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Old 06-29-2005, 11:11 PM
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Multiple carburation and progressive linkage?

Hey guys , here goes another noob question. I've seen some pretty rad 4x2 and 6x2 carb setups on traditional style rods in the past and was always amazed by how they worked. I undestand they all have to be synched perfectly with a custom made linkage, so they all open up the same, right? But then whats "progressive linkage". I understand there is a specific ratio for these? For example when 2 carbs are at 1/2 the other ones are at 3/4 throttle, but when the gas pedal is floored they all have to be wide open....why? How does this work, and how do you figure out a ratio etc...



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Old 06-30-2005, 12:05 AM's Avatar Moderator
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Progressive linkage is just what it sounds like. If 3-2s were connected to all open and close together, that would be the same as a big double pumper - way too much carb at low speed and bad bottom end performance. Old-timers got a round this by hooking up the linkage on the carbs so the middle one had a choke and the idle tuned to run like a single deuce and acted directly with the foot throttle. the other two carbs ran indirectly from linkages from the middle carb and the actuating boss on that carb slid on shafts going to them so they opened late. The ratio on the levers to the second two carbs wer shorter so they opened late compared to the middle one but came on a lot faster so they all three opened to max opening at full throttle.

I think they still sell progressive linkages for 3-2s. Nothing very fancy, just a few 3/16" rods, some slider clamps and some lever arms to screw to the throttle shafts. I guess you can do the same for 2-4s but I never fooled with those. Actually pretty lo-tech.
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Old 06-30-2005, 03:09 AM
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The above illustration (blow up) should help you understand the principle of progressive linkage. The center carb is connected directly to the accelerator linkage. The outboard carbs are connected to the center carb by way of rods and these have slides that allow when the outboard carbs will cut in.

So you can drive off the center carb and open the outboard carbs when you want by way of linkage adjustment, both at the same time or just one outboard and then the other depending on how much fuel you need. It can be adjusted to open all three simultaneousy, but this would be for competition only.

Now some 3X2 setups had vacuum motors on the outboard carbs (similiar to 4V) (CHEV CORVETTE 427/435 and MOPAR) and the outboard carbs opening rate was controlled by differing return spring pressures as you tune a vacuum secondary 4V HOLLEY.

The 2X4 had a similiar arrangemant with the linkage in that you worked the primary 4V directly off the accelerator linkage. The secondary 4V was attached to the primary by an adjustible linkage rod. You could run the car off one primary carb primary or synch them opening together. The carb secondaries were controlled by vacuum motors and return spring pressure.

Now Tunnel Rams had two HOLLEYS mounted (usually DP's) and the throttle linkage could open either simultaneously or slighly progressive depending on the amount of fuel needed at the start. You can probably see the Tunnel Ram Linkage on the HOLLEY site.
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Old 06-30-2005, 02:02 PM
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Geesh Nightfire, dont you know anything?
Actually, I didnt know it either so Im glad you asked. HG
I was built with the body of a God! To bad it was buddah.

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