Linkage Plate

[jhirasak]

Because my carbs face in the opposite directions between the drivers side and the passenger side, the cross link must be placed on a diagonal between the the two sets of throttle shafts. This geometry results in slightly different opening rates between the two banks of carbs depending upon throttle shaft angles. If all of my carbs were mounted in the same direction and all the throttle shafts rotated in the same direction, I would agree that a single cross link would work as good as a center bellcrank. Perhaps I am missing something? Suggestions for modification of a single cross link mechanism would be appreciated.

[Rick Parker]

I believe Cushman Motorsports has the correct linkage pieces to allow the single link to work. The arms have to be shorter on one bank than the other because of how they are clocked. Something similar to this was on the GT40's??

[bobcowan]

Originally Posted by Rick Parker

I believe Cushman Motorsports has the correct linkage pieces to allow the single link to work. The arms have to be shorter on one bank than the other because of how they are clocked. Something similar to this was on the GT40's??

You're trying to rotate two shafts at exactly the same rate. If one arm is shorter than the other, that will never happen, no matter how they are clocked. The throttle arms must be exactly the same length from the center line of the throttle shaft to the pivot point of the actuating rod.

Take a look at a progressive throttle linkage - like for two 4 barrel carbs on a common plenum manifold. You make them progressive by changing the pivot points so they are not equal. If you set up 4 Webers (or EFI throttle bodies) like that on an IR manifold, it will never run right. Your balance will only be correct at idle.

You do have to pay attention to how they are clocked. You are changing a linear motion tgo a rotational motion. Both pivot points need to move through the same arc in relation to the actuating rod. Ideally, at 50% throttle opening, you should be able to draw a line through the center of the cross shaft and intersect the center of both throttle shafts. That doesn't always work, but as long as each pivot point is at the same point in the arc at the same time, it works.

I found that using off the shelf identical Inglese arms worked perfectly. With those arms and a double-D shaft, each arm has 4 mounting choices. That gives you 16 possible combinations. Only one of the 16 will work.

I had mine set up like this for years, and it was well balanced at all throttle openings.

I'v been working with my 8 stack EFI system for quite a few years now. One thing I learned early on was how important proper linkage set up really is. It has to be right, and it has to be very precise. If this basic step is not done the balance will never be correct, and no amount of tuning will ever get it to run the way it should. This is a tedious, time consuming, and often frustrating step; but it is vitally important you're going to get it right.

I found it was a lot easier to do this with the intake manifold assembled on the bench, rather than under the hood. Much easier to make measurements and adjustments that way. I also ended up making a few parts myself. I could have ordered a lot of them from Inglese, but that costs a lot more, and takes a lot more time. To get a new rod sent out takes at least a week. I can make one in about 30 minutes. And if it's not right, I can just make another one.

[Earl J]

Hi Rick and Bob,

I have that very linkage from the GT40. The Pivot points from the center bell crank are not centered. They are actually off set. The reasoning that was explained to me was to compensate and balance the butterfly position from idle to WOT (wide open throttle). This sits on a E&F FoMoCo C6AE (same as the C6FE GT40) manifold. See pics. This linkage doesn’t apply to pmangelos original post question as his carburetor orientation is different (mirrored carb arrangement, fuel line on the outside and throttle arms pull in differ rent directions).

On a different note I wasn’t happy with the throttle arms from Jay Cushman of Cushman Competition.

Back to pmangelos post question. I fabricated a chunk of steel with a few (3) mounting positions for the center bell crank. My “pull” rod runs up the center. In the photos I removed the #7/8 carb to expose my linkage. The chunk of steel has a pointer stamped on it. It points to a red mark on the #2 flange. It’s just a visual to let me know if my chunk of steel bell crank pivot mount has moved due to vibration (which it won’t because it is keyed to the cast post on the manifold).

As for a return spring, the springs on the carbs are NOT enough. You will need a true return spring! (I run a double return and a third off the back of #4) I made a special cross bar to curve around the distributor and still be far enough away from the bell crank to so the spring functions as it should. I even drilled and tapped it for a spare 10/32 threaded Heim joint can hang from it for the front mount of the spring. It was made from a billet aluminum Summit Holley carb regulator bracket. Check the photo for the remaining piece.

Anyway this is my contribution to this post. Sorry it’s so long but pictures don’t lie and I’m big on visual. Cross my fingers, let’s see if the pics load.

Cheers,
Earl J

[Earl J]

I'm doing this totally wrong. I don't know how to post multiple pics in one reply, so my appologies to the moderator for making single photo posts.
~Earl J

[Earl J]

Here's the linkage with the #7-8 carb removed:

[Earl J]

Here's the "chunk of steel" bell crank pivot:

[Earl J]

Here's the Return Spring bracket and remaining piece I cut it from. It is mounted to the most forward studs of each back between the carbs and it curves around the distributor and is far enough away to allow the springs to work.:

[Earl J]

View from above. Sorry for so many photos. The forum moderator can group the posts and images to correct my error.
~Earl J

[bobcowan]

The central bell crank connects to pedal to the carb linkage. The bell crank will not - if installed correctly - change the relationship between on carb and another. it will have no effect at all on balance.

You can easily change the pedal sensitivity (to a point) by changing the ratio there. You can also move the arms around to be more or less than 90* to fit a custom application. You can even move the pivot point around a little bit to make the throttle actuation slightly progressive. Less sensitive at lower throttle opening, and more sensitive at the upper ranges.

The kind of bell crank you have is probably the best, as you can make a lot of adjustments between the carbs and the pedal to get the exact right arc of movement that you're after.

[*13*]

The best setup I've seen is to have the carbs all facing the same direction & run linkage between the banks. Sourcing that manifold may prove difficult. I run a center bell crank & it has uneven arms. Not perfect, but if you set it properly I performs well. Running the stock linkage that most provide in a kit is much worse. Do your vacuum readings just off idle & at about 2500 & you'll see how far out of whack things get.

[Rick Parker]

[IMG][/IMG]

Pivot point had to be relocated about .500"-.750"

Arms are same length, & syncronized throughough range.

[*13*]

I need to do this to mine at some point. I can't for the life of me figure out why they would put a pivot point that wasn't centered.

[jhirasak]

To solve my problem of syncronization between the two banks of carbs, I recently purchased a center bellcrank with a needle pivot bearing from Inglese and mounted it on a machined 1/4 inch aluminum plate located between my carbs. Since my Webers are mounted in a mirror symmetric arrangement, they now stay within one unit of flow as measured by my Unisync from idle (1000 rpms) to 3000 rpms. For anyone who is using a single diagonal crosslink between the two banks of carbs arranged in a mirror symmetric configuration and having trouble with syncronization or transition response problems, I would highly recommend them to change to a center bellcrank arrangement.

[*13*]

Nice solution!

[big deuce]

That looks great, awsome work!

[Caprimaniac]

Yea, Jirasak... Great solution. I do have the bellcrank mounted on a pole in the manifold floor. However... the pole is not straight, not even centered. I'll move it to a plate like that...

[Caprimaniac]

Well- back again after 7 years..... Returning to Webers on my street car and letting the EFI go on the racecar. Made the bracketry as Jirasak above today. Need to make a New throttle wire bracket, and then comes the vacuum lines for Power brakes.

Rediscovered the thread last night searching for better linkage solutions...