Post by chanito on Aug 12, 2007 13:55:56 GMT -5
I decide to put all the info i had on the ruckus/metropolitan carburator and make it a sticky
First thing first, i am not a CV carburetor expert, so this is just the way i do it after pestering some pros i know and reading most of the info on this kind of carburetors, so if you disagree please post your way to do it so we can find the easiest
Doing research for this thread i found out that the word carburetor was develop from the French carbure, that means mix with carbon, which i think was a great way to create combustible back in the 18 century, anyway later on some miners develop one of the most clever ways to light up an underground mine and that kind of lamp was still used not to long ago as it last way longer than battery operated ones, this lamps used a metering jet to allow water to drip into a container full of carbide pellets, which them released a gas, that was metered again thru a jet and lighted to give a very bright light, they used to called this lamp carburetors so the first carb used on cars adapted that name
The carburetor on our scooters is what is called a bleed type CV carburetor, which stands (according to who you ask) for "Constant Vacuum" or "Constant Velocity"
They are called bleed type because they have this small holes on the sides of the jets that allows air to be premix with the gas, a process called emulsification, which makes this carb way smoother than the old style were the jet holders were solid (did not have the holes)
These holes get their air supply from two different air jets, the one for the main jet is on the front of the carburetor and gets its air directly from the intake.
The slow jet gets its air from an air jet located on the floor of the diaphragm chamber and is very easy to get clogged, and that is the reason i think is a very bad idea to connect the PCV discharge hose to the fresh air hose to the carburetor diaphragm chambers as some aftermarket air filter do
Our carburetors have 3 different fuel jets, but we can only change two, they are the main jet and the slow jet, the third jet is the cold start jet, but it is just a hole drilled on the carb, so the only adjustment would be to drill that jet bigger, but, i do not recommend doing it, because if you mess up, the carb is useless. This are the gas jets from top to bottom, main jet, slow jet and SE jet
One of the things i found during this research, was how misunderstood the slow system is.
The main purpose of the slow system is to supply mixture at idle. It will continue to provide fuel during the entire throttle range, but after about 1/8 of the throttle the percentage of fuel supply by this system becomes insignificant, as more and more fuel is supply by the way bigger MAIN SYSTEM. At idle the butterfly valve is covering some very small holes, these are called transition holes and allow some fuel to be suck into the engine at very small throttle openings, so when you adjust the idle speed with that small screw on the side of the carb, you are adjusting the covering or uncovering of these small holes
as how more or less air goes pass the butterfly, further back toward the engine there is a bigger hole that allows fuel to be send into the engine, that is controlled by the idle mixture screw, this hole also allows the SE valve to dump extra fuel on cold starts, this SE (Start Enrichment) system gets its fuel from the SE jet and is allow to dump it into the engine until the ECM senses the temp going above 35 degrees Celsius and send a voltage to the SE valve which heats up and closes this circuit. The slow jet is always on, but is feeding gas thru the idle mixture discharge hole (the one facing the engine), and the transition holes, some times modifications make the engine produce more vacuum so the signal to this holes is greater, and they discharge more gas than before (that is the reason why in theory, installing a freer flowing exhaust pipe should require a smaller main jet) as more gas is being suck from the slow jet.
The trick is just keep correcting your idle speed with any major adjustment of the idle mixture screw, all my numbers indicate that the stock slow jet should flow plenty even for a big bore, but in any case, if you need to adjust your slow jet more than 1 turn out from the factory setting, go to the next bigger size.
So when an engine is warm all the fuel is supply by the slow jet and the main jet I hope you are still with me so far
I really need to sit down and go over this carburetor here, but it will be long, i will start with some boring stuff so it will be out of the way. your slow jet feeds the idle mixture screw and the transition ports, this ports are small holes before and after the butterfly, so they will discharge gas as you open the throttle, however due to the size, most of the fuel discharge is done by the idle mixture port, this is important because adjusting the idle mixture will cure a lot of problems now blamed on the slow jet, one of the most common is the stumble at take off when switching intakes, usually 1/8 to 1/4 of a turn is all it takes to have that fix, modifying the exhaust will make the carb more sensitive and it will actually run rich at part throttle, that is why i think adjusting the mixture is way better than start playing with the slow jet
Before due to the prohibitive cost of the tool, adjusting the mixture was hard, now with a cheap tool alternative not adjusting it is a crime.
This is my opinion, so if you disagree, i respect your opinion, but my experience backs mine. Reading spark plugs, is not easy, they can give you a lot of information, but it requires a lighted magnifying glass to really read one properly, and usually you need to start with new ones every time (it can get really expensive)
Also a lot of people are under the impression that the needle moves out of the way totally at wide open throttle, it does not, doing it will mean a poor choice of needles, you need the needle to stay on the carb, so when you close the throttle the needle does not go out of place and get stuck out of the hole preventing the slide from sliding down the taper and size of the tip is what determine the efficiency of the needle. How the carb works at part throttle is the work of the spring and the hole at the bottom of the slider, hope this clarify some of the mysteries of the carb
I would love to tell you, the guys at Honda did a poor job and a quick adjustment will bring the scooter to life, but in reality this is a very well done 50 cc scooter and it runs perfect stock, so it takes a lot of work to improve the performance of the engine My goal with this article is just let you guys know how the carb works so when you do any kind of job to increase performance you will know what is involved
First thing first, i am not a CV carburetor expert, so this is just the way i do it after pestering some pros i know and reading most of the info on this kind of carburetors, so if you disagree please post your way to do it so we can find the easiest
Doing research for this thread i found out that the word carburetor was develop from the French carbure, that means mix with carbon, which i think was a great way to create combustible back in the 18 century, anyway later on some miners develop one of the most clever ways to light up an underground mine and that kind of lamp was still used not to long ago as it last way longer than battery operated ones, this lamps used a metering jet to allow water to drip into a container full of carbide pellets, which them released a gas, that was metered again thru a jet and lighted to give a very bright light, they used to called this lamp carburetors so the first carb used on cars adapted that name
The carburetor on our scooters is what is called a bleed type CV carburetor, which stands (according to who you ask) for "Constant Vacuum" or "Constant Velocity"
They are called bleed type because they have this small holes on the sides of the jets that allows air to be premix with the gas, a process called emulsification, which makes this carb way smoother than the old style were the jet holders were solid (did not have the holes)
These holes get their air supply from two different air jets, the one for the main jet is on the front of the carburetor and gets its air directly from the intake.
The slow jet gets its air from an air jet located on the floor of the diaphragm chamber and is very easy to get clogged, and that is the reason i think is a very bad idea to connect the PCV discharge hose to the fresh air hose to the carburetor diaphragm chambers as some aftermarket air filter do
Our carburetors have 3 different fuel jets, but we can only change two, they are the main jet and the slow jet, the third jet is the cold start jet, but it is just a hole drilled on the carb, so the only adjustment would be to drill that jet bigger, but, i do not recommend doing it, because if you mess up, the carb is useless. This are the gas jets from top to bottom, main jet, slow jet and SE jet
One of the things i found during this research, was how misunderstood the slow system is.
The main purpose of the slow system is to supply mixture at idle. It will continue to provide fuel during the entire throttle range, but after about 1/8 of the throttle the percentage of fuel supply by this system becomes insignificant, as more and more fuel is supply by the way bigger MAIN SYSTEM. At idle the butterfly valve is covering some very small holes, these are called transition holes and allow some fuel to be suck into the engine at very small throttle openings, so when you adjust the idle speed with that small screw on the side of the carb, you are adjusting the covering or uncovering of these small holes
as how more or less air goes pass the butterfly, further back toward the engine there is a bigger hole that allows fuel to be send into the engine, that is controlled by the idle mixture screw, this hole also allows the SE valve to dump extra fuel on cold starts, this SE (Start Enrichment) system gets its fuel from the SE jet and is allow to dump it into the engine until the ECM senses the temp going above 35 degrees Celsius and send a voltage to the SE valve which heats up and closes this circuit. The slow jet is always on, but is feeding gas thru the idle mixture discharge hole (the one facing the engine), and the transition holes, some times modifications make the engine produce more vacuum so the signal to this holes is greater, and they discharge more gas than before (that is the reason why in theory, installing a freer flowing exhaust pipe should require a smaller main jet) as more gas is being suck from the slow jet.
The trick is just keep correcting your idle speed with any major adjustment of the idle mixture screw, all my numbers indicate that the stock slow jet should flow plenty even for a big bore, but in any case, if you need to adjust your slow jet more than 1 turn out from the factory setting, go to the next bigger size.
So when an engine is warm all the fuel is supply by the slow jet and the main jet I hope you are still with me so far
I really need to sit down and go over this carburetor here, but it will be long, i will start with some boring stuff so it will be out of the way. your slow jet feeds the idle mixture screw and the transition ports, this ports are small holes before and after the butterfly, so they will discharge gas as you open the throttle, however due to the size, most of the fuel discharge is done by the idle mixture port, this is important because adjusting the idle mixture will cure a lot of problems now blamed on the slow jet, one of the most common is the stumble at take off when switching intakes, usually 1/8 to 1/4 of a turn is all it takes to have that fix, modifying the exhaust will make the carb more sensitive and it will actually run rich at part throttle, that is why i think adjusting the mixture is way better than start playing with the slow jet
Before due to the prohibitive cost of the tool, adjusting the mixture was hard, now with a cheap tool alternative not adjusting it is a crime.
This is my opinion, so if you disagree, i respect your opinion, but my experience backs mine. Reading spark plugs, is not easy, they can give you a lot of information, but it requires a lighted magnifying glass to really read one properly, and usually you need to start with new ones every time (it can get really expensive)
Also a lot of people are under the impression that the needle moves out of the way totally at wide open throttle, it does not, doing it will mean a poor choice of needles, you need the needle to stay on the carb, so when you close the throttle the needle does not go out of place and get stuck out of the hole preventing the slide from sliding down the taper and size of the tip is what determine the efficiency of the needle. How the carb works at part throttle is the work of the spring and the hole at the bottom of the slider, hope this clarify some of the mysteries of the carb
I would love to tell you, the guys at Honda did a poor job and a quick adjustment will bring the scooter to life, but in reality this is a very well done 50 cc scooter and it runs perfect stock, so it takes a lot of work to improve the performance of the engine My goal with this article is just let you guys know how the carb works so when you do any kind of job to increase performance you will know what is involved