The carb setup is rochester 3 x 2gc. Does anyone know where are the correct vacuum port for the vacuum connection to the dizzy? On the rochester there is 3 ports, one on the lower base at the front on all the carbs (A1,A2,A3), one at the side half way up the carb (B) and one at the top by the filter (C). I have tried the one on the side (B) but I am not sure its the right one. Can anyone confirm? I have also used the front A1 port for brake servo vacuum and A2 for the PCV valves on each rocker cover.
Put your thumb over each port. If the port doesn't receive vac at idle, but does when you open the throttle; that's your port. It's been a while, but I think the winner will be "B" in the second photo.
I prefer to use manifold vacuum for the distributor. It greatly increases idle quality. The early Chevys with the 2GC's had a hollow stud that held the carb down that had a ****** on it where a hose hooked up to go to the DIst. Others had a 1/8" npt fitting on the throttle plate for this purpose.
I disagree. All the 60's Chevy's that I worked on required you to remove and plug the vacuum advance tube while setting the initial timing at idle indicating that there is vacuum to the dizzy at idle. If you didn't plug the vacuum advance tube it wouldn't start because of the vacuum leak. When you hook it back up the light jumps up off of the timing tag at idle. That lets you know that the vacuum advance is working. Most all the GMs worked this way. At least that is the way the factory GM dizzies were set up.
I always run distributor on ported va***n. That would mean upstream from the throttle blades.Regular va***n would just advance the distributor as soon as the engine starts . It would work but not as intended.
On mine, there's a smooth 3/16" diameter hose ****** that goes uphill out the back of the center carb. I think that's the ported vacuum port and that's what I connected to my distributor. I haven't run it yet though. There are larger threaded holes down on the bases of my 2Gs and they are manifold vacuum. I plugged the manifold vacuum ports on the front and rear carbs and put an elbow ****** on the center carb for hooking up to a PCV valve. The ported vacuum port has some vacuum at idle depending on where your idle speed is set, and the vacuum dances around a little with a hot cam. That vacuum helps to advance the timing to make it run smoother at light throttle. When you're at wide open throttle, that drops to almost no vacuum. When you set the timing, you're supposed to disconnect and plug up that vacuum line so that you're only looking at mechanical advance. The ported vacuum smooths out the engine by advancing the timing only at very light throttle. It also helps with gas mileage when driving slow around town. Sometimes you can get away with hooking the vacuum advance to the manifold vacuum, but sometimes that'll cause it to ping if you get too much advance at light throttle.
Some newer carbs have ports close to the top of the carburetor that are just a place to get clean air from. Like for an air injection reactor (AIR) pump for smog control ****. If the port just goes into the airhorn near the top, then that won't give you any vacuum. I think the ported vacuum ports go to a small pinhole that's right near the ****erfly when it's at idle or slightly above idle. The manifold vacuum ports go to an opening down at the base of the carburetor -- I think it's right in between the two bores in the center rear.
Don't guess. Get a cheap vacuum guage from the parts store. You want the port on the middle (primary) carb that shows no vacuum at idle but reads vac as the RPM comes up. As stated above, that is ported vacuum. It does this bacause the port is above the ****erfly at idle and exposed when throttle is applied. And that is when you want your timing to advance.
GM used manifold vacuum from the first vacuum advance in the stovebolt, uintil they installed emission controls in the late 60s. The reason they changed to ported vacuum was to retard the spark at idle to reduce NO at idle. The Cal. ARB came up with that fix. Manifold vacuum works better, idles better, and reduces overheating in traffic. but you do it your way.
Use the B port. That goes directly down through the base and into manifold. It will have manifold vacuum at all times and that is what you want.
Not at full throttle nor does the ported side. I prefer ported but I have seen the factory use either way. Mostly I have seen them use ported. Lets ***ume you have 8 degs base timing and then you hook up the vacuum advance to manifold vacuum. As soon as you start the car, you woul have 8 degs base and then about 16 minimum vacuum plus the 8, so 24 at idle. Then with a slow RPM increase, the vacuum would start dropping off and lose timing through the vacuum advance until the mechanical weights kicked in so while the vacuum is pulling vacuum off at rpms, the mechanical would be adding vacuum. This seems counter productive. So hooking it up to ported, as the rpm increases, so does the vacuum advance until the weights kick in for mechanical and by that time the vacuum would most likely be as high as it gets. Floor it and the Vacuum will drop on ported and manifold vacuum.
That is the way you do it, even on 80's/90's engines. The reason is that if the throttle blades are not set right, you will be getting vac through the ported hook-up at idle, thus creating a vac leak, or if you had it hooked up, it would be changing the timing.
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