THE SECRET LIFE OF CARBURETORS: A/K/A CARB THEORY AND OPERATION MADE EASY: Or, why carbs suck: At first glance, you might read the title of this article and say "so what?" You're not really interested in why carbs operate as they do; you just want them to perform properly. But wait, grasshopper: in order for you to appreciate both the what and the why of proper carb rebuilding techniques, then you need to also have a basic understanding of what a carb is trying to do, so you'll have a clearer understanding of why you need to pay close attention to what might seem like the incredibly picky details and procedures that are recommended as part of a proper carb rebuilding process.......which you can read all about here: http://xjbikes.com/forums/index.php?threads/14692 But before you go there, please continue reading here...... I'm Already Denying It; I Should Have Been A Politician: We started out by saying that "carbs suck". Well, as we all know, that's not really true. It's the engine that sucks, and it's that "vacuum signal" from the engine----that "suck"----that controls and tells the carb "when" and "how much" they're needed to do in order to correctly perform their dirty, fuel-and-airish deeds. And if your carbs are not ready to properly respond, or if they tend to respond in a non-appropriate or non-reliable or non-coordinated manner to that sucky signal, well, that's the root of all evil, now isn't it? Nature Abhors One: All About A Vacuum: Actually, there's no such thing as a "vacuum" in nature (no, not even in the new lightweight 8-pound Oreck model that you can try at home for FREE! for 30-days........), but there are areas of higher or lower fluid pressure........."higher" or "lower" being meaningful only in comparison to some other nearby or related area........and what everyone likes to refer to as a "vacuum" really means "an area of lower pressure". Areas of low pressure----just like you see and hear about on TV when the weather-person is talking about weather systems (and a hurricane, as you may know, is the "mother of all low-pressure areas")----will "draw in" a fluid---and physically-speaking, air is a type of "fluid"----from the surrounding areas of "higher pressure". Note that we haven't said anything at all about HOW MUCH pressure we're talking about.......nothing about p.s.i . or inches-of-vacuum or the like.......no, all that matters is that one area is at a lower relative pressure to some other area. Now, in an engine, the monster that is responsible for the creation of a "lower pressure area" is the piston and it's cylinder. As the piston is mechanically (via the crankshaft and the connecting rod) drawn down into the bowels of the cylinder, the ring-tight seal of the piston to the cylinder walls evacuates the area of the cylinder above it. Well, even the word "evacuate" isn't good, so let's try this: when a piston is at the very top of it's stroke, and before the piston moves down into the cylinder, there was, let's say, 3cc's of total area between the top of the piston dome and the cylinder head combustion chamber. And, there was 3cc's total volume of air in that space, too. Good. Now let's say the piston moved all the way down to the bottom of that cylinder. Now, there's maybe 120cc's of total area in that cylinder (between the top of the piston dome and the cylinder head combustion chamber), but still only 3cc's of air volume to fill this newly expanded area! That air thus becomes "thinned", in essence, and now we ram head-on into one of the basic laws of physics: A fluid (and once again, air is a "fluid" in physical terms) will undergo a decrease in pressure as it expands or is "thinned". Hey, I didn't make up the laws of physics, I'm just reporting them....... So the downward movement of the piston within the cylinder "stretches" and "thins" the amount of air that's in the cylinder. And this "thinned" air loses pressure. Bingo! A "low pressure area" has been created. Your piston, in a conspiritorial bond with both it's rings, the cylinder walls, and the cylinder head, in a plot worthy of a Tom Clancey novel, has just created a "vacuum", better thought of as an "area of low pressure".