Honda C90 – carb idle circuit

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The idle (or “pilot”) circuit controls the mixture of fuel and air at low throttle speeds. Here are the bits involved in the pilot circuit 1)it is common to use the term “idle circuit” in the UK but we’ll stick to “pilot” from now on, as that is what Honda uses in their documentation :

The pilot jet in the Keihin PB25A carburettor used on the Honda C90 z2 is marked with its size (#38) and is combined with an emulsion tube as a single part. It is an interference fit and it appears Honda did not intend for it to be removed or replaced (the pilot jet is a screw fit on later versions of the carburettor). The orifice in the pilot jet is tiny and may well need to be cleaned if the bike has been standing for a long time and In practice you can remove them by gently twisting and pulling them out – mine was a tight fit and I had to grip it with pliers, causing a bit of cosmetic damage in the process.

Honda do not sell a replacement part, but you can get a Keyster copy from NRP

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the pilot jet and emulsion tube are combined into a single part
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These two screws adjust the amount of fuel and air at low throttle speeds (see below for how they work):

the pilot adjustment screw is often called the “idle adjustment” in the UK
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Given all these parts are carefully machined to deliver exact amounts of fuel and air, why is adjustment needed at all? The reason is that the idle circuit only uses small amounts of air and fuel and is therefore sensitive to small changes in the fuel and air mixture. These changes can arise due to various maintenance issues, for example the reduction in suction caused by poorly adjusted valves or worn cylinders/piston rings, dirty air filters, air leaks, blocked jets ; wear on the needle jet etc

Let’s see how the idle circuit works2)keep in mind that Honda used several different carburettor designs over the years and, as we shall see, although they are all variations on a theme, the PB25A is has a different idle adjustment mechanism than most of the other versions installed on Honda’s step through bikes.

As with the main circuit, there is an air passage that feeds an air bleed connected to the pilot jet emulsion tube:

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Idle circuits are designed with either a mixture-adjusting screw (sometimes called a “fuel adjusting screw”) or with an air-adjusting screw.

The Honda C90 z2 Keihin PB25A carburettor uses the former. As far as I can tell from the catalogues available on CMS, carbs with mixture adjusting screws were only used on c90s made between 1978-1982 (the z2 and zz models) and all other models use air adjusting screws.

You can see the route of the air passageways in the underside of the top casting:

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Air is drawn from the opening on the filter-side of the carburettor into the air bleed passage – where it is mixed with the fuel in the pilot jet – and exits through the pilot hole shown in the picture below:

How does it work?
As you can see in the picture below, even when the throttle is closed a small amount of air can pass underneath the slide:

When the slide is closed small amounts of air can still pass under the bottom edge and through the pin hole in the slide

Although air is passing through the venturi when the bike is idling, the air does not have sufficient velocity to create a low pressure area capable of drawing fuel from the chamber below. Instead a vacuum is created downstream of the throttle slide because of the suction created when the piston descends in the cylinder and this causes fuel and air to be drawn through the pilot hole:

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suction created in the cylinder draws air and fuel through the pilot jet and out through the pilot hole

We can now see how the pilot adjustment screw works on this carburettor: when it is screwed inwards it closes off the passage to the pilot hole and this reduces the amount of fuel/air mixture that is pulled into the air supplied via the venturi, resulting in a mixture leaner:

Progression
You may have noticed in the photo earlier on that there is another small hole upstream of the pilot hole. This is described as a ‘progression hole’ in the Dellorto manual where I cribbed much of this theory from 3)a tuning manual is also available from Keihin is also available, but it is not very good.

As the throttle slide rises the velocity of air passing across the venturi increases to the point where the low pressure area is strong enough to pull fuel through the progression hole and the additional fuel/air mixture smooths the transition between idle and the point that the needle jet begins to deliver fuel.

The last part to look at is the throttle stop screw – this is used in conjunction with the mixture screw to adjust the mixture at idle and low speed: turning the throttle stop screw clockwise raises the throttle slide slightly and increase the engine speed (it is some times called the “idle speed screw” for this reason)

One last bit to go – the choke.

References   [ + ]

1.it is common to use the term “idle circuit” in the UK but we’ll stick to “pilot” from now on, as that is what Honda uses in their documentation
2.keep in mind that Honda used several different carburettor designs over the years and, as we shall see, although they are all variations on a theme, the PB25A is has a different idle adjustment mechanism than most of the other versions installed on Honda’s step through bikes
3.a tuning manual is also available from Keihin is also available, but it is not very good

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