Dual vacuum advance distributor
A couple things... first dennis111, if you 'plug' the rear port, would that make the vacuum diaphram unable to move because air is trapped in the rear chamber? I haven't ever taken one apart so I don't know, but it seems like the best thing to do would be to just leave it open to the atmosphere instead of plugging it.
Also, there are two places you can hook the vacuum advance, to the timed vacuum port on the carburetor, which only gets manifold vacuum after the throttle is cracked open off idle, or just hook directly to manifold vacuum. I've heard many arguments for hooking it to manifold vacuum, such as reduced idle temp, reduced emissions at idle, etc... but I think it's easier to tune and get a consistent idle with it hooked to the timed vacuum port on the carb.
'97 F250HD 4x4 460; '85 Ranger, 350hp 289, T5, firstname.lastname@example.org
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