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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