My two cents, but first you have gotten some really good suggestions with the prior respondents, nothing I would disagree with or take with a grain of salt (or two). Here is the web address for Holley's carb selector tool. Be sure to fill in the cubic inches and maximum redline at the top, then select the options below those entry fields. When you click the See Results button you will be presented with an array of possible carbs.
As for your more specific questions re: features, my first response is to get vacuum secondary carb, especially if you decide to get a carb a little larger than recommended for your engine displacement and maximum redline. It is hard to not fall prey to the temptation to get a larger carb than recommended. With vacuum secondaries the rear throttle blade set will only open when it get a vacuum signal indicating the engine can handle more A/F mixture. And, you will be likely to get a bog on heavy, WOT acceleration.
I like square holes vs Spreadbore holes. I have heard arguments on both sides of the fence, and for me it is a matter of practicality when it comes to being able to get more common parts.
I also vote in favor of electric chokes. But, be aware the White wire with a Black stripe feeding the stock/oem carburetor choke cap terminal is NOT 12 volts DC current (assuming it is the stock 2100 two barrel carb with electric choke). The after market carbs, to include Holley, "require" a 12 volt switch DC current to power the electric choke, which means a little repairing for you to do. Nothing bad or difficult, but if you get this wrong you will end up with a choke that will open too slowly or incompletely, which will result in fuel fouled spark plugs and puffing black smoke out your exhaust as the engine warms up. It is not hard to clear out fuel fouled spark plugs once the A/F ratio is stabilized, but you could end up with a speeding ticket when doing so.
There are few "correct" ways to get 12 volts of switched power to the electric choke, and several incorrect ways. I will cover a few of the more common incorrect ways I see occur - provided so you do not end up doing something that looks right, but are wrong:
- The first one I mentioned above is using Circuit #4 (White wire with Black stripe) being used on the factory carb electric choke cap. The problem is Circuit #4 is powered directly from the alternator stator terminal. It only produces power when the alternator is truning and outputting current for the battery and electric accessories. The problem is the stator voltage is about 1/2 of alternator output voltage, inadequate for a system requiring 12 volts. The other issue, which is a lesser issue, is the stator output is AC current, not DC. You do not want to use AC current on devices designed for DC use. For electric chokes they re durable enough to work with either AC or DC current, but the voltage is too low for the Holley carb, hence the choke opening will be too slow and/or not open enough.
- The next incorrect way to provide current to the electric choke cap terminal is to tap into one of the smaller terminals on the Starter Relay. The smaller terminals are marked with an "S" (Start) on one, and "I" (Ignition) on the other. The terminal I see folks use for 12 volt switched current is the "I" terminal. The problem is when the engine is running the "I" terminal is powered by the ignition coil's voltage from the ignition switch. The reason that is a problem is the ignition wire leading to the positive terminal on the coil, and to the "I" terminal on the Starter Relay, is coming through a resistor wire, where the voltage is less than 12 volts despite being DC current. As with the Stator current, the voltage is too low to let the choke as fast or as far as it needs to. But, I have seen lots of folks do it that way, and it seems to work okay. It just is not working like it should.
There are three ways I like to use to get full voltage switched power to the choke cap terminal. But, be aware that your choke cap may have two blade terminals. One is meant to go to ground. Do not apply voltage to that terminal. The other blade terminal is the one designed to receive full battery voltage (usually stated as 12 volts, but for a car that is running the alternator will produce a little more than 12 volts). Here the ways I select from:
- If you have a 1971 through 1973 Mustang the wiring harness provides a wire that is meant to connect to a carburetor Throttle Position Solenoid (TPS). If is powered by Circuit #640, which is a Red wire with Yellow hash marks. Your engine may or may not have a TPS on the carburetor, but either way the electrical harness near the carburetor should have a socket meant to connect to the pigtail lead for the TPS were it present. You can safely tap into Circuit #640 to use for the electric choke. I suggest tapping into the wire on the solenoid side of the connector near the carburetor to prevent altering the original wiring harness insulation.
- As alternative connection is to tap into the Circuit #63 wire for the windshield wiper motor. That is a red wire, and I suggest you tap into the wire on the motor side of its connector, again to prevent altering the original wiring harness.
- The third way to get 12 volts to the electric choke is to install a relay that uses with circuit #4 (stator), circuit #640 (TPS), or circuit #63 (wiper motor) as the trigger voltage to cause the relay to then send battery voltage from the positive side of the starter relay to the electric choke terminal. This is bit bit more involved than the first two options, but I have used it successfully in several cases. Using the Circuit #4 Stator current to activate a relay does work, but I really try to not use it for that purpose, simply because Circuits #640 and #63 are plenty easy to access - so why bother with the Circuit #4?
I have attached some schematic snippets for the 1971 - 1973 Mustangs for the circuits I have mentioned above. I included some schematics showing the oem Electric Choke circuits and Ignition circuits just so you can see how those are laid out, not to encourage you to use the resistor wire ignition or stator circuit to power the Holley electric choke..
I also have a few YouTube videos that you may find useful where I cover these circuits being used for an aftermarket carburetor electric choke. The video at
is oriented to providing 12 volts of switched power for a Pertronix Ignition System, but you can use what is shown in the video to power the electric choke instead if you like.
The video at
shows how I used a windshield wiper motor Circuit #63 to provide power to an electric choke. The only reason I did not use Circuit #640 for the TPS is because someone owned this Mach 1 before me, and either cut off the harness lead tot he TPS, or placed it somewhere I could not find. It was easier for me to just use Circuit #63 as opposed to dig through the harness to try to find Circuit #640's terminal wiring.
I think that ought to get you on track. If you have any questions you cam email me at
[email protected].
Best of luck to you. And, Circuits #640 and #63 are protected with proper fusing. Circuit #4 (Stator) is not. Also, if you use a relay be sure to provide proper fusing on the load wiring between the battery voltage and the relay so any wiring you introduce is properly protected from overload...
