[A Snake]

I'll throw another wrinkle in to think about. First, I've used both mechanical and vacuum secondary carbs on 428 engines. The original CJ came stock with a 735 vacuum secondary carb. You have to remember that these engines were put in Galaxies and Mustangs which weighed at least 1,000# more than a Cobra.

In these two instances, the vacuum carb worked better, because the driver could not inadvertently put his foot to the floor and get a giant case of the bogs. The engine took only what it could handle, based on the vacuum signal.

Now, with a 2,300# car like your Cobra, throttle response in moving the car is very quick compared to a big Galaxie or Mustang. This simply means that it is much less likely to"over throttle" the car.

With that said, my preference is buying a "prepared" 750 cfm double pumper Holley from Barry Grant or The Carb Shop. This carb will flow like an 800, and has four corner idle which is a real blessing with a bumpy cam.

9:1 will work, but you would like 10:1 much better and still be able to use pump supreme. Using the original Med. riser 427 solid lifter cam in a 428, works like a charm. Nice chuckle in the idle and torque that will snap your neck......and that combination likes the double pumper.

If you've had a double pumper under your foot before, you know that you can feel when the secondaries are coming on. You can't gauge that with a vacuum carb and therefore actually have less control of power output. That's a big reason no racers use them on the track. The same thing applies to a few blips of the throttle on a curvy road. The last thing you need is the back half of the carb coming in right in the middle of a curve. Believe me it can happen and will.

As you get used to the car and it's power output, you're going to want tighter control of the throttle than you can get with the vacuum carb. To me they feel mushy and unpredictable. One time you stick your leg in it and the car goes like stink. The next time it makes a lot of noise and doesn't do much. Depends on air temp and density and engine temp as well as engine rpm as to when the back half begins to work.

Just a few thoughts from an old coot who's run a good many FE engines over the years....including in a Cobra.

Al

Fuel mileage? In a Cobra? Good grief! If somebody wants mileage, buy a four banger with four doors!