How stuff works: air cut-off valve
Posted: Fri May 15, 2009 11:21 am
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
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
spring presses diaphragm with the small rod down
the rod keeps this valve open (shown here pressed down)
suppying air to slow running circuit)
..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:
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
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
spring presses diaphragm with the small rod down
the rod keeps this valve open (shown here pressed down)
suppying air to slow running circuit)
..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: