Found another write up on Dell needles:
This is one of the most comprehensive I've come across concerning PH series carbs.
www.apriliaforum.com/forums/showthread.php?280947-Carb-Parts-and-FunctionsDue to some recent Threads I figured it would be a good time to Post the "next installment" ...
Needles:
Needle "tuning" is not as straight forward as swapping Jets around, there are 3 separate areas of the needle that can be altered and all will have an effect on the way in which the Carb functions.
First is the little Clip that holds the Needle into position on the Slide.
Needles for the Dellorto PHBG have 4 positions for the Clip and depending on which groove you place the Clip into will alter when the particular characteristics of that Needle will begin to take effect.
This is accomplished because the Needle's taper is being raised / lowered in relation to it's insertion into the Atomizer outlet.
Fuel is metered into the Atomizer Tube via the Main Jet, this fuel is then drawn up into the air stream.
The amount of fuel that is able to be drawn up will be relative to the diameter of the Needle at the opening.
Moving the Clip on the Needle usually adds up to this, the LOWER the Clip the more open and sooner the taper will be effective and is considered "richer", the HIGHER the Clip the more closed and later the taper takes effect and is considered "leaner".
Top Notch = LEANEST
Bottom Notch = RICHEST
Standard = 2nd from Top
Next we must consider the 2 other variables of the Needle which are directly related to each other, Tip Diameter and Taper Length.
For the PHBG Carb we are given a Tip Dia. range from .50mm to 1.80mm and a Taper Length from 15mm to 26mm.
The combination of these two elements will determine the fuel metering "characteristics" of the Needle.
This again is due to the fact that the diameter of the Needle at the Atomizer opening will determine the amount of fuel that can be drawn up into the air stream at any particular Throttle position.
The Taper Length provides the rate at which the difference between the Tip Dia. and the Barrel Dia. are presented throughout the Throttle range.
The LONGER the taper the more gradual the transition will be, it will usually be an overall "richer" and "earlier" Needle.
This is because the Needle will begin metering more fuel sooner in the Throttle rotation.
The SHORTER the taper the less the length of transition between Tip and Barrel diameters, this is considered an overall "leaner" and "later" Needle.
Inversely, this is due to the Needle metering fuel later on in the Throttle rotation.
The Tip Diameter is considered the "starting point" of the Taper and is the final variable in the transition to the Carb metering fuel entirely based on the size of the Main Jet.
The LARGER the diameter of the Tip the greater the amount of fuel will suddenly be metered at the transition to WOT.
This would be considered a "leaner" Needle at 3/4 Throttle.
The SMALLER the diameter of the Tip the less the difference of fuel being metered at the transition to WOT.
This would be considered a "richer" Needle at 3/4 Throttle.
This is an important characteristic of the Needle and should be considered if one is having fueling issues in the final positions of the Throttle before WOT.
The Tip Dia. in combination with the Taper Length determine the overall "characteristics" of the Needle's ability to meter fuel throughout the Throttle rotation range.
This is also how Dellorto identifies the Needle designation W**.
VERY IMPORTANT:
NEEDLE NUMBERS ARE NOT SEQUENTIAL LIKE JETS !!!
USE CHART BELOW TO SELECT A NEEDLE BASED ON TIP DIA. AND TAPER LENGTH
At any given Throttle position the diameter of the Needle at the Atomizer opening, at that time will determine the amount of fuel able to be drawn out.
Again, the LARGER the diameter of the Needle at the Atomizer opening the less fuel will be metered "leaner", the SMALLER the diameter the more fuel "richer".
A Needle with a LARGER Tip and a SHORTER Taper would have a LARGER diameter of Needle at the Atomizer opening at any given Throttle position in contrast to a Needle with a SMALLER Tip and LONGER Taper for the same Throttle position.
So, a Needle with a LARGE Tip and a SHORT Taper is "leaner" compared to a Needle with a SMALL Tip and LONG Taper which is "richer" at the same Throttle position.
Another way to visualize all of this working together is to think of the Needle as the faucet on the kitchen sink ...
The Needle Diameter is how far open the faucet is and the Taper is how fast or slow you open it.
O.K., with all that said use this Chart to select a Needle of your "tuning" pleasures ...
He also explains slides pretty well I think...Slides come in a wide range of shapes, sizes and means of actuation (Round,Guillotine, Cable, Vacuum, etc.) and all have 2 basic functions.
(This is a little long winded but I hope it will prime everyone for a greater understanding of the collective functions of the Carb.)
The primary (and least discussed) function of the Slide is to regulate the amount of air flow through the Venturi relative to Throttle position and Engine load and in turn produce the required pressure differentials needed to move liquid fuel up into the air stream.
This is obviously done by varying the size of the opening in which air can pass through to the Engine.
The larger the opening is, the stronger the vacuum signal from the Engine must be in order to lower the pressure enough to draw up the required liquid fuel from the Float Bowl for that size of an opening.
This is true for all of the fuel delivery circuits, be it the Slide in the Venturi or the Needle in the Atomizer.
The Slide's effect on fuel delivery can be noticed up to about ¾ Throttle, after which the obstruction to the air flow retaliative to the rapidly decreasing area / volume of the Slide in a round bore is benign.
This primary function of the Slide is intended to meter the proper amount of air / fuel into the Engine as it is called for by the Engine's needs at that time.
On Constant Velocity type Carbs the Slide's position in the Venturi is solely determined by pressure differences presented against a spring loaded diaphragm and not a direct cable connection.
These types of Carburetors typically offer a more smooth linear progression through varying Throttle positions and Engine loads.
However, these types of Carburetors would require a slightly different conversation here at “Carb Parts and Functions” then what has been discussed so far and as such I will focus on direct cable connected units from here …
A direct cable connected Slide has an inherent flaw when talking about it's primary function. The Slide can be moved from fully Closed to fully Open as fast as you can twist the Throttle.
This creates a massive positive (bad) pressure change within the Carburetor as the Engine is incapable of producing the vacuum signal needed to inhale the amount of potential air flow that has been instantly presented to it by the larger Venturi area.
As such the functions of all the other progressive pressure based fuel delivery circuits are left trying to play catch-up and the Engine bogs or even stalls.
When referring to an Engine bogging it is important to understand what form of “bogging” you are experiencing, too much fuel or not enough.
In the above case it would be not enough.
This is because the air pressure in the Venturi is almost equal to that which is acting upon the liquid fuel down in the Float Bowl.
As the Engine speed rises the pressure in the Venturi begins to drop and more fuel is able to be drawn up and out through the various circuits.
How fast your Engine recovers from a scenario like this will tell you a lot about how well you have balanced the transitions between those circuits and what your Engine's particular set-up is capable of ingesting.
Typically though we “roll” through the Throttle and the transition between the circuits is gradual (relatively speaking).
This is when the primary function of the Slide truly shines through.
In these terms the Slide “manages” the delicate relationship between what we want the Engine to do and what the Engine needs to oblige our demands, manipulating low pressure regions within the Venturi to the vacuum signals produced by the Engine and ultimately providing the correct A/F ratio at any given Throttle position for any given Engine load, tall order indeed …
One aspect of the Slide that may not be as obvious is that the shape plays a major role in the formation and amplitude of the pressure regions that are presented to the fuel delivery circuits on the downstream side of the Slide.
I will however save that for a future installment on the Venturi itself, where it will be more relevant to the topic.
With all that said and out of the way it is time to move on to the most commonly asked about function of the Slide, the Cutaway …
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The Slide Cutaway has a pressure varying effect similar to that of the primary function but imposed on a very narrow window of the over-all throttle movement.
Here the leading edge on the bottom of the Slide has a taper cut into it.
The purpose of this tapered edge is to tailor the transition between the Idle Circuits and the Main Circuit.
When the Slide is closed, air and fuel are supplied via the Idle Circuits which are powered by the weak vacuum signals from the Engine at idle.
As the Throttle is opened the vacuum signals from the Engine are still too weak to lower the pressure in the Venturi enough to draw fuel up through the Main Circuit and the Idle Circuit cannot supply enough fuel for a clean uniform progression.
This is where Carburetors equipped with a Progression Hole under the Slide come into play.
Through this Progression Hole additional fuel is able to be supplied as the Throttle is opened and more air is allowed to pass through the Venturi.
The amount of fuel that can be supplied by the Progression Hole (as with all circuits) depends on the area of low pressure above it.
By changing the height of the Cutaway we can adjust how the vacuum signal is presented to the Progression Hole on the initial throttle movements ( up to ¼ ) by raising and lowering the pressure within the Venturi.
A larger Cutaway will allow more air to flow through which lessens the effective vacuum signal, in turn lowering the amount of fuel that will be supplied (leaning the progression).
A smaller Cutaway allows less air to flow through and a stronger vacuum signal is presented, this allows for more fuel to be drawn up (enriching the progression).
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The Slide Cutaway also effects how fuel is metered by the Atomizer from ⅛– ½ throttle.
This is accomplished by the same means as that which effects the Progression Circuit.
Here the interaction is spread over a larger throttle movement area and the impact of this will be relative to the selection of Atomizer, Atomizer Bushing and Needle used.
As noted in a previous Post regarding Atomizer & Bushing …
The step in the bushing creates a restriction in the air flow through the venturi and generates an area of high vacuum behind it and can be used to enhance fuel delivery.
The taller the step the greater the restriction / vacuum generated which in turn enriches the mixture and the opposite is true of a shorter step in the bushing.
In addition to the interaction with the Atomizer Bushing one must also look at the Needle they are using, as the initial taper profile metering characteristics (relative to the Atomizer size) will be effected by altering the strength of the vacuum signals as well.
With that taken into consideration it should be readily apparent that an extreme lean or rich condition can be created in the first half of the Throttle's opening by fitting a Slide with an incorrect Cutaway for your Atomizer / Bushing / Needle set-up, so use caution.
Also Worth Noting:
Always make sure that your Slide is fully seated when the Throttle is closed and there is a little slack in your Throttle Cable.
Always make sure that your Slide is open completely at WOT.
Do both of these every time you refit the Carb.
This may sound silly but I can't tell you how many times I have seen it...
maybe you can try the snipping of one of the wires... I know Brent had a thread about CDI restriction removal. Possibly the pink/red wire? sure you can put it back if it doesn't work...
