The idle circuit, sometimes called the pilot circuit, controls the mixture of fuel and air at low throttle speeds. There are not many components involved, read on to see how they work together to ensure the engine ticks over smoothly at low revs.
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. 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. The jet is an interference fit into the carburettor body (a screw fit was used on later versions of the carburettor) and – despite the fact that it seems Honda did not intend for it to be removed or replaced – In practice you can remove it by gently twisting and pulling until it comes free, taking care to minimise the damage in the process.
Honda do not sell a replacement part but you can get a Keyster copy from NRP. Note that the the jets included in carb repair kits are not always the same size as the originals and (even when they are marked the same size). To avoid upsetting the jetting in the carb it is safer to stay with the original Keihin brass parts unless the originals have been damaged – for example by over enthusiastic cleaning – or you are deliberately adjusting the jetting for some reason.
Throttle Stop and Pilot Adjustment screw
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 slight 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 etc
Let’s see how the idle circuit works. Keep in mind that Honda used several different carburettor designs on the C90s over the years and 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 and this is connected to the pilot jet emulsion tube. Air bleeds are so called because air is ‘bled’ into the fuel passages in the carburettor to improve fuel atomisation.
Pilot adjustment screw
Pilot circuits are adjusted with either a mixture adjusting screw or with an air adjusting screw. As the names suggest these screws adjust the richness of the mixture at low engine speeds by either metering the amount of aerated fuel going to the engine or the amount air.
Most motorcycles from this era used air-adjusting screws, but the carburettor used in the Honda C90 z2, the Keihin PB25A, uses a mixture adjustment.
Looking at 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:
How it works
As you can see in the picture below, even when the throttle is closed a small amount of air can pass underneath the slide
The pilot hole is located beyond the throttle slide and, although some air is passing through the carburettor bore when the bike is idling, it does not have sufficient velocity at this point to create a low pressure area capable of drawing fuel from the chamber below. Instead, each time the piston descends on the intake stroke a vacuum is created in the cylinder and – because the throttle slide obstructs the bore – the volume of air that can pass is insufficient to fill the vacuum. The resulting low pressure area over the pilot hole causes atmospheric pressure in the bowl to force fuel through the pilot hole.
The mixture adjustment screw constricts the passage leading to the pilot hole and, when it is screwed inwards, it reduces the amount of aerated fuel that is mixed with the air entering through the bore of the carburettor and this results in a leaner mixture.
Note that air adjustment screws work the other way round: when they are screwed inwards they reduce the amount of air mixed with the fuel and thus make the mixture richer.
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 1)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 aerated fuel smooths the transition between idle and the point that the needle jet begins to deliver fuel.
Note that many carburettors do not use a progression hole and instead have a single discharge for both the idle and low speed and in these cases the orifice is located directly under the lip of the throttle slide where air speed is greatest.
The last part to look at is the throttle stop screw – this is used to vary the speed of the engine when it is idling: turning the throttle stop screw clockwise raises the throttle slide slightly and increases the engine speed (it is some times called the “idle speed screw” for this reason)
One last bit to go – the choke.