How stuff works: air cut-off valve

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octane
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How stuff works: air cut-off valve

#1

Post by octane »

The air cutoff valve....will cut off air....hence the name.-)
It cuts off air to the slow speed circuit
and it does that ONLY when the throttle is CLOSED.

In all other situations the valve is redundant.

Look into the carb (carb for cylinder 1, that is)
Please note the small hole with the blue circle
Image

That hole is on the INTAKE-side of the throttle butterfly.
The minute you close the throttle, the vacuum from the cylinder
can only 'suck' from two holes;
(the bypass ports (red circle) will be BEHIND the throttle butterfly)
1) the pilot inlet (yellow circle)
and
2) the small hole with the blue circle.

THAT, and that only, creates the vacuum that lifts the cut off valve diaphragm, by leading the vacuum from that hole up to the air cutoff valve.
(actually when it lifts, it closes the air supply to the slow running fuel system) and you get a richer fuel mixture on 'over-run'
(throttle closed) so the fuel mixture is rich enough to burn in the cylinders-----> no "after-burning" in exhaust.


Normal position:

under the diaphragm there's the rod
Image


spring presses diaphragm with the small rod down
Image


the rod keeps this valve open (shown here pressed down)
suppying air to slow running circuit)
Image

..when throttle is CLOSED the vacuum from that tiny hole shown above (blue circle) is lead to the top over the diaphragm...sucks it up---->
valve / air supply closes -----> richer mixture


Again:
..the air cutoff valve is there only to fix the back-fire problem.
..It does not influence the system when the throttle is open
(so no influence on mpg)
..IF the air cutoff valve diaphragm is broken it will ONLY influence
the 'throttle closed' "after-burner" problem

.
Here's the schematics:

Image

Image
Da' Supercharged Bulldog

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"A designer knows he has achieved perfection
not when there is nothing left to add
but when there is nothing left to take away"

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octane
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#2

Post by octane »

For the fun of it:
Let follow the vacuum from where it is created
to where it works it's stuff:

Throttle CLOSED (see from the intake-side)

Air is sucked out of the arrowed hole: engine STILL sucking
on decelleration

Image



..goes through the carb-body:

Image



...through the float-bowl gasket and on
to the little hose:

Image



...into the valve-'body' through this hole:

Image


...through the 'lid'
up into the cut-off valve top- vacuum chamber
above the diaghphram

Image



...seen here with all the parts:

Image



...the thus created vacuum lift the diaghphrame
and thereby the 'rod'

Image


...thereby closing off the air-supply (creating a richer mixture)
by closing the valve that was, beforehand because of the
spring, open
(seen here in the open position, screwdriver simulating
the rod under spring-pressure)

Image


...get it?

.-)


.
Da' Supercharged Bulldog

Image

"A designer knows he has achieved perfection
not when there is nothing left to add
but when there is nothing left to take away"

Antoine de Saint-Exupery
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Neil
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#3

Post by Neil »

Thanks Octane, that was well put together, I even understood it :-D
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#4

Post by rogue1000 »

Octane, that was excellent! Too bad I forget it five minutes after I read it! I'm waiting on a rebuild kit for the air shut-off valve and this was very helpful to my tiny brain.
Brian Crow
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octane
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#5

Post by octane »

Thanks guys.
rogue1000 wrote:... I'm waiting on a rebuild kit for the air shut-off valve ...
On rebuilding cut-off valve,
have a look here: CLICK
(..and scroll half-way down)
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#6

Post by AC/DC »

That is fantastic :twisted:
Stiff upper lip.
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