Need to replace the idle passage plugs in your Honda CX or GL’s carburetors? This is one of the easiest bits of the carb rebuild!
The idle passage plug sits over the idle jet, which is pressed into the carb body on early carbs but on later carbs the idle jet has a slot for a screwdriver and is screwed in. There are two sizes of plug, and they seem to correlate with the type of idle jet – the pressed in sort uses a slightly smaller plug, the screwed in sort uses a slightly larger plug.
The plugs are often old, hard and shrunk by the time they get to you. Old on the left, new on the right (new may be brown or black in our experience):
The different sizes of plug aren’t compatible, either – the small will be loose in the larger hole and the large one won’t fit into the small hole. They’re not just held in place via an interference fit – there is a flat spot on the float bowl that presses against the top
So what do they do, anyway? The idle jet underneath the plug (or above the plug, if you’re looking at the carb bodies as they sit when they’re installed) draws it’s fuel supply through one of the main jet passageways; if the plug is shrunk and allows fuel to get past the idle jet then draws fuel straight from the carburetor bowl, which means that the idle circuit gets far too much fuel. It’s a removable plug so that you can take it out and access the idle jet for cleaning.
Installation is super easy – with cleaned carburetors, simply press in the new plug over the idle jet. It won’t require any more than finger force.
If you don’t have a plug and want to get creative with a solution you could tap a thread into the passageway – not too far in, though, or you’ll obstruct the hole that allows fuel in from the main jet passage – and install a brass bolt. You’d have to be very careful with your swarf and, since the plugs are not all that expensive, you’d have to really want to do it differently…
People have also used things like ball point pen ends to fill the hole – again, be careful with blocking the drilling that allows the fuel through – but also be mindful that Viton idle jet plugs are going to cope with being immersed in petroleum, where plastic pen parts generally aren’t and may end up causing you further problems if/when they disintegrate.
Head back to the carb cleaning page.