In this article I would like to address dual carbs and linkage. To this day, I still get customers that refuse to run dual carbs because they say they always go out of sync. The reason is usually because of the linkage. In the old days, we had twist linkage with a post in the middle and two rods that go out to the carbs. If you have ever had this type of linkage you know one rod is longer than the other and you usually had to bend it to get to the carb. Keep in mind these air cooled motors grow width wise with heat. With that type of linkage you would sync your carbs with the motor cold and find that when the motor was warm they were out of sync. Down linkage cures this problem when set up right. (I have to note here that dual carbs that open the same direction (left to right) ie: Kadrons, and have a single bar that goes from carb to carb are ok.) As a single bar opens the carbs the same amount even if the motor gets warm. On a drag car twist linkage is fine because we don’t care if the carbs are out of sync at an idle or partial throttle, all we care about is that they are both all the way open at full throttle.
So why does down linkage work better? If you look at the down linkage it has a bar that goes from carb to carb. This allows for expansion of the motor. The linkage that comes off that bar to the carb should push down the same no mater how much the motor grows. Porsche had this style linkage early on and when set up right it is the way to go.
How do we set it up right? Most kits come with a bar that is too long. They do this because most motors are different widths. You will need to cut the cross bar to size. What I do is set up the carbs making sure the nuts that mount the carb to the manifold and manifold to head are tight. Then mount your linkage bases to the top of the carb and snug on the nuts or bolts. Now, screw in one pivot end with the jam nut all the way either in the bar or the linkage base depending on the type of down linkage you have. Now place the bar acoss to the other carb. Set your pivot into the base with its jam nut. Lay the linkage on top of that and scribe a line on your cross bar. This is the amount you will need to take off. Lets just say it’s and inch. Whatever the measurement is, I take it and divide by two and take that amount off of each end (In this case ½ inch off each side). Why? A lot of the time the cross bar ends only have threads so far down. By taking a little off each end I usually don’t run out of thread. Before you thread the ends on, slide your down rod arms and cable arm on the bar. Next, thread both ends in the bar with their jam nuts. Remove one side linkage base and pull it up enough to get the bar end in. Don’t forget your springs that center the rod. Remount the linkage base to the carb. Now we should be looking at two carbs with a bar going across. Now, grab the bar - we want a little play here side to side to allow for the motor to contract a little (about .030 play). Lock down your jam nuts and install your down rods. I usually thread the hiem ends in all the way at first leaving the jam nuts loose. Make sure you have the left hand thread end both on the top or the bottom. We don’t want one at the top and one at the bottom. Now thread it into cross bar arm. The important part: before you lock down the arm on the cross bar, slide it over to get the down rod as straight up and down as you can. If one down bar is at a different angle than the other, the carbs will not open the same. Spend some time here, we want both down rods to be at the same angle. I find it is not uncommon to have to slide the arm over the jam nut. With that said you have to tighten the jam nut hex to match the hex of the cross rod. If you look from the side of the motor you won’t be able to get them perfectly straight up and down, but it’s important to have them the same angle. If they’re not the same angle you will have to use a spacer from the down rod to the carb. On most dual carb set ups the 3 and 4 side is farther back than the 1 and 2 side. So it only makes sense that the spacer be longer on the 3, 4 side that goes from the down rod to the carb. These spacers come with your kit. If you don’t have them you will need to try and buy some or make some.
Now you’re ready to start the motor and sync the carbs. Once you sync them, lock down your jam nuts on your down rods. You’re good to go!
If you’re removing your carbs to get the motor out here is what I do; I take the down rod off the top at the cross bar arm. Take my fuel line off and remove the two nuts that hold the manifold to the head. Pull the whole carb and manifold off the car. Don’t lose the spring that is in the end of the cross bar. Put them back on in the reverse order and you will find that you’re either right on with the sync or damn close.
Last note here, I have found the bushing wear out over time for the cross bar. On some kits you can now get bearings instead of a bushing- A great way to go.