few engines, like the early 1960's 327 V8's, used full manifold vacuum to the advance for some reason. I've never seen that setup on a stovebolt six with factory carburetion. Jerry
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if i want to try to hook the advance to the manifold, can i split that port on the manifold and use the same manifold vacuum for bolth the pcv and the advance?
