Max, methinks the float needle controls a potentially larger
volume flow rate than the carbs suck in, and somewhat more
continuously than the on/off step function you envision. Secondly,
some of the carb throat (or butterfly squeeze) vacuum must be used
to increase the potential energy of the fluid - i.e. the higher
you gotta lift the gas the more vacuum needed. Ergo, float level
has got to affect the mixture. Most manuals have very precise
specs and instructions to be precise. Also most carbs do not have
the SU mixture nuts, which if used to for compensate for wrong
float level at idle will then leave you out of spec mixture at mid
and full throttle.
Mike L
60A,67E,59Bug
Interestingly, most bike carbs have adjustable needle height
adjustment (4 or 5 grooves), for altitude and exhaust mods.
----- Original Message -----
From: Max Heim <mvheim@studiolimage.com>
To: MG List <mgs@autox.team.net>
Sent: March 7, 2001 3:40 PM
Subject: Re: SU Adjustments (LONG!) was: Float Height Issue
> Very interesting. I have always thought of the float bowl as
containing a
> reserve of unpressurized fuel, which flowed into the carburetor
as allowed
> by the needle jet opening. In this view, as long as the level is
not so low
> as to run dry, or so full as to flood, it shouldn't matter where
the level
> is at precisely at any moment (it being generally in a state of
transition
> -- either filling or being depleted). The float level serves as
a trigger to
> regulate the influx of pressurized fuel from the pump -- when
the level gets
> too low, the valve is opened allowing more fuel to flow in; and
when the
> level is sufficiently high, fuel flow is cut off (momentarily).
>
> Now this is basically a static view of the situation. I realize
that in the
> real world, instantaneous demands for fuel based on throttle
position,
> mixture setting and needle profile may result in fuel starvation
if the
> level is set too low, even if that setting works at idle or at
part
> throttle. But once you have a level setting that is "high
enough" without
> resulting in flooding at low throttle settings, how does any
further, more
> precise adjustment of the float level make any operational
difference?
>
> It seems to me that for your hypothesis to be correct, you would
have to say
> that, for a given mixture adjustment, throttle position, and
needle, that
> the instantaneous state of the float valve and fuel level (open
and filling,
> closed and depleting, or in-between) makes a difference in the
fuel mixture
> at the carb throat. I don't see how that can be so (barring
starvation, of
> course). Given any fuel at all in the float chamber, the pipe
going to the
> jet will be filled, and the only thing that controls the flow
rate to the
> carb throat is the needle/jet annulus (I love that word!). Am I
missing
> something?
>
> I don't have an ax to grind, here, I'm just trying to understand
it...
>
> on 3/7/01 11:18 AM, Dave Munroe at dave@munroe.ca wrote:
>
> > Hey Max;
> >
> > Sorry to bust in here, but I have to tell you this: I had the
opportunity a
> > number of years ago to spend a day with noted tuner and engine
builder,
> > Jerry Branch, who, amongst other things, rents out his dyno
room to
> > motorcycle factory distributors to test after-market motor
accessories.
> > This involves, for example, installing, tuning for max.
performance, and
> > removing as many as a dozen or more exhaust systems for each
model they want
> > to test. With most of these bike having 4 carbs. and all that
entails, he
> > estimated that they spent 80% of their time installing,
removing, and
> > changing jets and needles, and 20% on the dyno.
> > He came up with a neat device to cut down on the number of
times they had to
> > go into the carbs to change jetting: he hooked up the float
bowl drain tubes
> > to a small header that led into the control room where a
vacuum/pressure
> > pump was controlled by a sensitive valve/switch through a VERY
large meter.
> > Over time he was able to develop a scale that very accurately
related small
> > elevation changes in the float bowl fuel level to % change up
or down in
> > jetting. This was so accurate that he could predict the jet
change required
> > almost without fail and limit the required adjustment to a one
time only
> > operation.
> >
> > So, it appears that, in the case of some carbs. at least,
float level does
> > affect mixture.
> >
> > So from my experience Rick Lindsay is not too far off the
point.
> >
> > Dave Munroe
> >
> > ----- Original Message -----
> > From: "Max Heim" <mvheim@studiolimage.com>
> > To: "MG List" <mgs@autox.team.net>
> > Sent: Wednesday, March 07, 2001 2:10 PM
> > Subject: Re: SU Adjustments (LONG!) was: Float Height Issue
> >
> >
> >> That was a good write-up, and I think your proposed
experiment would be
> > very
> >> valuable, but it still seems to me that the float bowl fuel
level does not
> >> directly affect mixture richness, in that it is modulated by
the needle. I
> >> mean, in extreme cases, float bowl fuel level can cause
starvation, or
> >> flooding, but in between those extremes, the amount of fuel
supplied to
> > the
> >> carburetors is modulated by jet/needle annulus, as you said.
Possibly the
> >> "mean" between the two extremes is a very small range,
however, and any
> >> insight your experiment gives into that would be useful.
> >>
>
> --
>
> Max Heim
> '66 MGB GHN3L76149
> If you're near Mountain View, CA,
> it's the red one with the silver bootlid.
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