Basic Explanation
  In-Depth Explanation

Carbs / Fuel System - In Depth


More In-Depth Explanation

Basically, there are usually three carburetor systems in one carburetor. The first, or primary, provides the proper gas/air mix when the bike is idling (or when it's at a very low rpm), the next provides gas/air as you open the throttle (the middle range), and the last provides the proper mixture as you scream by that geek in the BMW at full throttle who thinks he can out run you at 120 mph (the writer of this article admits to breaking no laws, just dreaming about it!). The three carburetor parts are called the pilot jet (or idle jet), the needle jet, and main jet. The pilot and main jet are just fixed nozzles, while the needle jet has a needle that slides up and down, depending on the amount of vacuum provided. The throttle, which more accurately is called the "butterfly valve" (because it has a central pivot point and two "wings" off of the center that essentially form a circular disk), is primarily responsible for letting the air flow, and hence the vacuum, increase and decrease in the carb. When the throttle is "closed," there still is air that comes in through the "bleed, or pilot screw" and the gas comes up through the pilot jet. So, even when you think the throttle is closed, there still are bypasses in the carb to allow the air and gas to flow to the engine (otherwise your engine would die).

As you open up the throttle, more gas is sucked up by vacuum through the needle jet. The rate at which the gas travels up through the needle jet is determined by the position (and condition) of the jet needle (different than the needle jet!), which travels up and down inside the needle jet as you open (up) and close (down) the throttle, or more accurately, as the vacuum in the carb venturi increases and decreases. The main jet takes over usually around 80-90% throttle open. It is the largest of the jets (and has the largest hole), and therefore lets the largest amount of gas pass through. Think of the jets in a carb like a nozzle on an aerosol can, just in reverse. Instead of pressurizing the can and forcing the liquid through the nozzle and atomizing into the air, a carb provides vacuum to suck liquid through a finely tuned nozzle and atomizes it. The diameter of the holes in the jet nozzles varies from less than a millimeter to maybe a couple or few millimeters. You can see why getting gunk in there really messes things up! Take a 1 millimeter chunk of goop. That’s all it would take to fully clog one of your pilot jets on your bike. Now the bike might not stop running, but you may notice that it is idling very poorly. Ok, so you basically understand the innards of the carb. So how does the gas get into the carb and up the jets? The gas is sucked up through all of the jets from the float bowl, or float chamber. Think of it as a mini gas tank for each carb. When the fuel level in the float chamber drops to a certain level, the floats will fall down by gravity and pull on the bottom of the needle valve (again, different than the needle jet and the jet needle…don’t blame me, I didn’t name them!). The needle valve is a small (usually less than a half inch long and a quarter inch in diameter) metal needle that pushes up and seals on a rubber "seat". The rubber "seat" is pretty much just a small rubber O-ring. Anyhow, the floats pull down on the needle valve to let more (liquid) fuel into the float chamber. The fuel will then flow by gravity from the fuel tank (as long as your fuel valve is on PRIme or the vacuum of the engine is holding the fuel valve open). You can see why dirty carb parts make for a poorly running carb – there are a lot of little pieces that operate on nothing but gravity and air to work! The choke. The choke used to always be literally a choke. Originally, it was a smaller butterfly valve that sat at the inlet of the carb. On the small engines for a lot of yard equipment, and for many motorcycles, this is still how the carbs are choked. By closing the choke, you "choke" off some of the air coming into the carb, thereby forcing the cylinder to pull a higher ratio of gas into the engine, making it easier for a cold engine to start and run while it was warming up (but gives poorer performance at higher rpms). Some newer (actually, they’ve been doing it for years) bikes will use a small needle valve (separate from the one that lets fuel into the bowl) to allow more fuel to flow through the carb using small passage-ways that bypass the regular method of metering the gas. Regardless of the method of choking, they are still usually controlled by a single cable that is attached to a cam at the carbs via a set screw (to set the amount of slack in the cable – and there should be some slack – usually less than 1/8") and run up to your handlebars somewhere where there is a lever. 


A simplified Carb Diagram (click for a larger view)


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Last Update: Wednesday, December 14, 2005 11:53 PM