1929 Cad - What is an 'Automatic throttle' plate?

[m-mman]

I am trying to figure out the operational theory of the Johnson carb on my 1929 Cadillac. As if any mere mortal could possibly do so.

 

Above the linkage operated throttle plate is something called the 'automatic throttle'. (arrow) it is a spring loaded plate that is half solid metal and half swiss cheesed with holes.  It is kept closed by a spring that is adjusted by using a weighted tool (shown at bottom) The tool is of course unavailable and nobody seems to have any specs about how much weight the spring should be adjusted to equalize. . . . 

 

My question is, what the heck is this automatic throttle thing supposed to do?? 

 

Why would you even have or need a second (spring loaded) throttle plate directly above the linkage controlled plate???

[hidden_hunter]

I assume it’s a valve that’s dampened or something which would adjust air/fuel ratio. 

[carbking]

It was spring-dampened. Its function was to increase A/F velocity when the throttle was quickly opened. Think of the "automatic throttle plate" like the auxiliary air valve on a modern Carter AFB secondary. The driver can open the throttles wide open with the footfeed, but the secondary will not open completely until the engine had increased speed to where it could ingest the additional air/fuel. The "automatic throttle plate" works exactly the same way, only opening (assuming correct adjustment) as quickly as the engine can use the air/fuel. The restricted opening keeps the air/fuel stream velocity at the desired level. Air velocity is one of the most important elements in carburetor metering.

 

And I still don't like the carburetor!

 

As far as adjustment: try rotating the adjuster until the automatic throttle plate just touches closed with no tension; then rotate another 1/2 turn (180 degrees) and tighten the lock screw.

 

Jon.

Edited March 29, 2020 by carbking (see edit history)

[m-mman]

1 hour ago, carbking said:

And I still don't like the carburetor!

 

Neither do I  😞 

But I understand there are no simple substitutions.  Although some have converted to downdraft. . . . 

 

 

[edinmass]

It was set correctly last year! 🤔

 

Hope all is well, stay safe. Ed

[m-mman]

Hi Ed,

Digging the car out for the now cancelled show season and after I fixed the fuel leak. (how can a tight pipe plug even allow gas to pass?) it was back to its old symptoms of acting like the throttle was still closed, even with the pedal down. 

 

Being stuck at home makes you dig back into things.

I loosened it up such that the spring barely closes it and it now accelerates more consistently and was able to hit 45 regularly. 🙂 

 

It is inspiring more confidence now. As you say not a car for beginners, but since I got it, I gotta fix it. 

 

[Mikefit]

Looks somewhat like Ford's vv carburetor of the 60s