We’ve built and tuned so many supercharged engines over the years, we’ve lost count. And every time we build a Roots-blown engine, the thing we least look forward to is setting up the carburetor linkage.
If the engine is running a single carb, it’s an easy job. Dual carbs mounted inline is almost as easy. But when we’retalking about side-mounted double pumpers, getting the carb linkage set up properly can be difficult and frustrating.
This is the carburetor setup that works best on blown engines. The double pumpers also look the coolest sticking through your hood with all of the rod ends, stainless bolts, fuel pump to carburetor lines and anodized tube nuts running around the top of the blower.
But setting all of that up isn’t fun.
The blower manufacturers don’t always help mucheither. The instructions they send with the linkage kits usually just include a parts list, a photo of the linkage setup on a blower, and maybe an exploded diagram showing how all the pieces go together. But that’s only marginally helpful, especially if you’ve never done this before.
Over the years, we’ve discovered some tips and tricks that will increase your chances of setting up carburetor linkage right the first time—and maybe help you actually enjoy the process.
Here’s a Step-by-Step Guide to Make Dual Carb Linkage Assembly a Better DIY Experience
First, take a look at this Weiand 7166-6980 Supercharger Carb Linkage document(printing it out might help) for an exploded view of the parts (there are several!) as well as what proper assembly looks like.
Then, flip through the slideshow photos below to see and read step-by-step instructions for taking the frustration out of assembling dual carb linkage.
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The first thing to do when you get your linkage kit is to lay out all the pieces and organize them by size, length, etc.
The next step is to completely close the throttle blades by backing the idle speed screws out of both carburetors until the throttle arms stop moving.
If the rod ends did not come threaded onto the aluminum hex shafts, that is the next step. There should be an equal amount of right- and left-hand male threaded rod ends. Hex shafts should have one end marked with a groove cut around it, like the upper ends in the photo. This typically indicates left-hand threads. Left-hand threaded nuts might be a different color too, like the brass nuts shown. Don't cross-thread the aluminum shafts by forcing the wrong rod ends into them! If they don't start threading in easily, try the other end of the aluminum shaft. Turn all rod ends all the way in, and make sure they are all close to the same length and are easy to turn in and out. Light oil or anti-seize on the threads can help.
If a rod end can't be turned using just your fingers, mount it in a vise and turn the aluminum shaft or opposite rod end back and forth with a 5/16-inch wrench until it’s easy to turn.
Your kit should come with two large male rod ends. Screw them all the way into the black anodized linkage support brackets as shown in the photo, then back them out a half-turn. Mount the linkage support bracket in a bench vise and tighten the lock nut using a ½-inch wrench while holding the rod end with needle nose pliers. Make sure the rod end remains perpendicular to the bracket.
The throttle cable tower bracket with its small pressed-in bearing—aka the "T Bar" or "bell-crank" bracket—has off-set mounting holes. Be sure to mount the bracket with the holes closest to the blower as shown. This will provide clearance you'll need later.
Using a solvent tank or a pan with some paint thinner in it, take the time to "hone" the linkage arms on the stainless steel main shaft by sliding them up and down the splines repeatedly.
Next, deburr the main shaft's splines using a wire wheel on a bench grinder. Go with the grain of the splines, not across them. You'll see a slight color change and the splines will feel considerably smoother when you’re done. You'll thank us later if you take this extra step.
Most linkage kits do not come with carburetor mounting fasteners. Most guys like using studs, but the studs in a regular kit will be too short for the two linkage support brackets. We prefer using 11/4-inch long stainless socket cap screws and lock washers instead. They're easier to work with than carb studs when setting up the linkage. We use a long, quarter-inch ball end wrench to install and remove them. You can swap to studs after setting the linkage up, but trust us—socket cap screws are the way to go.
It's important to have the fuel lines in place when setting up the linkage. Otherwise, you may have to readjust everything after you put the lines on.
Your linkage kit will come with several 10-24 thread stainless socket cap screws. Two should be shorter than the rest. Don't mix them up.
You should also have two ½-inch longer blower studs, shown on the left, to mount the main blower case bracket.
Remove the two center blower studs on the left side of the engine. Use doubled nuts if they're stuck. Then install the longer studs from the kit, leaving about 1 inch above the washer as shown.
Most blower cases have flat, counter-sunk mounting holes that cause havoc with your main blower case bracket. Leave the existing washer underneath the bracket as shown. You don't need to add another washer on top of the bracket when you tighten the stud's nut again. This method will also raise the bracket to clear tall valve covers.
Use a torque wrench to tighten the blower studs to 10 ft.-lbs.
The linkage kit comes with several 10-24 nylon lock nuts. Don’t use these for mocking up the linkage. Use regular 10-24 thread hex nuts like these. This is another tip you'll thank us for later. But you don’t have to. It’s our pleasure.
Two of the splined lever arms go on the long end of the splined shaft, which faces the rear of the engine and connects the bell crank to the main shaft assembly. The arm at the rear of the shaft (marked C) connects to the rear carburetor. The front arm (B) goes to the vertical link from the bell crank. Set the distance between them (V, as we call it) at a minimum of 1 1/4 inch to no more to 1 1/2 inch apart as measured at the screw-hole centerlines. You might not be able to get it exact, but close is good enough.
Assemble the rest of the splined shaft components exactly as shown. Pay close attention to the orientation of all parts—flip one around and you'll be in for headaches later on. The two splined lever arms on the ends marked A and C need to be as close to parallel as possible. In all of our years of doing this, we've never found one shaft that has the splines cut exactly equal end-to-end, so just get them as close as possible by laying the assembly down on the bench as shown and making sure both arms are close to touching the worktop. Do not tighten the lock screws on any of these pieces yet. They need to slide around for now.
Splined lever arm marked A in the previous photo is for the front carburetor. Attach one rod end to it and the other end to front carb's throttle arm using a pair of the brass linkage pilots that should be in your linkage kit.
Mount just the rear linkage support bracket (arrow) loosely to the rear carburetor. You should be able to slide the shaft assembly up and under the fuel lines to get the long splined end into the support bracket's rod end (right hand in photo). You can loosen the fuel lines a bit to help. This might take some wiggling, but it will fit. Note the forward carb linkage support bracket we’re holding with the left thumb and forefinger is not secured yet. You can bolt this bracket down after the splined shaft is in the rear bracket.
Attach rod end C (arrow) to the back side of the rear carb's throttle arm as shown. This is a spot where many people get messed up and accidentally attach it to the front of the throttle arm. Repeat after us: "Back carb, back side. Front carb, front side."
Attach the other rod end on the back carb's shaft to the back side of the splined lever arm as shown. Note we’re holding the front splined lever arm down with the left thumb to make accessing the back arm easier. (Image/Mike Petralia)
Here's where one of two small brass rod end spacers gets used (arrow). It goes between splined lever arm B and the rod end to prevent binding. This is important so don't leave it out. Sometimes it's easier to thread this screw in with the linkage pulled open as shown before. This way you can also check that the splined lever arms clear the fuel lines at wide open throttle too.
And here's why we mounted the throttle cable tower bracket with the offset screws pushing it away from the blower. At wide open throttle, we will have just enough clearance for the socket head to clear the blower's support rib in the side of the case. This is where the other small brass rod end spacer gets installed.
Now's a good time to fine-tune each threaded rod by hand, watching the carb's linkage arms for any signs of movement. Hold the splined lever arm steady and twist just the aluminum shaft very carefully. You want to take up any slack, but not move the arms when turning the aluminum shaft. You'll probably have to do this a bunch of times, switching between carburetors. Each time you’ll get a better feel for taking up all the slack. This step is necessary to make sure one carburetor doesn’t open sooner or later than the other.
Set up the bell crank lever arms' spacing as shown. This should give you enough travel for wide open throttle operation without anything binding.
Here's the lever arm shown in its most rearward position as it should appear at wide open throttle.
After cycling the linkage several times, it will settle into proper location. Now you can go back and tighten the two 5/16-inch I.D. locking collars using a small 3/32-inch Allen wrench. If anything binds, loosen and adjust just one item at a time to see if that fixes the problem.
