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MG MGB GT V8 Factory Originals Technical - Vacume or Mechanical Secondaries?

I'm shopping for a 4 barrel carburetor right now between 500 to 600 cfm for my 289 Ford engine. Generic advice is mechanical secondaries for light cars and vacume for heavy cars. The problem is there seems to be WAY more vacume secondary carbs for sale out there, and I've seen these types of carburetors in light V8 cars as well. Those of you using carburetors, what are you using and what are your thoughts? Any downside to using vacume secondaries? I'm slightly Holley biased, but haven't ruled out the Demon or Edelbrock just yet.

Scott Wooley

Scott--My B has a 5.0L Ford and I am using a 500cfm Edelbrock which I am happy with--I did have a 600 cfm Edelbrock on previously and felt it was overcarbed even tho I had jetted it down.I have a rebuilt Carter 500cfm that I tried but it is about 3/4 inch too high to fit on my set up and it worked fine when I tried it but couldnt close the hood properly and rather than redo all I went for the Edelbrock.
Way back when--- I had a new Pontiac GTO in 1964 with 3 -2bbl carbs and a vacuum set up --I never liked it as it was too much dumping of gas on full throttle. I had a 1966 GTO subsequently with mechanical linkage on the 3-2bbl setup and found it much better.
Hope this is of some help to you.
Any questions just ask
Good luck
Gil Price

Go vacuum. They tend to do better for tuning and cruising. The Mech are best for racing or people that want to constantly tinker with thier carb...
Larry Embrey

FYI --I put over 15K miles on this car a year and never tinker with the carb --set it up and forget it !
Gil Price

1350 CFM.

Vacuum operation, never stumbles, always smooth. Try that with mechanical!
Bill Spohn

The reason I ask is I had a 600 Holley on my 5.0 Mustang with mechanical secondaries and I loved it. Once it was set up right, I never messed with it either. There just seems to be an abundance of vacume operated carbs out there, and there must be some reason for it. Just getting opinions, they really matter to me. Thanks for your replies.

Scott Wooley

Drag strip days are a bit in the past, but if I recall, there are two types of mechanical secondary carbs. The progresive opens the secondaries only after you get your foot way into the pedal. The other one (as we had on the race car) opened all four throtle plates at the same time. I do not think you would want to run that one on the street!


My vote goes with the vacuum secondaries. If the springs are selected carefully to match the engine demands then there shouldn't be any flat spots or stumble in the acceleration which is usually the case with mechanical secondaries.
Bill Young

Scott - go for the Demon! I had one on a vehicle last summer, and it was amazing how much more performance it offered compared to the previous carb. They are also very easy to adjust, and the power valves aren't as problematic. I think they recommend the mechanical secondaries on cars with manual transmissions. You can also get the direct linkage rather than progressive linkage for the four barrels. Holy Cow does that give you throttle response!!! But at the loss of rpm range. Its definately a drag race setup - NOT to be used on the freeway!

The Edelbrock was a more difficult carb to tune - the needles can be tough to reinstall. I don't think the power potential is there either and you can't externally adjust the floats. These are a step down from a good Holley, and a step up from a used Holley. I've heard after 30,000 miles they wear out as well.
Jeff Schlemmer

This thread was discussed between 04/01/2006 and 06/01/2006

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