Max;
We are pretty much talking here, I think, about the SU carb. A number of the
carbs on the bikes in question are from the same family as the SU, being of
the constant velocity type, and many have idle jets that bleed into the carb
throat through a very small passage, a virtual pinhole, and these are very
sensitive to the fuel level in the float bowl.
The HIF SU's have an internal passage that feeds the cold start enrichening
device which is always full of fuel, and is also float-level sensitive.
I think this discussion is valuable, as it really brings together various
levels of theoretical and practical experience, and we all will go away
richer for it. The SU is magnificent for its incredible range of
adjustability in its simplicity. I have marvelled at the chutzpah of the
Bros. Skinner to tackle what must have been a formidable engineering
challenge in their day to mass produce those multi-station needles to the
nearest thou. Just amazing.
Hoist a beer to those boys next time you pass the fridge!
Dave Munroe
----- Original Message -----
From: "Max Heim" <mvheim@studiolimage.com>
To: "MG List" <mgs@autox.team.net>
Sent: Wednesday, March 07, 2001 4:40 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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