Pilot screws

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mikenixon
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Pilot screws

#1

Post by mikenixon »

Carburetors have discharging passages called "circuits" because like electricity these passages all begin at one place -- the float bowl. All atomizing circuits have three legs. 1 One is fuel sourcing, another is air sourcing, and the third combines the two and outputs fuel and air together. This is important and basic to carburetion. Yamaha calls this the "Y-System", for obviously self-promoting reasons, though it is handily descriptive -- if you turn the "Y" upside-down.

All carburetors have idle mixture screws. This is because at idle the engine is its least efficient in using fuel properly. Therefore, tiny adjustments for altitude and many other environmental differences are necessary in the idle circuit. Even outboard motor carburetors had idle mixture screws. It's a very sensitive circuit.

In the earlier days of motorcycling, almost all idle mixture screws were mounted close to the air filter, almost assuredly because it was an intuitive placement and a very easy way to manufacture the carburetor. This screw type simply metered the idle circuit's air bleed, that second of the three legs making up the idle circuit. So the early type idle mixture screw simply metered (adjusted) the air leg of the 3-legged idle circuit. Crude though it was, it worked well. Until maybe the late 1970s, nearly all powersports carbs were like this.

But the feds eventually said there were two things wrong with this method of idle adjustment. One, air is compressible while fuel is not, thus overall circuit adjustment was not precise. And two, the air type screw's tip was fairly blunt, meaning a tiny fraction of a turn resulted in a huge change, also causing very crude adjustment, especially at the hands of the unknowing. So, when exhaust emissions began to be an issue on motorcycles decades after it had on cars, manufacturers changed the motorcycle's idle mixture screw just as they had with the same part on cars. The screw moved to the engine side of the carburetor and no longer metered the idle circuit's air bleed but now metered the idle circuit's third leg, the one that rises upward having fuel and air discharging together; the discharge leg. This was far more accurate as now the media being controlled was more substantial (having less compressible mass), and consequently the screw's tip was completely redesigned with an almost needle-like shape that required many turns to make much of a change in output. More better. Happy feds. (Of course, they then mandated hiding the screw so few homebrew mechanics could hope to find it, but that's another story...).

Since the old style screw metered only air, it was named by some the "air" screw, and since the later type metered air/fuel mixture combined it got the label "fuel screw" which as you can see isn't entirely accurate; the later screw meters air and fuel already mixed, not just fuel. The roles of the two types can be seen to be reversed also, you should note. Turning an air screw inward richened the circuit while turning a fuel screw inward leaned the circuit. Again, this is very basic. However, instead of calling one an air screw and another a fuel screw, I like to say "idle mixture screw" or "pilot screw". (When I am scribbling notes on a customer's project ticket, I default to shorthand, "pilots").

It is interesting to note that even some brand new carbs still have the old style "air" type idle mixture screw. That is because these carbs are found on, or intended for, bikes that are not emissions regulated: offroad machines and vehicles designed for other forms of closed course use, including road racing. This would include the number of exotic carbs available for streetbikes, expensive carburetors which despite their cost oddly employ the cruder "air" metering idle mixture screws. An anamoly, that. And now you know why.

Did you know there is a carburetor having both types of screws in the same carburetor? You'll find that on one model of Kawasaki, presumably because the carb manufacturer envisioned the bike as a sporting model benefitting from finer tuning capability. 2 Related to this is the interesting fact that tuning the fuel side and the air side of a given circuit has been proved to give slightly different (and thus more opportune) results even when the percentages of change are equal. Whole 'nother subject, but the well-informed are aware of this.

The GL1000 carburetor, though of a very early design and originating on a Honda car, never had the air type pilot screw, being subject as it was to emissions regulations in the car context long before it was put on a motorcycle. Thus it had the later style "fuel" screw from its inception. So, in for lean, out for rich.

One thing folks struggle with is seized and broken pilot screws. This is more endemic on certain models than on others. The Mikuni carbs on many Yamahas wherein the screws are vertically positioned at the carburetors' topside, for example, are bad for this. Presumably their positioning attracts more moisture and debris leading to the screw jamming, often associated also with corrosion of the casting. I "chase" most carbs' pilot screw tower threads for just that reason. But even horizontally oriented (GL1000) and vertical but bottom located (GL1100) screws are somewhat at risk of seizing. I think many times simply not being careful while adjusting the screw, letting it bottom too forcefully before turning it outward, is part of the reason the screws seize and even snap. Aftermarket pilot screws having a slightly different taper from stock and likely made of inferior material are also possibilities. But for sure the almost hypodermic shape of the later type screws when compared with the earlier blunt ones has to be largely to blame for its reduced durability.

The tips of broken pilot screws, embedded in the carb casting, are murder to get out. The difficulty is not as much getting the part out as it is doing so without augering the factory's carefully sized discharge outlet the errant part is jammed into. 3 This is what is nearly impossible. Because once that hole is even a liitle bit larger than factory spec, you have lost control of the idle mixture. If you can see the difference between it and its twin in an adjacent carb, the casting is ruined. Using a whole grab bag of different techniques, from specially made tools to heat to whatever, I have been successfully only about 30 percent of the time in removing broken pilot screws. Not great stats, that. And with the idle discharge outlet being essentially a blind hole, I'm not sure even the save-all EDM method would work. It's a tough one.

Another interesting fact many not making a living in the industry are unaware of is how much the idle circuit contributes to midrange and even high speed carburetor function. Though at idle and up to about quarter throttle the idle jet is by far contributing the most to the engine's operation, one, the idle curcuit is the only one of the main circuits that never shuts off, and two, on a very large bike having multiple carburetors and running in top gear, the idle circuit is in fact doing a huge amount of the work of the carburetor. 4

Notes:
1 To atomize means to break into fine droplets. The carburetor's most critical circuits pre-atomize, that is, mix with air before discharging. Some carburetor circuits don't atomize until after discharge. An accelerator pump circuit is one example. Non-atomizing circuits don't have the traditional three-way construction.

2 If memory serves, this was the Kawasaki KZ650 four.

3 The tiny hole in the carburetor casting's bore found at the tip of the pilot screw.

4 The contribution percentage of each significant circuit has traditionally been illustrated by the classic "fishook" graphs. The only problem is, these ubiquitous graphs are really only valid on one carburetor type.
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robin1731
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Re: Pilot screws

#2

Post by robin1731 »

Yes, it was the KZ650. ;)
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mikenixon
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Re: Pilot screws

#3

Post by mikenixon »

:)
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gltriker
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Re: Pilot screws

#4

Post by gltriker »

Ok. I'm biting.

What does the classic " fishook " graph look like?

Ubiquitous, means what?
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mikenixon
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Re: Pilot screws

#5

Post by mikenixon »

Image

Ubiquitous means "observable all over the place". Plentiful. Common.
Last edited by mikenixon on Fri Oct 04, 2019 11:41 am, edited 1 time in total.
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mikenixon
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Re: Pilot screws

#6

Post by mikenixon »

Image

Even more classic, this one is overall fuel use, and the one most people think of.
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Re: Pilot screws

#7

Post by mikenixon »

In fact, the curve directly above is the one programmed into fuel injection computers. I used to be able to speak intelligently about this, extrapolate, confab all day long. I find that I have forgotten much... Old age. The point, going back to the multi-curve example, is that fuel contribution isn't the linear thing we like to think. And, specific to the article, that the idle circuit plays more a role than we assume.
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Re: Pilot screws

#8

Post by flyday58 »

tumb2 tumb2 tumb2

If I remember from A&P school in the 70s, the carb theory books had similar curve diagrams that showed the relation between fuel metering and altitude changes. I wonder if any powersports manufacturers ever made a graph for those who use their products in places other than sea level?

Great stuff, Mike.
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Re: Pilot screws

#9

Post by mikenixon »

:)
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