What happens in those carbs of ours, airflow/fuel wise?
starting from engine-end we look into the carb
and first we see the pilot outlet
witch lets in fuel regulated by the pilot screw
right behind it (not visible on photo)
we find the bypass ports
Next are (left) the outlet controlled by the main nozzle
and (right) the outlet controlled by the jetneedle attached to the vacuum piston.
So what causes the fuel to flow into this space and on into the
Well, because the diameter of the passage through the carburetor
lessens right here, constrictting the passage of air; the venturi effect
comes into play
and causes fuel to be sucked into the venturi.
Now how is this vacuum thing working for the vacuum piston/needle.
Take a look at the piston/(and needle)
or seen from 'inside' the vacuum chamber, through the piston anddown into the venturi (blue light inside venturi):
so here we have tree holes to be sucked at !
yeah, but no fuel or anything in there to be venturi-sucked out through those holes, you might say?
Oh yes; air
will be sucked out, in effect creating a vacuum inside
the (surprise) vacuum chamber
Take a look here:
DARK blue is air being sucked out through the holes in the piston
Yellow is the vacuum thus created.
Note the small air passage right under the space
to witch the white plastic thingie forms the top 'lid'.
This drawing shows the situation with the trottle valve (right side)
1/4 to 1/2 open.
(PS: LIGHT blue is fuel)
What happens is that when you open the trottle more and more
---> more and more air sucks--->more vacuum--->
piston is moved upwards because of the vacuum--->
more air AND fuel is lead into engine (jet needle, controlling
rate is attached to piston..remember)
(for more on what
, please look
at this, and the pages following
Now back to that tiny air passage at the top of the
vacuum chamber (here back-lit through the pastic thingie
as you can deduct from the drawing shown above this passage is
needed to 'vent' the space created by the hollow piston 'rod'
and the tube in witch it slides (also shown here back lit through
so yes; you are in trouble if your plastic thingies are not intact.
Not only will the air passing through a hole in the plastic,
lower the vacuum in the chamber, and thereby in effect
lower the position of the piston (lowering the intake of fuel/air)
but 'false' air will enter through
hole--> vacuum chamber--> holes in piston--> venturi-->engine
Still don't believe me ?!
For the sake of investegating this facinating subject
I put my health at risk and lit up a cigaret
..grabbed a vacuum chamber with a holed plastic top
..inserted a piston/needle (a bit of tape to make up for
slack in piston/side of chamber)
..and blew smoke into the hole
yes indeed; the smoke passed
hole in plastic--> vacuum chamber--> holes in piston-->
(and would pass on to; venturi-->engine)
Conclusion: replace chambers with defect plastic tops.
BTW another thing came up:
The plastic thingie does in fact act as a
upper-level-stop for the pistons.
Not only are traces of the piston rod/tube clearly visible at the underside of the plastic:
but I did measure vacuum chamber-tube/piston-rod/
piston etc. to confirm this.