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Look on the carb for a small vacuum source above the throttle body. You would want one that has no vacuum at idle.
vacuum lines,there are three one for each petcock and one for the vacuum advance, the two for the petcocks go to the carb boots, one on each side,and the one for the vacuum advance goes directly to the no. two carb.
It might have four hoses? Pcv and Brake booster go to separate large ports, the pcv usually to the carb base and the booster to the manifold fitting. Vacuum advance goes to one of two small ports on the carb. Test to find the one that has no vac at idle but as soon as you touch the throttle you get vacuum. Cruise goes to either the small port on the intake or the carb base. It could be split off the vacuum advance.
Check your timing and your vacuum advance.
It depends on the year of the car.....but if you are running HEI ignition (74 and later) it will run to the carb. It goes to a ported vacuum signal. You will find a ported vacuum on a fitting that is above the carb base plate. It should have no vacuum when the engine is idling but will have vacuum when you rev the engine.
Any place that has constant vacuum Usually on the back side at the base of the carb.
you need manifold vacuum. Either on one of the intake runners or at the base of the carb.
Connect the line to a source that is above the throttle body on the carb. It should be a source that has no vacuum at idle.
There should be a vacuum source on the carb above the throttle body that gives spark ported vacuum. You can connect a tube from this source to the distributor.
There should be two small and one large vacuum connection points on the front of the carb, hook it to one of the small ones, that will be for your vacuum advance. the big one is for crankcase ventilation. one of the smaller ports is manifold vacuum, the other is timed vacuum. they are not the same, but you will use one of these, usually the "timed" port.
Could be, Accelerator pump in carburetor is not working, (rebuild carb) Timing is off, (adjust timing) Vacuum advance not working, (replace vacuum advance unit) Points are in need of replacing or adjusting,
When you get ready to run the vacuum line from the turbo 400 trans modulator to an Edlebrock Performer 800 CFM carb, attach the vacuum line for the transmission vacuum modulator to the bottom constant vacuum source or the plug that is below the throttle plate. After that, install the vacuum advance to the port that is signaled or right above the throttle plate. Once that is completed, install the large vacuum line to the back of the carb for the power brake.