Hawk Talk

Ariel Red Hunter · 05-01-2019, 09:41 PM

Getting acquainted with carburetors. The thought crossed my mind that there is a large percentage of new riders that have never owned anything with a carburetor, unless it was a weed whacker or a lawn mower. And those carburetors work different. The motor cycles we are going to talk about use piston valved carbs. Instead of having a "butterfly" throttle plate, they use a round piston, which is raised or lowered to control the amount of air/fuel mixture to get more power or less power output. So, perhaps the carburetor can be described as the instrument that mixes the gasoline and air in the correct proportions to support combustion. While every body knows that gasoline is extremely flammable, not everyone knows that there is a mixture of volatile gas vapor with air that is virtually explosive. That mixture is between 12 to 1 to 13.5 to 1. That is 12 POUNDS of air to 1 pound of gasoline. Or 13.5 Pounds of air to one pound of gasoline. The carburetor can do this job very nicely, although it took a long time to work it out. About 25 years, believe it or not. All of the carburetors that come on China bikes are capable of performing the fuel mixture function very well. And why not? The basic design of today's carbs were applied and understood by 1927. Then they spent the next 92 years figuring out how to make it cheaper, but still funtional, and they have done a pretty good job of it. Yes, China bike carbs come too lean, but that is to pass emmission regulations. Basically they break down as two tubes, one if which is open to the atmosphere on one end and open to the engine on the other end. At 90 degrees to the first tube the second tube, threaded on one end, is attached about in the middle of the first tube through a hole cut in the horizontal tube, the joint soldered up and a cutter used to trim the vertical tube so that the interior of the first (horizontal) tube is very clean and clear. Next a cast brass venturi is sweated in the horizontal tube with pre-cut holes or ports designed to fulfill various funtions which we will explore later. A piston is made to fit in the vertical tube. It was made to accept the end of a cable that aallowed it to slide up and down a lite spring helps to close the piston, which goes far enough down the vertical tube to shut off all air trying to get to the engine. This worked but not very well, so a tapered needle was hung from the bottom of the piston in order to lean out the part throttle over richness of the mixture. Underneath the horizontal tube was hung (literally) a float bowl in which there was a brass float, all soldered together which controlled the height of fuel in the float bowl. As the fuel was consumed, the float dropped, allowing more fuel to flow into the bowl controlled by a tapered rod at the bottom of the float bowl. No needle and seat like now. Attached to the bottom of the horizontal tube was a small brass tube that was in perfect alignment with the tapered needle hanging from the piston valve. Although that brass tube is called a needle jet it's not really a jet at all. It could be called a jet holder, beause the main jet is screwed to the bottom of this tube. But it is called the needle jet because originally that is the way it controlled fuel flow, but by 1927 they figured out how to use the needle it self as the restrictor, AND so make it possible to have the midrange fuel mixture controlled by a fixed needle in the piston valve, instead of the earlier way of controlling mixture by setting the air for the power you wanted first, and the adjusting the fuel flow by manipulation of a second lever, controlling the needle. Next a pilot jet was added to give better control of the mixture below 1/4 throttle. The main jet feeds just after the main restriction in the venturi, the pilot circuit and idle air or fuel jet feeds a little farther into the venturi section. Air rushing through the venturi suffers a loss of pressure, so the fuel comes up through the various jets because THE PRESSURE IS HIGHER IN THE FLOAT BOWL THAN AT THE VENTURI. Hopefully, that's more than you needed or wanted to know about your carb...ARH

For your kind attention.

05-01-2019, 09:41 PM	#15
Ariel Red Hunter Join Date: Jun 2016 Location: akwesasne, NY-13655 Posts: 2,220	For the new guys. Getting acquainted with carburetors. The thought crossed my mind that there is a large percentage of new riders that have never owned anything with a carburetor, unless it was a weed whacker or a lawn mower. And those carburetors work different. The motor cycles we are going to talk about use piston valved carbs. Instead of having a "butterfly" throttle plate, they use a round piston, which is raised or lowered to control the amount of air/fuel mixture to get more power or less power output. So, perhaps the carburetor can be described as the instrument that mixes the gasoline and air in the correct proportions to support combustion. While every body knows that gasoline is extremely flammable, not everyone knows that there is a mixture of volatile gas vapor with air that is virtually explosive. That mixture is between 12 to 1 to 13.5 to 1. That is 12 POUNDS of air to 1 pound of gasoline. Or 13.5 Pounds of air to one pound of gasoline. The carburetor can do this job very nicely, although it took a long time to work it out. About 25 years, believe it or not. All of the carburetors that come on China bikes are capable of performing the fuel mixture function very well. And why not? The basic design of today's carbs were applied and understood by 1927. Then they spent the next 92 years figuring out how to make it cheaper, but still funtional, and they have done a pretty good job of it. Yes, China bike carbs come too lean, but that is to pass emmission regulations. Basically they break down as two tubes, one if which is open to the atmosphere on one end and open to the engine on the other end. At 90 degrees to the first tube the second tube, threaded on one end, is attached about in the middle of the first tube through a hole cut in the horizontal tube, the joint soldered up and a cutter used to trim the vertical tube so that the interior of the first (horizontal) tube is very clean and clear. Next a cast brass venturi is sweated in the horizontal tube with pre-cut holes or ports designed to fulfill various funtions which we will explore later. A piston is made to fit in the vertical tube. It was made to accept the end of a cable that aallowed it to slide up and down a lite spring helps to close the piston, which goes far enough down the vertical tube to shut off all air trying to get to the engine. This worked but not very well, so a tapered needle was hung from the bottom of the piston in order to lean out the part throttle over richness of the mixture. Underneath the horizontal tube was hung (literally) a float bowl in which there was a brass float, all soldered together which controlled the height of fuel in the float bowl. As the fuel was consumed, the float dropped, allowing more fuel to flow into the bowl controlled by a tapered rod at the bottom of the float bowl. No needle and seat like now. Attached to the bottom of the horizontal tube was a small brass tube that was in perfect alignment with the tapered needle hanging from the piston valve. Although that brass tube is called a needle jet it's not really a jet at all. It could be called a jet holder, beause the main jet is screwed to the bottom of this tube. But it is called the needle jet because originally that is the way it controlled fuel flow, but by 1927 they figured out how to use the needle it self as the restrictor, AND so make it possible to have the midrange fuel mixture controlled by a fixed needle in the piston valve, instead of the earlier way of controlling mixture by setting the air for the power you wanted first, and the adjusting the fuel flow by manipulation of a second lever, controlling the needle. Next a pilot jet was added to give better control of the mixture below 1/4 throttle. The main jet feeds just after the main restriction in the venturi, the pilot circuit and idle air or fuel jet feeds a little farther into the venturi section. Air rushing through the venturi suffers a loss of pressure, so the fuel comes up through the various jets because THE PRESSURE IS HIGHER IN THE FLOAT BOWL THAN AT THE VENTURI. Hopefully, that's more than you needed or wanted to know about your carb...ARH For your kind attention.
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