The idle jet

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mikenixon
Early 'Wing Guru
Early 'Wing Guru
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The idle jet

#1

Post by mikenixon »

Purely from a circuit standpoint, the idle jet is the most important focus of carburetor rebuilding. On GL1000s and GL1100s, the engine actually starts on the idle jet. Starts, as in not even yet running. On all carbs, the quality of the engine's idle and the linearity and responsiveness of the throttle depend on it. So it's important. And yet, on 1975 (GL1000) through 1981 (GL1100) Wing carbs the idle jet can be a challenge to remove for cleaning. 1 And wouldn't you know it, this important yet tiny jet virtually always needs that cleaning.

On the GL1000 especially, there is a trick to getting the idle jet out even after it has been unscrewed. 2 The jet's tower is nearly the same diameter as the outside of the jet, and with the usual light corrosion making the fit even tighter, it takes some doing to get that itty-bitty jet out. The jet is so small and so lightweight that mere upending the carb or even shaking it does not do it. The trick is to blow compressed air into the idle air bleed passage on the side of the carburetor (after the brass elbow has been removed). The jet will fly out onto the bench, so watch for it. Did I mention it's very small? If you sneeze, you will never find it.

Speaking of smallness, if your eyes are anything like mine you will have to squint to tell the idle jets from the air bleed jets. They're close in physical size and shape, though the idle jets have heads on them and the air bleed jets don't, and of course the two jets' threads are different as well.

You simply *must* "size" the idle jet after it is removed and cleaned. This means inserting a 0.013-0.0135" (0.33-34mm) wire -- a light gauge high E guitar string works well -- into the jet for gauging purposes. Don't reinstall the jet until it is clean enough to go easily onto the string. 3 When reinstalling it, thread the jet into place and only very gently tighten. You don't know what frustration is until you have encountered an over-tightened GL1000 idle jet. It will not inspire. Blow out the idle jet tower with compressed air to finish the job.

If someone before you has left signs of a struggle getting the jet out, that is, the inside of the jet tower is pretty scarred up, it may be that the tower will no longer properly accept the rubber plug. The plug will be too loose. You can't really fit an oversize plug as it will be both too fat and too tall, though you may be able to razor-blade it to size. There is still another option however so contact me if this happens to you.

A slight digression. Goldwing jets are sized in hundredths of a millimeter (0.01), as are all Keihin streetbike jets. But there are actually two sizing systems when it comes to carburetor jets. One system such as the Goldwing's simply measures the jet orifice's diameter. The diameter-based system will be metric on metric carbs and Imperial on American and Brit carbs. But still diameter-based. That's one system. The other jet sizing system marks the jet as to its flow rate. A few Keihin racing carburetors use this system, and practically all Mikunis both street and racing. The flow system is based on the rate of a test liquid dropping through the jet into a beaker, that is, quantity versus time. The flow-rate jet marking system has its origin and its purpose in racing. Here's why. When you are racing a carbureted motorcycle and suddenly you decide you have a change in conditions requiring a change from the present jetting and you can make a pretty good guess as to percentage, the flow rated jet is your friend. It is designed to make that easy, as you simply apply the percentage against the jet's number. The diameter rated jet just won't work that way, inasmuch as diameter does not directly relate to flow. It can still be done, but it requires more math. Since the diameter-rated jets are mostly used in street bikes that isn't an issue. They're fine. But the flow system is more easily adapted to racing use. By the way, eons ago Mikuni and Keihin decided to use different thread systems to make it harder to confuse their jets, but I have seen them mixed up anyway, and the last time was only a few months ago.

The rubber plug that fits over the GL1000 idle jet forces the jet to get its fuel from the primary main circuit. You will note the horizontally drilled hole between the two jet towers. This is a very old carburetor technology that Keihin and other carburetor manufacturers pretty much abandoned at the end of the 1970s. Various qualities of rubber plugs are available. It's best to use the highest quality you can find, as the cheaper rubber tends to deform and vulcanize (glue itself) inside the jet tower. When I see this I have to remove it using a carefully-sized drill bit rotated against the plug by hand; basically Roto-Rooter the plug out in globs.

It is almost never a good idea to enlarge an idle jet. You might think you are safe to do so, thinking that the pilot screw's adjustability will cover up any size issues. But it won't. First, the pilot screw's entire adjustability range is less than that of a change in jet size. 4 Second and more importantly, the idle circuit's complexity means the pilot screw does not control all of the idle jet. Some of the idle jet's output circumvents the pilot screw. Look at the bypass holes in the carburetor casting. These are fed by the idle jet. Yet they are parallel to the pilot screw and not part of its flow. 5

1 The 82-83 GL1100 has conventionally-removable theaded idle jets.

2 Use a carefully chosen or even specially ground screwdriver to remove and reinstall idle jets.

3 The idle jet will also have to be sized just to make sure the size is correct if it is an aftermarket jet.

4 Which by the way in Keihin jets is two hundredths (0.02) not five (0.05).

5 In other words, the pilot screw intercepts only the mixture going to the idle discharge pipe. It does not intercept the mixture going to the bypass ports.
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