Paul,
Very good point!
A is more compact than a round opening sliding over  a round opening.

Although B is shorter than A is doesn't give sufficient room for the
bearings spread that I wanted - but more importantly the opening is far
less
linier, in that it opens to the widest part (centre) in a linier fashion (
incrementally), but then the 'additional- after centre' area starts to
decrease ( incrementally) until it reaches the back of the opening.

This is more difficult to match fuel flow to, and is what I refer to as
less
linier opening.

The half round slide over a round opening gives the shortest slide with
the
most linier opening.  The only thing better than this is a square slide
over
a square opening. Next best is a round hole over a round opening ( given
it
will take up more room), because the round hole in the slide actually
reduces any surrounding space to zero as the opening is fully open (WOT).

That's the best I can explain it Paul.
George ( down under)


The last thing is what you want. A round hole in the slide with a round
opening. An aircraft engine spends over 90% to 95% of the time at WOT
and that gives the smoothest runner walls when WOT.

Paul Lamar ...No rotor no motor.

Paul,
I accept your argument, you may well be right, especially considering the
thicker slide that I'm using - who knows what turbulence is involved. The
problem is that I wouldn't be able to use either the round needle or the
flat ground needle as the needle needs to recess into a jet at the bottom of
the round opening - can't do it with that configuration you suggest.

As usual the design is a trade-off considering what is available to us and
what the individual considers most important. As this design is in current
use with the Aero carb, I feel confident it will work well enough.

There is also an additional configuration that is available to us and that's
the opposite to A., which is even better than B - it has very little
surrounding area and you can still use a needle, but it would have to be
elongated at the top to accommodate bearings. The incremental opening would
also be greater at the critical low RPM levels ( i.e less control at idle
levels).
Sorry to be so boring!
George (down under)


Yes that is true about the needle and all the Mikuni and other similar carbs have
the same problem. I have a new appreciation of the problems carb have on tuned
manifolds after our recent bad experience with the Weber. I suspect most carb are
not used on tuned manifolds so you could be in for a surprise or two. You won't
know until you try it.

As far as linearity is concerned I doubt seriously you will notice the difference
among the various designs as the load on an aircraft engine with a fixed pitch
prop is highly non-linear.

If I were doing a single throat carb I would do it this way.
It is perfectly smooth inside when WOT. The most important configuration.

BTW why did you not use solid Teflon for the slide? It eliminates the needs
for bearings.

Paul Lamar ...No rotor no motor.

Paul,
Again, another good point !  I did think of this just as I sent the last
e-mail i.e. hold the needle and move the slide.
This leads to other configurations, like having the needle behind the slide
(say in another block of material). This could be used if you wished to
introduce a venturi just before to the needle. This could (like a particular
brand of slide carb) allow for a very thin slide - bearings could run on the
block attached /behind the slide.

There are some interesting designs out there.

I notice however there is a tendency for slides to jam from fod or
misalignment or galling of 'like materials' like Aluminum, most likely
because of the tight tolerances - hence my use of bearings. My concern on
synthetic material (since you asked) was my concern of the tendency for
synthetics to expand at a higher rate than Al when heated- further
exacerbating the tendency ( in my mind) for jamming.

I envisage that one set of linear bearings on the slide side running in a V
channel in the body of the carb Or Vice Versa, would be an optimum design
IMHO. This would set the tolerances in all directions, however difficult for
me to produce in the home workshop.

I hope Doug is getting all this cross discussion on carbs.

George (down under)

FOD is NOT necessary for slide throttles to jam. When you allow the engine
to over run, a vacuum is created on the engine side of the slide.
The engine becomes a large and powerful vacuum pump.
The vacuum can be as much as 15 psi BELOW atmospheric pressure.

On a two inch hole that pressure difference will generate a force of 6.28 X 15 PSI or
94 pounds!!!!!!!! If the friction coef. between the materials is one
the slide will lock up and that 94 pounds is the force required to move it.

The coef. of Teflon is not one. It is .06 so the force to move the
slide under these conditions becomes 5.6 pounds. I guess you missed
this message a year or so ago when we were discussing this.

The temperature of the slide is always very low as the process of the fuel mixture
evaporating actually chills the intake manifold. Therefor temperature
expansion of the slide is not a problem. Also Teflon is soft enough
to accept small FOD such as sand.


Paul Lamar ...No rotor no motor.

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