A question for the Zen masters

[Joe Flanagan]

I rebuilt my carburetor some years ago and set it on the shelf wrapped in plastic for the big day when I could actually install it. I put it in the other day but before I did, I double checked everything. I was reading in the manual about how these things work and there was a section about a little device called the step up rod (or step up piston). It's intended to open up and give you an extra squirt of gas when you're accelerating suddenly.

When the engine is running, the vacuum from the intake manifold pulls downward on this little rod, holding it in the closed position. But when you stomp on the accelerator, the vacuum in the manifold collapses, which releases the tension on the step up rod and allows it to rise to the open position, thus letting in a burst of gasoline.

Here's my question: How can the vacuum in the intake manifold collapse? It seems to me that the vacuum would increase when you accelerate. I take Chrysler Corp's word for it that that is actually what happens, but I can't quite grasp the whole collapsing thing.

Not that it matters. It's just that the grasshopper wants to know.

[Don Coatney]

Here's my question: How can the vacuum in the intake manifold collapse? It seems to me that the vacuum would increase when you accelerate. I take Chrysler Corp's word for it that that is actually what happens, but I can't quite grasp the whole collapsing thing.

Not that it matters. It's just that the grasshopper wants to know.

Joe;

The carburetor throttle butterfly is what regulates the manifold vacuum. With the butterfly closed the manifold is mostly sealed from the atmosphere. When the butterfly opens atmospheric pressure is introduced into the manifold collapsing the vacuum. This is why vacuum windshield wipers dont work under heavy acceleration.

[Plymouthy Adams]

the shooter (accel pump) should be spring operated and allowed to shoot fuel through mechanical travel dependent on the throttle position...with an engine is not running, the shooter will still supply gas thus ruling out your vacuum theory..the stronger spring on the throttle return is the resetting factor on the shooter.. it is for the transisition from the idle to the run circuits this is needed and of course any travel left in the throttle will continue to displace on acceleration...

WOT acceleration eliminates vacuum..your maximum vacuum reading will always be in decel with throttle closed...your engine is not sucking enough volume through the now enlarged opening in the manifold (throttle plate) and as the engine comes up to speed in relation to the throttle position so will the vacuum

[greg g]

Tim I believe joe is speaking of the circuit that is sometimes referred to as the power circuit or jet. And in the carter applications it is regulated by engine vacuum. The acc pump is the mechanical device that starts the process but after that initial squirt, the vacuum operated power circuit take over to enriche the fuel charge and continue the rpm increase. As you look at the Typical carter single with the float bowl toward you, it is located o the left side of the throttle bore It comprises a screw type retaining device a small flat bar, a spring in the center, and a thin brass rod suspended from the bar. This thin rod goes into the jet. The spring pushes the rod out of the jet opening it. High vacuum at idle and cruise, pulls against the spring keeping the rod in the jet blocking the gas flow. When vacuum is lowered, the spring raises the needle releasing fuel into the circuit, to meet the power need until, the throttle position is stablilized against the current RPM need therby re establiching the vacuum signal. As you clean and service the carb, you should be able to freely press the spring down seating the needle against the spring tension and a bit of friction. the assembly should move up and down smoothly.

There was a discussion a day or so again, regarding assuring thea the vaccum passage between the carb and the top of the intake manifold mounting surface was esstablished and aligned to the vacuum port inthe base of the carb. If that signal is not picked up through the carb, the valve will remain open providing a to rich mixture, and negatively effecting fuel economy, as the jet will be wide open via the spring at all runing conditions.

As for the vacuum science, you would need to conduct some research into those two guys, Bernoulli and Archemedis. There might have been another guy involved also

Edited December 24, 2009 by greg g

[Plymouthy Adams]

I was thinking he was asking about the accelerator pump...the step up enrichment system is as you say..loss of vacuum provides the extra fuel needed to get the engine up to speed to match the demand (load) or speed (throttle position)..and as mentioned earlier..when the speed of the engine is enough to overcome the atmospheric pressure the throtttle opening has created (load induced also if you will) then the step up is returned to closed position..

[Don Coatney]

Actually below is his question.

Here's my question: How can the vacuum in the intake manifold collapse?

[Plymouthy Adams]

simple, though induction of atmospheric pressure disportionate to the speed of the engine..(WOT)

[Dodgeb4ya]

I rebuilt my carburetor some years ago and set it on the shelf wrapped in plastic for the big day when I could actually install it. I put it in the other day but before I did, I double checked everything. I was reading in the manual about how these things work and there was a section about a little device called the step up rod (or step up piston). It's intended to open up and give you an extra squirt of gas when you're accelerating suddenly.

When the engine is running, the vacuum from the intake manifold pulls downward on this little rod, holding it in the closed position. But when you stomp on the accelerator, the vacuum in the manifold collapses, which releases the tension on the step up rod and allows it to rise to the open position, thus letting in a burst of gasoline.

Here's my question: How can the vacuum in the intake manifold collapse? It seems to me that the vacuum would increase when you accelerate. I take Chrysler Corp's word for it that that is actually what happens, but I can't quite grasp the whole collapsing thing.

Not that it matters. It's just that the grasshopper wants to know.

Here is a pic of the insides of the Carter 1BBL.

[Joe Flanagan]

Greg described the device I was referring to. Thanks for all the answers. It's another way to look at what's going on inside the engine, which I sometimes don't understand.

As for the vacuum science, you would need to conduct some research into those two guys, Bernoulli and Archemedis. There might have been another guy involved also

I believe the other guy was named Freddy.