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.
>
> on 3/7/01 7:10 AM, R. O. Lindsay at rolindsay@dgrc.com wrote:
>
> > Max Heim wrote:
> >
> >> Are you sure the carbs are not original, or just the floats? The
original
> >> style floats are NLA, so everyone winds up using the new style. You can
> >> still set them to 1/8" by using extra washers under the valve.
> >>
> >> Anyway, this shouldn't really affect richness. You need to adjust the
big
> >> nuts under the carb bodies for that.
> >
> > This topic deserves a little chat.
> >
> > We are talking about three major adjustment ideas here -- all
> > interactive: (1) The float level adjustment, (2) The air/fuel mixture
> > adjustment, and (3) The profile of the needle, although not part of
> > Max's original note. And this does not even touch balancing
> > multiple carb setups.
> > I am not an expert on carburators but I am a physicist and I have
> > to understand HOW they work to be contented. Beyond that is the
> > concept of proper tuning and that is a mixture (forgive the pun) of
> > science and magic (as fluids don't move exactly as we model them).
> >
> > (1) The float level adjustment: The specifications state that the float
> > is adjusted so that there is a certain clearance (1/8" in this case)
> > between the nub on the top of the float and a line parallel with the
> > top of the chamber (or similar words). However, that is NOT
> > what is being adjusted. Rather, it is simply a convenient place to
> > make a measurement. What matters, and this is really ALL that
> > matters, is the height of the fuel in the bowl as THAT is what sets
> > the level of the fuel within the jet. Think about it; The fluid in the
> > float bowl seeks it's own level with the fluid in the jet. The two
> > are connected directly. Float level is probably the most under-
> > valued setting on the carb!
> > Adjusting a new float, such as one of the new all-plastic jobs,
> > to 1/8" below the rim does NOT guarantee that the fuel is at the
> > same level as it would be with an original float because the
> > geometry of the float is different. It simply says that there is
> > similar clearance. If the float is designed differently, and it is, the
> > clearance is necessarily wrong. The assumption is that the plastic
> > float is already correct and that the only reason for the old-style
> > float adjustment is that the metal lever arm is easily bent out of
> > adjustment! I think this assumption is too simple minded (kinda
> > like the idea of 'universal gaskets' or that the '25D4 distributors
> > with unknown advance curves are okay). Yes, they will work
> > but who knows if the setup is optimal?!
> > Here's what I plan to do for my car: I have one, good, old-
> > style float for my HS4s. I am going to install it correctly using
> > the 1/8" gap setting, which is correct for this float, and measure
> > the height of the fuel level in the jet! This is not hard to do with
> > the air piston removed and the jet lowered via the choke. From
> > there I can then install the new floats and re-measure the fuel
> > levels. If adjustments are needed to match the 'correct' level,
> > the added-shims suggestion is viable.
> >
> >[snip]
>
>
> > Rick Lindsay
> > Diamond Geoscience Research
> > 5727 S. Lewis Ave., Tulsa, OK
> > Voice: +1 918-747-3456
> > Fax: +1 918-747-8599
>
> --
>
> 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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