If you’re having trouble idling or starting, adjust the pilot screw! If you don’t know the structure, don’t miss it! | News

The old carbureted cars tend to get out of shape quickly, whereas the injected cars are free from problems all year round.
That’s not true! The majority of people who drive carbureted cars must have experienced engine (carburetor) malfunction.
In addition, if you touch the carburetor when you try to adjust it, you may find that it becomes even worse.

Shh!
There is no way to improve the condition of a precision machine if you don’t know what you are doing.

WebiQ is here to bring you a little happiness and motorcycle knowledge.
This time, I’d like to talk about the solution to carburetor malfunction.grossI’ll explain it to you!

Carburetors are precision machines

Whereas modern fuel injection “actively injects” fuel after a calculation process based on the values of various sensors, carburetors rely solely on the negative pressure of the engine’s intake to “passively draw up” the amount of fuel, which is completely mechanically controlled.

If you compare it to a watch, the injection is a G-Shock and the carburetor is a mechanical watch.
No kidding, it’s that different.

No matter how dirty it may look, the carburetor is a mass of precision parts and is designed to be worn down.
So it’s inevitable that you’ll lose your shape as you use it.

But I’m sure it happens! The carburetor is adjustable.
(Although some carburetors for moped scooters and other disposable carburetors are not adjustable.

However, when I tried to adjust it, there were so many unidentifiable screws and holes that I couldn’t figure out where to touch.
Even if you look at the information on the Internet, there are so many different things written by different people that I don’t know what the right answer is. ……
I’m too scared to touch it.I think there are a lot of people who think that.

A carburetor is, in other words, a “fog sucker

So.First, let’s review the carburetor.From.

If you can understand the principle, you will be able to judge who is right and who is wrong in the stories of setting that flood the Internet.
It’s a bit difficult to say, but it’s super important because if you don’t understand this, you won’t be able to do anything about it.

Well, I’m sorry to interrupt, but you have a mist sprayer, right?
The thing that sprays liquid into mist when you hold it.

In a carburetor, you hold the lever and apply pressure to force the liquid into a narrow passage and atomize it as it is sprayed.

A negative pressure is generated in front of the nozzle, and the liquid is pulled out by the negative pressure.is the basic principle.
Reverse of misting = mistwrapper
The negative pressure to pull it out is generated by the engine sucking in air.

As it is pulled through the narrow nozzle, the liquid is atomized and sucked into the engine …….
That’s what a carburetor is.

The problem is when the intake negative pressure is low.

Injection injects its own fuel, so it doesn’t matter whether the negative pressure generated by the engine is high or low.
As a result of the calculation, the optimum amount of fuel is this! It only injects the amount of gasoline calculated as
It’s exactly like a foghorn.

In contrast, the carburetor “atomizes” the gasoline by the negative pressure generated by the engine, but the problem is that the negative pressure of the engine intake is not constant.
Especially in the low RPM range, there is not enough negative pressure because there is not enough air to inhale, so gasoline cannot be atomized from the main fuel nozzle.

Worst of all, you can’t breathe gasoline at all, and if you can, it doesn’t make a nice mist, it just drips all over the place.
It’s the same logic that if you grip a misting bottle too weakly and there’s not enough pressure, the liquid will drip instead of misting.

If the throttle is fully closed even at low rpm, the negative pressure is actually high ……, or if the carburetor bore is too large even at high rpm, the negative pressure will be low ……. However, I will not talk about it this time because the difficulty level will increase dramatically.

It has three kinds of fog sucking nozzles according to the suction negative pressure.

As mentioned above, at low RPM (e.g. idling), the engine will not be able to suck mist properly, and the engine will become very sluggish, so there is a separate nozzle for low RPM (for when the negative pressure is low).
It is called “slow system”, and it is “slow jet” that measures the amount of gasoline sucked by such slow system.
By replacing the jet with one that has a slightly different hole size, the gasolinemaximum flow rateChange the
(The nozzle for high rpm and high negative pressure is the main system, and the main jet is what determines the maximum gasoline flow rate of the main system.

However, at very low rpm like idling, the hole is still too big even for slow system.
We need more fog sucking nozzles for lower RPMs (and lower negative pressure)!
Because it is ……, there is also a flow channel called “pilot system” which is targeted only near idling of the throttle fully closed.

mainthrowpilotIn.Total 3 types

I would say that the “main jet”, “slow jet”, and “pilot jet” …… meter the gasoline in each stream, but only the pilot jet does not exist.
I’ll explain why later!

The majority of malfunctions are pilot-based.

In fact, there is a little characteristic when a carbureted car goes out of tune.

Full throttle at 6th gear rev limit! Once the setting is found, it will not go out of tune.
This is the domain of the main system, but since it is sucking up a huge amount of gasoline at a huge negative pressure, even if the flow rate changes a little for some reason, it won’t affect many people.
In other words, if you don’t do some tuning (or whatever)You don’t need to change the main jet just because your engine is running bad.

The slow system is also called slow, but it is an area where there is a lot of negative pressure, so even if there is wear due to aging, it is possible to get by with a little sluggishness.
The correct way to get back in tune is to replace the worn jet needle and needle jet (the tube that the needle and needle go into) instead of simply replacing the slow jet.

The one thing you can’t do anything about is pilot systems.
This is where the whole engine starts to feel sluggish at once …… and this is what happens when a carbureted car goes sluggish.

I can’t start the engine, it just about starts but is difficult to start, it starts but won’t idle, it just about undulates but is very unstable, it is very jerky when getting up from an intersection, it makes a rumbling noise when fully closed, and it makes an afterfire (popping sound) when fully closed. This kind of problem is usually caused by the malfunction of the paitrot system.
In other words.In case of engine malfunction caused by carburetor, the cause is mostly pilot system.‘

Why no pilot jet?

as a matter of factThe pilot system is part of the slow system.That’s why.

In the slow system, the gasoline measured by the slow jet and the air measured by the slow air jet (or air screw) are mixed in advance inside the carburetor and then atomized by the negative pressure of the engine intake.

The pilot system is a specialized mechanism for metering the amount of fuel needed near idle, but the amount of gasoline needed at idle is sufficient only by atomizing a small portion of the fuel in the slow system.
If the demand is so low, it is beyond the range of flow control by changing jets, and it is faster to bypass a part of the slow system than to change jets every time.

The figure below is a simplified cross-section of a carburetor, with only the bypass passages for the slow and pilot systems extracted.
I tried to simplify it as much as possible by sucking gasoline from the slow jet, adjusting the amount of air to mix with the air screw, and adjusting how much fuel mixed with air to suck with the pilot screw.

As you can see, the pilot system bypasses part of the slow system, so it is somewhat affected by slow jet changes and air screw adjustments.
If you replace the slow jet, which should not affect the idle much, the idle may become sluggish for some reason because the slow system is directly connected to the pilot system.

The pilot screw controls the amount of bypassing.

パイロット系をもう一度確認です。

If you don’t understand this structure, you will never be able to adjust it well.
I’m just trying to figure out where to adjust.The key to getting back on track is to be able to imagine it in your brain.It is.

Pilot screws are ultra-precise parts

Looking at the diagram, didn’t you think that the taper for adjusting the flow rate on the air screw is a big triangle, while the taper for adjusting the pilot screw is awfully thin and long?
It’s not that I failed to draw a diagram.The pilot screw is such a precision partIt shows that it is

In the first place, it is the pilot system that has to atomize a very small amount of gasoline at an idle speed with extremely low negative pressure.
The aisles are super thin and it’s very serious.

If a small piece of dust gets in the hole or a scratch on the air screw, you are out of luck.
Scrubbing with a wire brush to remove dirt or gouging with a needle to remove clogged debris is strictly prohibited.

What happens when the pilot screw is tightened?

The pilot systemnarrow down the aislething.
Because the amount of gasoline (and air mixture) decreases near idle with the throttle fully closed,become thinThat’s what I’m talking about.

For example, if the idling area is good, but you want to make it a little thicker around the open area, you can tighten the air screw to increase the gasoline content in the slow system.
This way, instead of making the slow system better, it also thickens the pilot system, which is directly connected to the slow system.
To solve this, you can tighten the pilot screw a little …… and use it that way.

What happens if I loosen the pilot screw?

In contrast to the above, at very low speeds near idlingLarge aisle.As a result, the gasoline flow rate increases.thickenThat’s what I’m talking about.

Note that the amount of gasoline that comes out of the pilot system is very small, so no matter how thick you adjust the pilot system (loosen the pilot screw), the amount of gasoline that comes out is very small.Almost no effect except around idling.

The complication is that there is an air screw that looks very similar to the pilot screw from the front.
Furthermore, the air screw affects the entire slow range above idle.
on top of thatThe air screw is the opposite of the pilot screw in how it works dark/light.And the air screw should be tightened to make the mixture thicker.
It’s very easy to get it wrong, both in shape and how it works.So, make sure you do.

What is the adjustment range of the pilot screw?

It’s a very delicate part.When tightening, tighten lightly.I’ll try to do that.
If you tighten it sizzling like a normal screw, it will be destroyed by itself and cannot be re-installed.
At best, a new pilot screw is needed. At worst, the carburetor itself is damaged and the carb is replaced.

Tighten it lightly and loosen it from the fully tightened position, but as you can see, it’s just a screw.If it is loosened too much, it will fall off due to vibration while driving.I will.
(If it drops out, it will cause intense sluggishness not just near idle, but throughout the entire area.

In addition, all carburetors have the same number ofBack out 1 to 2 1/2 turns from the tightened positionis the basis.
There aren’t many exceptions.
If you are feeling well beyond this range, it is better to think that the cause of your discomfort is elsewhere.

For example, there is trash caught somewhere in the aisle.
The pilot screw tip is dirty, and so on.
The O-ring that goes with the pilot screw is broken, or something like that.
It’s a silly cause lurking in the background.

However, there are rare cases where “3 and a half turns back is the best”, so it is not an absolute no-no if you exceed the range.

Why does the pilot screw have a special tool for adjustment?

The pilot screw (with a very few exceptions on Honda cars) has a negative groove carved into it, so if you have a thin flathead screwdriver, you can use it without disassembling the carburetor.Easily adjusted from outsideIt’s like that.
Because you want to turn the screw and adjust it with the engine running and idling.

The pilot screw is adjusted by the number of revolutions it returns from fully closed (how loose it is), so it’s best to install it on the car and fine tune it while it’s actually idling!

There’s a…!
The area around the carburetor is often so narrow that there is no room to get at it …….
In a parallel four-cylinder engine, there are four carburetors in a row, and the two in the middle are absolutely impossible.
The engine’s hot, and I’m waiting to get burned!
To prevent that from happening.Insert it from the side and turn the pilot screw located at the back.There are special tools sold for this purpose.

The usage is as shown in the image below.
(The black bar extending from the lower right to the center of the image is the adjustment tool.
Insert it through the gap on the side of the car body, align it with the pilot screw on the carburetor body, and turn the dial on the hand so that the flat-blade screwdriver on the tip, which is connected by wires and gears, turns the same amount.

No, you don’t need expensive tools to make adjustments, right?
However, if you want to adjust it without special tool, you have to remove the whole carburetor.

There are many people who make their own tools because they want special tools but special tools are too expensive.
It’s interesting to see how creative you all are!
The image below is an example of a self-made “ultra-short screwdriver that can only turn the pilot screw” specially designed for “vehicles where there is almost no space to insert a screwdriver although you can fit your hand.

The pilot screw only affects the area around idle…?

That’s what they say, and there are many articles explaining it that way.
You’re absolutely right, and if you want to fix a problem with a stock installed carburetor, you’ll need aPilot screw adjustment range is only around idleYou’re definitely right.

However, if you are using a general-purpose racing carburetor that is not stock, you may not be able to use theSometimes common sense just doesn’t cut it.
Personally, I also feel that when a large displacement cylinder is combined with a small bore carburetor, it will affect the carburetor to a very slow system area.

As you can see, it’s a pretty different story depending on whether you want to get the stock carburetor back in shape or if you want to set up a racing carburetor.
It is also one of the reasons why different people say different things.
So, if you simply rely on the search method such as “how to adjust the pilot screw”, you may not understand what is going on.

Adjustment method for multi-cylinder engines

Pong it all together! It doesn’t go with …….
Adjustment one cylinder at a timeI will.

There are some troublesome things that need to be done before adjusting the pilot screw, such as adjusting the throttle opening and synchronizing the negative pressure with the vacuum gauge.
Only after all of that is done do I start adjusting the pilot screw.

First, tighten the pilot screw all the way closed on one cylinder (no fuel is supplied, so rpm drops), and then tighten the pilot screw all the way closed on the other cylinder (no fuel is supplied, so rpm drops)./It will open in four-turn increments.
Every time you open it, the idling speed will increase because fuel will be supplied more easily, so set it to the position where the speed is the highest.

If the speed is within the specified range (one to two and a half revolutions), it is OK.

The idle speed that has risen too high is lowered to the specified value with the throttle stop screw, and the next cylinder is …….
This is how you adjust one cylinder at a time.

1 after all adjustments/4 to 1/Some say it’s better to open it up 8 turns (to make it a little thicker), but this is your choice.

Also, since it’s highly unlikely that all cylinders are in exactly the same condition.The pilot screws don’t all have the same amount of return.is normal.
There’s no better way to get it right, though.

And this is not the ‘only right way’ to do it.
The way to do this can be a little different for each person and each car model.

Carburetors come in a variety of configurations.

This is a very annoying story, but I’ll tell you a super annoying story at the end.

As I’ve explained in many ways, there are carburetors that are not main, slow, and pilot.
Some inexpensive carburetors for small displacement engines may not have a pilot system, and some carburetors have a fourth path for starting (starter system).

I mentioned that the air and gasoline should be mixed in advance in the carburetor, but the amount of air to mix them may or may not be adjustable.
Even if it can be adjusted, there is a type of jet with a very small hole called an “air jet” that can be replaced, and there is a type that adjusts the amount of air by tightening a screw called an “air screw”.
There may be two air screws with separate main and slow systems, or there may be only one type.

Anyway, there’s a lot of variety.There’s no rule that says, “This is how it’s supposed to be.of.
However, the role of the pilot screw remains the same.
No matter how complicated your carburetor is.The role of the pilot screw is the same

Also, although the carburetor is a precision machine, it automatically adjusts the amount of fuel mist sucked in by negative pressure, so in a sense it is more forgiving of settings.
So even if the adjustment method is completely different from person to person, the result can be similar.
It’s so deep.of.

But if the engine is sluggish and doesn’t start well or idles erratically, why don’t you try turning the pilot screw anyway?
What’s nice is that the pilot screw adjustment is almost never necessary to ride and test ride (although technically it is).
As long as you note down the original rpm, you can get it back to normal at worst.
There is no need to buy various types of parts or jets, just turn the screw.

If you can get an engine that wouldn’t start to start and idle nicely, that alone should give you the accomplishment of going three bowls of rice!