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Old 12-21-2005, 06:13 PM   #1
meat popsicle OP
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Question Float Height Importance in Carb Tuning

http://www.factorypro.com/tech/float...procedure.html

I am surprised by this technical discussion on the influence of float height on carb performance by Factory Pro. Please check out the link above.

Here is a hook:
Quote:
Originally Posted by Factory Pro
...
Why adjust the float height?

Changing the float height changes the level of the fuel in the float bowl. The fuel height adjusts the full throttle, 2k-3k rpm and part throttle cruise, as in cruising around town, trying to be quiet... To give a scale of change, if the bike runs well when cold, but gets a bit sloppy when fully warmed up, lower the fuel level 1mm (i.e. go from 15mm to 16mm float height - remember the float measurement is "backwards").
...
I have never read anyone discuss this here.
Anyone?
More:

Quote:
Originally Posted by Factory Pro
...
CV Carbs: Too RICH at full throttle / low rpm (that's the most common in CV carbs)
Slide carbs (FCR's) - Float height controls 25% throttle at ALL rpms.

If the float height is too low (small float height measurement), bend the tab slightly to increase the height measurement. It's a 5:1 movement ratio.

CV: If the float sticks out further, down, into the float bowl, the carb will deliver less fuel (leaner), especially at low rpms and at cruise.
Slide carbs: Float height controls 25% throttle at all rpm's, even at redline.

You'd generally change the float height in 1mm increments when tuning. It will affect the topend slightly. Maybe 1.5mm leaner float height would require 1 size larger main jet to keep equal main jet fuel delivery.


CV Carbs: Too LEAN at full throttle / low rpm (not too common in CV carbs)
Slide carbs (FCR's) - Float height controls 25% throttle at ALL rpms.

If the float height is too LARGE, bend the tab slightly OUT to decrease the height measurement.

If the float sticks out further, UP, into the carb body, the carb will deliver MORE fuel (richer), especially at CV low rpms and at cruise.

It will affect the topend slightly. Maybe 1.5mm richer float height would require 1 size smaller main jet to keep equal main jet fuel delivery.

You'd generally change the float height in 1mm increments when tuning.

When you are done, a multi-cylinder must be all within .5mm (1/2mm) or .020" range of each other.
...
Is this something that should be set or only adjusted when a problem presents itself? With this modifier added to the others, the ocean is getting too deep for me; I feel like I'm drowning...
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Old 12-21-2005, 06:45 PM   #2
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I've never encountered that extensive a discussion - a wee bit overwhelming, particularly if you have ever tried, or had, to make float adjustments. My recommendation: don't mess with it unless you absolutely positively must. Float level adjustments are incredibly sensitive to imperceptible adjustments which make tracking and planning and actually effectuating adjustments difficult. In my experience, it is (fortunately) the rare time that one must adjust a float level. Once you do it, you'll probably join me in not wanting to chase that gremlin again.
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Old 12-21-2005, 06:56 PM   #3
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Float height is a great tool for fine tuning. Sometimes a 0.5mm or 1mm change can remove a slight surge or help you offset a slightly too-rich or too-lean condition without having to use shims.
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Old 12-21-2005, 07:21 PM   #4
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Thank you Catcus Dave for that very reasonable warning. I like the imagery: "Chasing the Gremlin". I consider myself warned.

But you potatoho, have posted next to nothing. You post has me imagining a fairly one-dimensional problem, yet we all know that is not the case! Aside from the macro-dynamics of intake and exhaust's impacts on one side of the stoichiometric equation,

and the interplay of the idle jet, needle jet (and jet needle) (and it's adjustment), and main jet (with the air jet modulating the previous paragraph) and settings (low speed idle screw and idle screw) in the usual carb adjustments that variably interplay in adjusting low, midrange, and the top end,

we now are discussing a further adjustment to the fuel half of the stoichiometry (which I was not aware of): the float height's impact on fuel delivery to the jets...

I think that this has past the third dimension and entered the fourth; one that is beyond my abilities to comprehend presently. And certainly one that is not facilitated by simplistic posts.

Perhaps someone would care to discuss why float height has such a non-linear effect upon fuel delivery, especially in a CV carb , and how this facet intertwines (lustfully so the boys at Factory Pro lead me to believe) with the other mignions that supply and relieve my engine.
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Old 12-21-2005, 08:31 PM   #5
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Quote:
Originally Posted by meat popsicle
Aside from the macro-dynamics of intake and exhaust's impacts on one side of the stoichiometric equation
Great Holy Jeebus Meat, are you trying to win the "I sound just like Hickson" award?
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Old 12-21-2005, 08:45 PM   #6
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Quote:
Originally Posted by ChrisC
Great Holy Jeebus Meat, are you trying to win the "I wish I sounded just like Hickson" award?
Fixed.

I was hoping that if I could spew forth enough multisyllabic words in obviously contrived and contorted phrases, perhaps dangerously confusing the subject he might feel obligated to sort me out.

Of course I know he is busy at the moment so I will try to be patient and enjoy the peanut gallery (and the few rational skeptics that show up).

All kidding aside, this float height factor really is news to me and I was hoping for some discussion. I was thinking that perhaps this variable may be the monkey wrench in many a tune up without ever being considered.
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Old 12-21-2005, 08:51 PM   #7
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I don't want anything to do with this thread... I'm not even here.
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Old 12-21-2005, 08:58 PM   #8
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Here's to you and your stoichiometry, Meat....

Liquid fuel is fed to the nozzle of
the carburetor venturi, and flows
due to the vacuum generated by the
air flowing past the venturi itself,
and from airflow pulsations generated
by the piston movement. The
calibrated jets placed upstream of
the spray nozzle itself control the
fuel flow reaching the spray nozzle.
Motorcycle carburetors are nearly
always of the needle type and have
a structural architecture as shown
in the accompanying illustrations.
The fuel arriving from the tank is
held inside a constant level float
chamber. The liquid pressure head
on the various jets is relatively constant.
The difference between the
float chamber fuel level and the level
that the fuel must be raised to
by the inducing vacuum remains
constant. The float chamber level is
kept constant by means of a fuel inlet
valve, actuated by a float that
follows free surface of the liquid in
the float chamber. When the float
chamber level drops, due the fuel
used by the engine, the float drops
and opens the valve, so that additional
fuel can flow from the tank.
The level of the fuel and float then
increases, and at a certain point,
closes the valve until the sequence
is repeated. The level in the float
chamber is therefore a calibration
element of the carburetor, since the
metered fuel delivery changes with
float level, and therefore affects the
mixture ratio. By having a high
float level, a greater fuel quantity is
delivered compared to the case with
a low float level, under all operating
conditions and for all of the carburetor's
circuits.
Adjustment of the
float chamber level is affected by
two elements: the weight of the
float (or of the floats) and the configuration
of the lever arm that connects
the float with the valve. By
installing a heavier float, the free
surface of the float chamber liquid
must rise before the float buoyancy
force balances the increased weight
making the float rise. The result will
be a higher float chamber level and
a richer delivered mixture under the
same conditions. On the contrary if
we install a lighter float, a lower liquid
level will cause sufficient
buoyant force to actuate the valve
and therefore the carburetor calibration
will become leaner. That is
why floats are classified according
to their weight (printed on them)
and calibration standards for their
position inside the float chamber
are prescribed in order to assure
correct operation. To modify the
float chamber level, if necessary and
when it's not possible to change the
Checking the position of the float inside the
float chamber is prescribed. According to different
carburetor models, the distance of the float
from the contact surface of the float chamber
needs to be measured float weight, in some cases it's possible
to change the angle of the lever
that operates the valve.
In this way, the float closes the valve
in advance (for a lower level) or
later (for a higher level) at equal
weight.
We must note, however, that too
low a level in the float chamber can
result in an insufficient liquid head
on the jets and therefore lead to the
risk of dangerous enleanment of the
delivered mixture.
This can occur when the fuel moves
inside the float chamber due to the
accelerations the vehicle undergoes.
In these cases (which mainly happen
on off-road motorcycles or on
the track, in the bends or under violent
braking), if the level is too low,
one of the jets leading to the carburetor's
circuits may be temporarily
exposed to air instead of liquid.
In some versions, special screen baffles
are applied near the jets.
These are called bottom traps and
their purpose is to maintain the
maximum liquid quantity around
the jets under all possible conditions.
A needle that closes on a seat,
which is inserted or screwed into
the carburetor's body, forms the
fuel valve. The needle is equipped
with a synthetic rubber element on
the tip.
This material is perfectly compatible
with normal commercial gasoline
but in the case of special fuels
such as those containing alcohol, it
is necessary to verify the compatibility
of the fuel and the seals in order
not to compromise the carburetor's
functionality.
Different versions of the needles are
equipped with a sprung tip in the
connection with the float, in order
to reduce the needle's vibration induced
by the motion of the liquid
in the float chamber and from the
motorcycle's movements.
The diameter of the needle valve is
a calibration element since it determines
the maximum fuel delivery
rate.
If the diameter is too small to accommodate
the fuel quantity that
the engine requires under certain
conditions (generally at full load)
the float chamber empties faster
than it can be replenished through
the needle valve! If this condition
should continue for some time, the
engine suffers from reduced fuel delivery
due to the fact that the level
in the float chamber is decreased
and therefore the carburation has
become too lean.
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Old 12-21-2005, 09:10 PM   #9
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You could have tried a little punctuation on that before you pasted and posted... ya' know.



























Still not here.
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Old 12-21-2005, 09:24 PM   #10
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Quote:
Originally Posted by ChrisC
Here's to you and your stoichiometry, Meat....

The level in the float
chamber is therefore a calibration
element of the carburetor, since the
metered fuel delivery changes with
float level, and therefore affects the
mixture ratio. By having a high
float level, a greater fuel quantity is
delivered compared to the case with
a low float level, under all operating
conditions and for all of the carburetor's
circuits.
The Factory Pro boys don't make it sound like that linear of a variable. In fact it sounds downright chaotic... I am starting to think there is some turbulence at work in there; perhaps variable fuel flow rates can significantly differ in their turbulent flow just like the intake air? Enough to matter?

Did I hear something? Sounded like... ah, nevermind.
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Old 12-21-2005, 10:24 PM   #11
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Hey Chris...

I just remembered where I had read your paste before... it's from the Dell'Orto "Carburetor basic principals" booklet. As soon as I saw the word "enleanment", that was the tip off.


Still not here...

Hey Meat... wana read some good carb stuff and not the usual basic bullshit? Just scroll down to Dell-Orto General Carburetor Manuals.
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Old 12-22-2005, 01:57 AM   #12
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This is what's good about EFI. And you can go buy nice Techlusion /whatever things for them.

... The day's coming - and soon - when carbs will seem as stupid, or archaic, as points ignition.
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Old 12-22-2005, 07:53 AM   #13
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Quote:
Originally Posted by creeper
Hey Chris...

I just remembered where I had read your paste before... it's from the Dell'Orto "Carburetor basic principals" booklet. As soon as I saw the word "enleanment", that was the tip off.

Fuckin'A Creep, I thought I could get away with looking like one of you smart guys with that post.... and then you outted me. Next time I'll make sure to steal info from more esoteric sources....
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Old 12-22-2005, 08:17 AM   #14
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Quote:
Originally Posted by ChrisC

Fuckin'A Creep, I thought I could get away with looking like one of you smart guys with that post.... and then you outted me. Next time I'll make sure to steal info from more esoteric sources....
Estoeric Sources?
Here ya go Chrissy: Dead Men Don't Wear Plaid
Enjoy! Especially the videos... gawd I love that movie

PS - Cat, beware of Technology and the revenge of unintended consequences...

PPS - reading creeper; thanks.
(EDIT: is that an 'index hint'? you bastage you... )
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Old 12-22-2005, 10:58 AM   #15
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Quote:
Originally Posted by meat popsicle
But you potatoho, have posted next to nothing. You post has me imagining a fairly one-dimensional problem, yet we all know that is not the case!
I just followed the carb manual (gives a range of settings). Hard to quantify it in terms of the course adjustments (jets, clip position). Lemme try to find the web page which I followed..

Oh haha, it was the same factorypro pages:
http://www.factorypro.com/tech/carbt...m_engines.html

I followed that method when I first got my bike, but I didn't want to mess with the float at first, other than setting it to the stock setting. When I started with aftermarket carbs I didn't expect the default float to be correct for my application, and it worked better at a different setting.
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