I asked this question months ago but didn't get much response, possibly
because I didn't know the name of the part in question, so here goes
I have rebuilt my Stromberg carbs but am not sure that I have tuned
them properly. There are four adjustable screws on each carb. The
idle trimming screw, on the same side as the temperature compensator,
is not one I need to mess around with as I understand it. I leave it
turned in all the way. On the other side of the carb are three screws.
The lowest one is the fast idle screw which is pushed out by the choke
cam when the choke is activated at start-up. That I understand. The
middle and top screws are the ones I don't understand. In my owner's
handbook, the top screw is called the idling adjustment screw. It
is a screw with a lock nut and it projects through a fat lobe in the
body of the carb and contacts another lobe which is part of a lever
referred to in the Moss catalog as the "float chamber vent valve lever."
The remaining screw (spring loaded) projects through this same lobe on
the vent lever and contacts the throttle stop lever which holds the fast
idle screw. My handbook doesn't mention this screw at all, but the
Moss catalog calls it the "slow idle screw." The Moss catalog refers to
the top (locknut) screw as the "vent valve lever screw." So, which
of these two screws is really the idle adjustment screw? The
interesting thing is that turning either one has the same effect on
the throttle lever. The middle screw directly pushes down on the
throttle lever while the upper one pushes directly on the vent lever
which in turn pushes on the throttle lever. The difference is that
only the upper screw pivots the vent lever.
I have been adjusting the idle by turning the middle screw (which does
not change the position of the vent lever). My handbook says to set the
idle by turning the top lever (which does change the position of the
vent lever). But the handbook doesn't tell where to set the middle
By looking at the carb and the various levers, it looks to me as if I
should first set the top screw to some setting that puts the vent lever
in the right position and lock it down. Then I should set the middle
screw to the correct slow idle. Because this middle screw affects the
throttle lever, it looks as though I should then adjust the fast idle
screw to give the correct clearance to the choke cam.
So, can anybody give me some advice? Have I got things right or wrong?
And what does the float chamber vent valve do?
Thanks for any help,