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FrugalNinja250 · June 7th, 2016, 08:12 PM

You need some basic information, your experience and knowledge of fuel and air mixing in carburetors is severely impaired.

The way a carb works is that it has a narrow area where the air goes through. According to Bernoulli's principle https://en.wikipedia.org/wiki/Bernoulli's_principle when you have the same amount of air going through a restriction the air speed increases while at the same time the pressure decreases. This creates a vacuum. That vacuum sucks fuel from the carb bowl through a series of air mixing jets and tubes, atomizing the fuel and mixing it with the air stream so that when it arrives in the cylinder it is mixed at the right ratio to get maximum combustion and heat. This is called stochiometric and with gasoline it's about 14.7 parts air to 1 part gasoline. If you have more or less fuel for the amount of air then it's rich or lean, either way the amount of fuel burned is decreased as is the power and efficiency. https://en.wikipedia.org/wiki/Air%E2%80%93fuel_ratio

In car carburetors the air flow restriction, or venturi, is always the same size. This means that as you open and close the throttle the speed of the air flowing through the venturi varies dramatically, and that variation also dramatically affects vacuum. Since that vacuum draws fuel from the bowl, car carbureters normally have several different jets and metering circuits to ensure the proper amount of fuel for the air entering the motor at differing speeds.

Motorcycle carbureters are fundamentally different in that instead of having a fixed venturi and many fuel circuits and jets, they often have instead a variable-sized venturi and just one jet. The goal of the variable venturi carb is to keep the air speed through the venturi always the same, keeping the vacuum the same. These are referred to as CV carbs, for Constant Velocity.

The slides are the means of changing the venturi size, and they are operated by vacuum, not by the throttle cables. If the slide diaphragms are damaged then all sorts of problems occur.

I think you will end up spending far more trying to get this bike to work on one carb than you would saving up for a year (if needed) and buying a proper set of carbs for it. If you cannot (or refuse to) get factory carbs, look for a pair of carbs with the same size bores as the factory carbs, but you will need a lot of jet tuning since the amount of fuel for the air is related to horsepower output. IIRC the Ninja 250 is rated around 35HP at the crank, so looking for carbs from single cylinder motors rated at 17-18HP will get you in the ballpark. You'll also need to come up with a system to join and synchronize the carbs, etc.

June 7th, 2016, 08:12 PM	#25
FrugalNinja250 ninjette.org certified postwhore Name: Frugal Location: Dallas/Fort Worth (DFW) Join Date: Mar 2010 Motorcycle(s): Several Posts: A lot.	You need some basic information, your experience and knowledge of fuel and air mixing in carburetors is severely impaired. The way a carb works is that it has a narrow area where the air goes through. According to Bernoulli's principle https://en.wikipedia.org/wiki/Bernoulli's_principle when you have the same amount of air going through a restriction the air speed increases while at the same time the pressure decreases. This creates a vacuum. That vacuum sucks fuel from the carb bowl through a series of air mixing jets and tubes, atomizing the fuel and mixing it with the air stream so that when it arrives in the cylinder it is mixed at the right ratio to get maximum combustion and heat. This is called stochiometric and with gasoline it's about 14.7 parts air to 1 part gasoline. If you have more or less fuel for the amount of air then it's rich or lean, either way the amount of fuel burned is decreased as is the power and efficiency. https://en.wikipedia.org/wiki/Air%E2%80%93fuel_ratio In car carburetors the air flow restriction, or venturi, is always the same size. This means that as you open and close the throttle the speed of the air flowing through the venturi varies dramatically, and that variation also dramatically affects vacuum. Since that vacuum draws fuel from the bowl, car carbureters normally have several different jets and metering circuits to ensure the proper amount of fuel for the air entering the motor at differing speeds. Motorcycle carbureters are fundamentally different in that instead of having a fixed venturi and many fuel circuits and jets, they often have instead a variable-sized venturi and just one jet. The goal of the variable venturi carb is to keep the air speed through the venturi always the same, keeping the vacuum the same. These are referred to as CV carbs, for Constant Velocity. The slides are the means of changing the venturi size, and they are operated by vacuum, not by the throttle cables. If the slide diaphragms are damaged then all sorts of problems occur. I think you will end up spending far more trying to get this bike to work on one carb than you would saving up for a year (if needed) and buying a proper set of carbs for it. If you cannot (or refuse to) get factory carbs, look for a pair of carbs with the same size bores as the factory carbs, but you will need a lot of jet tuning since the amount of fuel for the air is related to horsepower output. IIRC the Ninja 250 is rated around 35HP at the crank, so looking for carbs from single cylinder motors rated at 17-18HP will get you in the ballpark. You'll also need to come up with a system to join and synchronize the carbs, etc.