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Vaccum Secondaries vs manual

573 Views 5 Replies 4 Participants Last post by  Axnjxn
I have a 65 A code coupe that I bought with the motor in the box. I have the motor at the shop and am trying to figure out what to do for a carb/ manifold. The motor is a pretty much completely stock, just rebuilt 289. I don't have any throttle linkage or carb, but I do have a stock 4 barrel intake. I am wondering if anybody can recomend a carb and if I should keep the cast intake or switch to alluminum. I was thinking 600 cfm. I do have a knob under the dash for manual choke, but I don't know if I need manual of vaccum secondaries. And if I get vaccum, what do I need to hook it up?

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Are you sure that knob under the dash is for the choke, and not for the fresh air vent? Only K code cars came with chokes, IIRC, from the factory.
I'll try the easy part. The secondaries on the carb can either be opened through progressive linkage on the carb (manual secondaries, ie., a Holley Double Pumper) or through a vacuum linkage. For streetworthiness, vacuum secondaries are much easier to deal with, and will provide better mileage. If absolute, dragstrip performance is your goal, manual secondaries, when properly tuned, can provide a small margin.
Manual choke (the knob under the dash) doesn't make a performance difference that I can tell. Most carbs come with an automatic choke, activated by a bimetal strip. Again, works fine for the street, but may need to be set properly to keep the choke from staying partially closed.
600 cfm seems the right size to maybe a bit large. There are several posts describing the cfm calculation, but with stock A code heads and cam, something smaller than a 600 may provide better throttle response without hurting wide open performance. I'll leave it for others to describe their experiences with the various possible carbs.
The secondary linkage and choke set-up will come with the carb, so picking size first may be the best way to go.

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Yeah, the little knob next to the e-brake handle is the fresh air vent. You may have a second knob that someone installed for a choke cable.

For a stocker, probably the best way to go is with a stock A-code manifold and stock Autolite carb. IIRC it's 480 cfm, perfect for that motor. Now, if the engine had been "warmed up", with a good breathing cam, some head work, and headers, then you would benefit from a slightly bigger carb and aftermarket manifold. Yes, you can get some marginal gains from those now, but likely no more than a few HP over the stock configuration.
I know if isn't the vent. It goes to a cable in the engine compartment. So if I get electric choke, what do I wire it up to? Should there be a spot on the engine wiring harness?
you have to tap into the ignition switch. The easiest way is to pull out the instrument cluster. Then you have full access to the switch. On of the wires coming off it goes into a bullet connector, then into a pink wire. This is the pink "resistor" wire that you hear everyone referring to. You need to splice in right at the bullet connector, since that is the only 12 V switched source in the entire car. OR, you could wire up a separate switch I suppose, but if you didn't turn it off, your battery would die.
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