PRINCIPLES OF MODERN MOTORCYCLE CARBURETOR
Whatever the motorcycle or automobile, virtually all carburetors (or “carbs” for short and not to be confused with the “carbs” which can affect your waist-line) work on the same principles and use similar internal systems to deliver fuel in the proper air/fuel ratio to the engine. Depending on the manufacturer, the actual components within the carb(s) that use those principles do vary somewhat, but
their ultimate execution remains the same. They can be broken down into separate “circuits”. Like electrical circuits, they have defined paths of flow, cause and effect. The Road Star uses a Mikuni 40mm CV-type carburetor. “CV” stands for constant velocity and refers to the theoretically constant speed of the air that passes under the slide. But as you read further, you’ll see that the actual air speed does vary to some extent. At the outset, it must be mentioned that the OEM carb on the Road Star (and most emissions-legal street motorcycles, since 1978), being a CV-type carburetor, has a few significant design components that separate it from most pre-emissions era carbs and the so-called “race” or “high
performance” carbs which are still available today. Carburetors can use any combination of slide and/or throttle plate to control airflow into the engine. CV carbs have both, while most other designs use either a slide OR throttle plate. If they have manually controlled slides they’re typically called either “slide type” or “throttle slide” carbs. The designs that have no slide at all, but use only a throttle plate (or “butterfly”) to control airflow are typically called “butterfly” carburetors.
The OEM Mikuni 40mm Carburetor, as viewed from the intake side. Note: the needle has been removed in the carb above. If it were in place, it would protrude from the slide, down into the needle jet below the slide (and above the empty port at 6-O’clock). The throttle plate in a CV carburetor is a flat plate that pivots in the bore of the carb and, when nearly vertical, almost closes off the airflow into the intake tract, limiting intake flow to just what the engine needs to maintain a consistent idle. When it is opened all the way (directly inline with the carb’s
bore) it allows maximum airflow into the engine. In the case of the CV carb, the throttle plate is downstream of the slide, so maximum airflow requires that the throttle be fully opened and that the slide rises to its highest position as well. As a rule, those two requirements do occur at about the same time because you control the throttle plate with your right hand and the slide rises in response to the opening of the throttle butterfly. Non-CV carbs use either a rider-controlled slide as the throttle or they have no slide at all and use only a butterfly valve. Those carbs that have no slide at all are the simplest and oldest design of all carburetors and resemble the ones used on lawn mowers and other engines that don’t
require frequent changes in throttle control. Such designs are primitive, because they lack the precision control of fuel & air needed to pass emissions requirements or to give smooth well-controlled engine response, but they do work well in supplying massive amounts of air to engines made to make a lot of power (supercharged configurations being the best example).It bears mentioning that some of the old types of (mostly) pre-emissions carburetors have a choke plate near the mouth of the carb. It resembles the throttle plate of a CV carb, but it is upstream of any slide or throttle plate and it actually chokes off most of the air from entering the intake tract, hence the name “choke” which we still use to this day, even after the real chokes have been replaced with fuel enrichment systems.
Ken “the Mucker” Sexton
Specifically in application to the Yamaha Road Star)