Secret Ninja 250 Carburetor Passages

Here are the secret passages in the carburetors. They are kind of hard to figure out sometimes and this picture guide shows where they are for cleaning purposes.

When cleaning your Ninja 250 carburetors, all of these passages need to be cleaned so that gas can get through them unobstructed. You can usually do this with spray carburetor cleaner. In some cases, a very thin copper wire can be used for stubborn clogs.

It is recommended that the diaphragms and plastic caps be removed prior to cleaning to prevent damage to the expensive rubber parts.

There are three jets in each carburetor - pilot, main and choke. The choke jet is pressed into the body and does not come out. The main jet is easy to remove with a socket and a large flat head screwdriver. It is actually two parts. The pilot jet is a little more difficult and requires a thin but strong flat head screwdriver.

On both the pilot and the main jet assembly, there are little holes in the ends and sides that must be clear. They are easy to miss sometimes so look carefully to find all of them.  The choke jet can be cleaned with a thin copper wire poked through the end followed by a squirt of carb cleaner.


 Carb Pilot jet Circuit

Carb Main Jet Circuit

Carb Fuel and Overflow Drain

Carb Diaphragm Intake, upper choke and vacuum ports

Carb Choke Circuit

Not pictured is the coasting enrichener circuit. It usually does not need to be cleaned, but because it contains a small diaphragm, you should be careful not to get carburetor cleaner inside it.

Ninja 250: Petcock Problems

The petcock's job is to control the flow of gasoline from the tank to the carburetors. The gas in the tank flows by gravity so even after the engine is turned off, the gas will continue to flow unless something stops it. On motorcycles of yesteryear, the petcock was simply a valve that the rider had to remember to turn off after every ride. On newer bikes like the Ninja, a vacuum diaphragm is used to stop the flow of gas when the engine is not running.

The knob settings on the Pregen (1988-2007) petcock are: OFF, ON, and RESERVE. The OFF setting will shut the flow off completely regardless of the engine running status. There are two intake ports on the Pregen petcock. One is higher than the other. The purpose of this is to provide reserve fuel that is hopefully enough to get to the nearest gas station. The ON selection takes gas from the upper intake. RESERVE takes gas from the lower intake. Both ON and RESERVE require a vacuum from the engine to allow any gas to flow.

The Newgen (2008-2012) has only one pickup at the bottom. The knob settings are: OFF, ON and PRIME. The PRIME selection is a special selection that bypasses the vacuum cutoff and allows gas to free flow regardless of engine run status. PRIME is used to prime the carburetors if they are dry. You should not run the bike when set to PRIME. The ON and OFF selections work the same as the Newgen. There is no RESERVE selection on a Newgen.

Symptoms of a Bad Petcock

The most common symptom is fuel dripping from the carburetors or the crankcase filling up with gas. This is caused when the petcock fails to shut off the gas when the engine is off. This can be caused by several things. The diaphragm o-ring could be bad or the spring that pushes it into its valve seat could be weak. Sometimes the diaphragm itself could be bad allowing gas to leak through the vacuum port and into the carburetor.

A less common failure is when the petcock becomes clogged somehow and the gas wont flow at all or at a reduced rate. This will cause a no-start condition, a poor running condition or a situation where the bike starts fine, but then stalls a few minutes later. The clog can be anywhere from the intake ports to the vacuum port.

The diaphragm itself is a very thin membrane that can tear easily. When it goes bad, gas can be sucked into the vacuum line to the carburetor, or you could also see gas dripping from the petcock body.

In some cases, the rubber gasket under the knob can wear out. When this happens, the knob selections can operate incorrectly such as the ON position drawing gas from the RESERVE intake port or the OFF position allowing the gas to flow.

Caution

If the petcock has allowed gas to run into the crankcase, then you should not run the engine until you have repaired the petcock and changed the oil. Gasoline is a very poor lubricant and your engine could be damaged if you run it that way.

Testing the Petcock

To test the petcock to see if it is operating correctly, you can do the following procedure:

1. Turn petcock to ON (with plenty of fuel in tank).
2. Remove big hose. Gas should NOT come out - except for maybe a few drops that might be in the hose. There should be no continuous drip, but have a glass jar handy to catch any gas just in case.
3. Remove the small hose. There should be absolutely no gas come out of the small hose and port - not even a drop.
4. Attach a 3/16" diameter test hose, about two feet long, to the small petcock port.
5. With your mouth (or syringe if you have one), apply suction to the small tube (again, there should be no gas in the small tube). Gas should flow out of the big tube in a nice thick stream.
6. Stop applying suction. The gas should cut off.
7. For Pregens, switch to RESERVE and repeat the test.
8. The gas that comes out should be free from any debris. If its clear, you can pour it back into the tank.
9. Lastly, switch to OFF and apply suction to the small tube. This time there should be no gas come out.

If the petcock does not operate as described, then you need to remove it and repair it.

Rebuilding

If your petcock is bad, you can either rebuild it or buy a new one. I don't recommend buying used ones because they could be just as bad as the one you are replacing. If you rebuild, be advised that the parts aren't cheap.

There are some rebuild kits out there, but generally, its cheaper to buy just the parts you need from the dealer. Be advised that the Oldgen (1986-1987) used a different diaphragm so rebuild kits for it will not work with the Pregen (1988-2006).

Whatever you do, don't be tempted to use RTV to seal any part of the petcock. RTV is not gasoline resistant and will clog up the works and/or leak.

Finally, if you remove the petcock from the gas tank to rebuild it, be advised that the washers on the bolts that go into the tank are special washers that have been coated with a special gasoline proof rubber. These are sometimes not reusable and should be replaced. If you replace them with hardware store washers, expect them to leak. The main petcock gasket is usually reusable, but occasionally, it has to be replaced as well.