When I first decided to try to run a 2x4 carb set-up, all I had were two Holley 600s originally designed to work alone. In order to use a progressive linkage, I had to make several modifications. I thought this post might prove useful to others who are cheap like me…er……on a budget. I did in fact eventually get proper carbs, but they seem to work the same, so maybe money was wasted.

I can break this down into 4 parts:

1/ Secondary Carb Throttle Lever Mod
2/ Vacuum Secondary Connection
3/ Linkage adjustment
4/ Idle Mixture Modification

Before I start, take note of the set-up and names of parts from this pic of the dual carb set-up from my 1966 Ford Manual (there is 1 mistake…..the secondary throttle lever (bell crank) has been mis-identified as an additiional primary one):



　
1/ Secondary Carb Throttle Lever Mod
　
Below you can see the difference between a modified and an un-modified bell crank (modified on the left):



You drill the hole 5/8” from the throttle shaft, and in-line with a line between the center of the original hole and the throttle shaft. Here’s a close-up of the modified bell crank:



After the new hole was drilled, a standoff, spacer, and some washers were bolted on. Here’s the stand-off height along with more details:


　
2/ Vacuum Secondary Connection

You need to connect the vacuum secondary chambers together so the secondaries always open simultaneously. Holley sells the covers, or you can make your own.

Here’s the proper Holley vacuum covers:



Here’s the ones I made using grease fittings:



3/ Linkage Adjustment

The linkage is designed so that as you press the accelerator, the primary carb (front one) starts to open, but not the secondary. After the primary one opens approximately 30% (I’ve also seen the 25% figure mentioned, but I don’t think it’s critical), the linkage sloppy link starts to pick up the secondary carb (rear).

Because the secondary carb has it’s linkage attaching pint located closer to the throttle shaft, both carbs are intended to reach WOT at the same time.

Here’s the progressive linkage on my car:



You start by adjusting the primary carburetor rod to 4 and 3/16”. Different lengths may be needed dependant on how far apart the carbs are, which is based on intake manifold design. Mine is actually set to 4 and 5/16”. That may be my mistake, although I did it so long ago, I can’t remember for sure any more.

Next you hold both carbs wide open and adjust the secondary carburetor rod so that it just holds the secondary throttle wide open when the primary one is wide open.

Now go back and look down both carbs. Start opening the throttle and you should see the secondary carb pick up when the primary carb is approximately 30% open. If it doesn’t appear correct, play with the primary rod adjustment, re-adjust the secondary rod, and look again.

4/ Idle Mixture Modification

First of all, I should mention that when I am adjusting the idle, I set both throttle Stop screws loose, then turn both in until they just contact the throttle levers. Then I adjust both the same number of turns to get the idle speed I want. I think having both carbs open the same amount at idle is a good thing, otherwise one might be open too far, exposing too much of the transfer slot. I'm not sure of this, but it seems to work fine.

It appears that using two carbs makes the idle mixture too rich. Using a vacuum gauge, I could turn all 4 mixture screws completely in and still not hit my vacuum peak. But I might have. I just couldn’t’ go any further to see if the vacuum started to drop off. I seemed to get satisfactory performance by leaving them a little rich anyway….somewhere between 1/8<SUP>th</SUP> and ¼ turns out.

If that’s not good enough, here’s a pic describing a simple way to modify the idle jets for dual carbs by adding restrictions:

 

Good Read John... Never saw a grease fitting used in that manner..But Hey ! It worled !