On my older multi carb bikes, the fuel bowls each have a drain
plug. In situ, it's easy to remove that plug and adapt a fitting
spigotted to an open ended clear plastic tube. Then with petcock
on and holding the tube next to the bowl gives you an outward
visual location of the fuel level that's inside the bowl. Some
float levels are spec'd this way. Regardless its a great way to
check for fuel delivery, float needle failure or float level
consistency between carbs.
Theory is one thing, Lucas realities another. Sometimes vertical
movement of the tube does not result in maintenance of a constant
fuel level in the tube, as one would expect. Sometimes starting
empty and opening the petcock does not produce the same end result
in levels, when repeated with the same carb. I attribute this to
the Spirit of Meniscus Stiction and Gas Cap Gravity effects.
Mike L. P. Eng.
60A,67E,59bug
----- Original Message -----
From: R. O. Lindsay <rolindsay@dgrc.com>
To: Max Heim <mvheim@studiolimage.com>
Cc: MG List <mgs@autox.team.net>
Sent: March 7, 2001 10:10 AM
Subject: SU Adjustments (LONG!) was: Float Height Issue
> 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.
>
> (2) The air/fuel mixture adjustment nut: This adjustment raises
or
> lowers the position of the jet in the venturi bridge. It
also raises
> or lowers the mixture all across the spectrum. That is, a
lowered
> jet will increase the mixture at all engine speeds, not
because it
> lowers the fuel level in the jet, but because it moves the
jet with
> respect to the needle. The needle is tapered so the annulus
(is
> that a Bevis and Butthead word?) between the needle and the
> jet is LARGER when the jet is lowered. Therefore, MORE fuel
> is supplied into the constant air stream. Voila! Richer
mixture.
>
> (3) The profile of the needle: The profile of the needle is
such that
> as the air piston raises, in response to greater depression
caused
> by the opening butterfly, that the amount of fuel metered
> increases in proportion to the increase in airflow.
Therefore, the
> air/fuel mixture remains <roughly> constant. When this
assumption
> is violated, for instance, by an un-centralized jet or a
bent or
> abraded needle, the mixture CHANGES as the air piston rises
> causing stumbling or "flat spots" as the mixture deviates
from
> optimum.
>
> Therefore, the fuel level in the float bowl HAS to be right,
the
> air/fuel mixture adjustment needs to be right and the needle
must
> be undamaged and in the center of the jet for the HS4 to work
> properly. These carbs are stunningly simple machine but they
> DO have to be adjusted correctly to perform at their peak. But
> you knew that...
>
> Best regards on Wednesday,
>
> Rick Lindsay
> Diamond Geoscience Research
> 5727 S. Lewis Ave., Tulsa, OK
> Voice: +1 918-747-3456
> Fax: +1 918-747-8599
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