Idle Thoughts On Carbs

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

While it’s true that there exist carburetors that lack an idle fuel system separate from their main fuel systems, such as the Lectron, the long-ago Wal Phillips “injector,” and Mikuni’s short-lived zero-cutaway carb, the great classic racing carburetors such as Amal’s GP series and Mikuni’s VMs do indeed have idle systems. What some racing carbs do not have is a throttle-slide idle stop screw, whose job it is to hold the slide open just enough to admit airflow sufficient to maintain a steady idle.

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Are the Amal GP and Mikuni VM truly racing carburetors? The GP was the final development of a line of hollow-cylindrical-slide racing carbs originating in the 1920s, and was standard fitment on the racing engines from Norton, Velo, AJS, and more, which won so many postwar 350 and 500 TTs in the Isle of Man. The Mikuni VM was standard on the thousands of Yamaha TD and TZ production racebikes from 1969 onward; the carburetor won two Daytona 200s on 350 twins in 1972-73, and then every subsequent 200 until 1983 on the TZ750.

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Cylindrical-slide carburetors need an idle system because at nearly closed throttle the air velocity over their main fuel system, which is centered under the throttle slide in VMs and offset to one side in GPs, is too low to lift fuel from it. It is the downstream edge of the slide which can completely close, so at very low throttle the air velocity under that edge of the slide is very high. It is therefore obvious that in order to lift and mix fuel at very low slide heights, the place to do it is just downstream of the engine-side edge of the slide. And if you look there, you will find one or more tiny holes, connected to a separate idle fuel system, with its mixture controlled by an idle mixture screw.

The Legendary Amal GP

The main fuel system orifice, located under the central plane of the throttle slide, is in a “great hall” in an Amal GP, whose slide is hollow all the way to its top. Low-throttle airflow through that big volume is too low to pull fuel from the main system, so the GP’s idle system is located where air velocity is highest at low throttle, just downstream from the engine-side edge of the slide. As the rider begins to lift the slide, the very high—and I mean close to sonic—velocity under the barely open slide edge easily lifts and vigorously atomizes fuel from the idle orifice there.

The Amal GP carburetor (Amal Carburetor Company/)

In the Mikuni VM, the space above the main fuel system orifice is lower than in Amal’s GP, about 10 millimeters high. But the air velocity through it is also too low to lift and effectively mix fuel at low throttle openings. Therefore the VM also needs a separate idle fuel system, located just downstream from the engine-side edge of the throttle slide.

These separate idle systems continue to deliver fuel until the slide lifts high enough to raise air velocity sufficient to lift and atomize fuel from the main fuel system’s orifice.

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A similar situation exists in carburetors with shaft-mounted butterfly throttle plates. At small throttle angles, air velocity over the main fuel system orifice, located upstream from the throttle plate, is too low to lift fuel. But right at the edge of the throttle plate as it cracks open, very high velocity exists, ideal for lifting and atomizing fuel. So an idle fuel system has its orifice(s) located close to that edge, to take advantage of those ideal conditions. As in the slide carburetors, the idle mixture is adjusted by a screw that controls either idle air, as in the Mikuni, or idle fuel. If engine idling is necessary, as for street-operated vehicles, an adjustable throttle stop screw can hold the throttle plate open just enough to achieve that.

The Lectron: Full Vacuum All the Time Makes Its Appearance

The Lectron carb, which hit the racing scene in 1977, had the goal of smoothing the transition from a separate idle system to the main system, often a difficult period. The Lectron is a “single-point-metered” carburetor; in contrast with cylindrical-slide carbs, a Lectron’s main (only!) fuel system orifice is downstream of its T-section flat slide, so that it is subjected to full engine intake vacuum at all times. The needle is not tapered but cylindrical, and its downstream side bears a flat milled at a small angle. Because the needle closely fits into its bore in the carb body, the only pathway for fuel is provided by the tapered and milled flat of the needle.

A bank of Lectron carbs for drag racing, complete with intake bells tapped for nitrous oxide. (Lectron/)

When the slide is closed, the needle, which is attached to the slide, closes the fuel orifice. As the slide lifts, the thin end of the milled flat allows a tiny flow of fuel to mix with the air moving rapidly under the lifting slide. When the angling of the milled flat is correct, the fuel flows and air flows remain in reasonable proportion over the full range of the slide lift. No idle system is necessary, because the needle orifice is always under engine vacuum, assisted by high-speed airflow passing under the slide. Lectrons were noted for their excellent throttle response and part-throttle mixture.

Mikuni “Zero Cutaway” Solution

In 1981 Mikuni found a way to the same goal while retaining the original cylindrical-slide geometry in production VM castings. The cylindrical throttle slide in a zero-cutaway carb closes completely at its upstream edge (that is, there is zero cutaway) and a large square opening is cut into the bottom of the downstream slide edge.

Round-slide Mikuni VM carburetors (Mikuni/)

This places the main fuel system under full engine vacuum, as in a Lectron, and thereby is able to deliver initial fuel without a separate idle system. All the passages for the normal VM idle system were present, but the usual idle jet was replaced by a solid plug. The main system fuel needle was replaced with one shaped to work in the new way. The zero-cutaway carb’s major problem was that because the whole area under the slide was under engine vacuum, lifting the slide required considerable force. The carburetors otherwise worked very well indeed.

We may think that carburetors are outdated instruments, but understanding what goes on inside them helps us to understand larger principles of how our engines work.

 

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