Sweet, sweet, 40's...Center Cable Linkage ?'s

[brianstj]

I got my DCOE's all mounted up - loving how they look! I got the car fired up just for a minute, but something was obviously out of whack - and then the car wouldn't shut off, but that's another story!

So I got the cable linkage that pulls from the center from the bottom side. I love the simplicity of this setup, but I am wondering about the adjustability. I initially set the synchronizing screw to where both carbs would hit full throttle at the same time. When I fired it up I couldn't get it to run below 2500 RPM even with the idle speed screw turned all the way out - something obviously not right. So in looking in to the linkage, I found that the "slave" carb was opened up quite a bit at "closed" throttle when I adjusted the synchronizer screw for hitting full throttle at the same time.

So I readjusted to make the carbs equal at closed throttle. When I operated the linkage by hand the left carb would hit the stop at full throttle, but then the right hand "slave" carb could still be pushed further to its hard stop - leaving up to about a 1/16" gap between the lever and synchronizing screw.

Is there a way to adjust this linkage to make sure both carbs hit closed and full throttle at the same time? Or do I just sync 'em up at idle and not worry about it?

[bingbong]

you should have a balancer on your lever that you can adjust some how. I'm not sure as I don't touch my 40's because they scare me!

[Scott Sislane]

nice catch can.

[Tommy]

If I remember correctly there should be a throttle stop screw on each carb. It sounds that your "slave" carb allows the butterfly open too much. If they are in sync at idle they should be in sync at WOT too. Butterfly can open past 90 degrees but the stop should be adjusted so that it wont.

If I'm thinking more, the stop screw is probably removed from the slave to prevent stress on shafts and linkage. Then it should just follow the master.

Conclusion: sync at close/idle and don't worry about WOT.

Tommy

[MollysDad]

... if the two carbs are synced when the butterflies are closed (which would probably be well below comfortable idle), and if they both open at the same moment, and if they're a matched pair, then they should still be in sync at full throttle, as the butterflies would rotate the same distance when you pressed the throttle.

First, make sure that the idle speed adjustment screw isn't causing the carbs to open at all. Then get 'er started and adjust the mixture until you've got the smoothest idle. Then adjust the idle to a comfortable rpm. Then use a synchronizer to make sure that the two carbs are pulling the same amount of air at idle (where the progression circuit has just started to come in), at 1700 rpm (where the progression circuit is turning the job over to the main circuit), and at 3000 rpm (where the car is running on the mains only). If you can do all that, then the carbs should pull evenly all the way to full throttle.

[TobyB]

as the butterflies would rotate the same distance when you pressed the throttle.

Aha- David, I think you found it. Are the carbs exactly aligned with each other?

If they're not, the center- pull won't necessarily open the 2 at the same rate.

I use parts of that linkage- and a plenum intake arrangement that makes

sure the 2 can't wiggle independently. And I do use the isolating mounts,

but more to adjust for the misalignment of the carbs than for vibration...

That said, as long as the first 20 degrees of rotation agree, after that it's not

particularly critical, as Tommy says.

Then use a synchronizer to make sure that the two carbs are pulling the same amount of air at idle (where the progression circuit has just started to come in), at 1700 rpm (where the progression circuit is turning the job over to the main circuit), and at 3000 rpm (where the car is running on the mains only). If you can do all that, then the carbs should pull evenly all the way to full throttle.

Err, I have to disagree a little bit here- on the carbs that I've been using,

the idle screw is the only thing feeding mixture when the throttle is closed.

I looked. You can, too- just pull the drilling cover off the progressions, and

you can see the edge of the throttle plate.

On my setups, the plate is completely past them at idle.

And while the switchover points from progression to mains is gradual, as you

note, it's not all that rpm- dependant- it's more a function of airflow

and throttle position. If I wind the stop in on mine to get a 3k high idle,

it's running on just the first progression drilling!

But if I'm lugging it pulling out of a corner at 3k, I have the throttle wide open-

and that's all mains...

just what I've found...

t

[brianstj]

I was thinking maybe slight carb misalignment too, making them open at slightly different rates. I have the new Ireland 1 piece manifold, but that doesn't necessarily guarantee perfect carb alignment. But assuming a carb misalignment would also assume that the hard stops on the carbs are machined in the perfect position and that the linkage levers are also set up perfect to hit the stops at the exact same rotation point. Could be asking a lot to think that everything is that well lined up. It seems like the only way to really tell for sure would be to mock up a little degree wheel to see if each shaft actually rotates the same amount, or if there is in fact some misalignment causing them to open at different rates. Hmmmmmmm.....

I think what I will do at this point is get them all synced up at the "closed" position. It makes sense to me that a difference in opening rates would be less pronounced at WOT. I am thinking it might not be a good idea to start bending up linkage based on trusting that all the little stop tabs are bent and placed perfect. Maybe if I run in to any noticeable driving issues I will think a little more about how to verify if both carb shafts are rotating exactly the same amount...