Slide valves are quite effective for use where the valve is lubricated
by the fluid flowing in the system.  The fluid seals the slide due to
viscosity effects in the very small clearance gap between the slide and
the guide.  They require close tolerance machine work to be effective.
The larger the valve the more difficult the construction.

Have you considered cutting the opening in the slide in a teardrop shape
to make the idle adjustment less sensitive?

Mark LaPierre
Registerd Linux user No #267004


Yes we tried that and decided it was not worth it.
These are normally punched out with a CNC punch to keep the cost down.

http://www.rotaryeng.net/__slide-kit.jpg
<http://www.rotaryeng.net/slide-kit.jpg>


Paul Lamar

I don't know if this issue is worth discussing any more, but just for
argument sake I'd like to defend the ball-valve.

True ... a partially open ball valve creates "huge turbulence". Is this
bad? (It might even be a plus and serve to prevent a "wet" charge from
entering the combustion chamber).  Would ball-valve turbulence
necessarily be any more than that created by a slide-valve?

As for the statement "Tiny movement results in large flow percentage
change", that too is true.  But my mathematics (I will skip the calculus
here) tells me that with a slide valve the rate of increase in the open
area depends on the distance across a circular opening, which for small
openings is increasing very rapidly (some reciprocal square-root
relationship).  With a ball-valve the rate of increase of the area of
the opening is determined by the overlapping areas of two circles, and
that rate-of-increase calculates to be less than that of a slide-valve
(I think).

Also ... ball-valves work well under huge pressures, and the
pressure-drop on an intake manifold can never be more than about 15 psi,
so I suspect seal problems would not be a major issue.

My only experiences with slide-valves were in a paper-mill, where they
are used extensively for low-pressure applications on large pipes of
lightweight stainless. They are probably comparatively cheap to make in
SS, but I seem to remember that most of the slide-valves in use had a
big puddle on the floor beneath them; they leaked a lot.

The biggest negative I see for the slide-valve in use for throttle
purposes is that they take up quite a bit a space (lengthwise).

R Berglund

Yes they do but the throttle linkage is super simple.

BTW I just realized you get an accelerator pump for free
as one of the Holly float bowls has it built in. Many
aircraft carbs do not have accelerator pumps so you have to be
careful about shoving the throttle in too fast.

The Holly design for this is extremely ingenious and and simple. All
it does is add a little volume to the float bowl and momentarily
raises the fuel level to richen the mixture when the throttle is
jammed wide open.

We can also use the stock Holly jet collection which gives us a huge
amount of mixture options.

Paul Lamar


Paul,

How do you propose dealing with the leakage that occurs when the locking
screw that secures the needle/seat is loosened?  There's a gasket under
the head and another under the adjustment nut that seals the fuel from
leaking.  If you loosen it when there is pressure in the fuel system it
will leak with a vengeance.  Obviously, we wouldn't want that to happen.

Mark S.


We don't loosen that screw. We move the entire float bowl up and down which gives us a far larger range of mixtures than would be possible
by merely changing the needle valve seat height. Everything from dribbling fuel out of the float bowl (flooding it :)) to idle cut off.

Speaking of leaking there is enough meat in the Holly float bowl to have an NC mill put in an O-ring groove.

Paul Lamar