What is carburetor ice?
Carburetor ice is the term for a condition where ice accumulates in the throat of a carburetor. It's most commonly associated with piston-engined light aircraft, but can occur in any engine which uses a carburetor. The Ninja 250 is susceptible to carb ice in certain conditions. Fuel injected engines are practically not susceptible to icing up, although it's theoretically possible to accumulate some ice under extremely adverse conditions.
For the Ninja 250, it appears that a range of about 20° F to 50° F (-7° C to +10° C) with visible moisture (fog or rain) can cause carb ice. Pod-style air filters may exacerbate the problem, since they can allow more moisture into the carbs than the stock airbox, however the shorter intake tract may yield some benefits when considering strictly atmospheric moisture.
If you are riding in conditions where carb icing is frequent, you can learn to predict it quite accurately by carefully observing your local weather conditions. Factors affecting carb icing include:
For example, it has been shown that noticeable carb icing will occur at 450′ ASL approximately 90% of the time when
1. The current temperature is less than 6°C above the dew point
Additionally, for the same altitude, it has been noted that if it is snowing big fat snow flakes, carb icing will almost certainly occur; whereas little tiny powdery snow flakes indicate that it will probably not occur. Larger snowflakes generally correspond to higher relative humidity.
Why is carb ice a problem?
Carb ice is a problem because it constricts the caburetor, choking off the engine in a very literal sense. As the ice builds up, it closes off the passage through the carburetor, reducing the fuel/air mixture the engine can get. Under appropriate conditions, carb ice can entirely kill the engine. Even before that, it reduces the engine's performance enough to be dangerous in any real-world traffic situation. Carb ice is closely analagous to asthma in a living creature.
Carb ice can stall your motor, and there's nothing you can do to get it running again while riding along in traffic! The only cures for carb ice involve many minutes of waiting.
Severe carb icing on the Ninja 250 will also cause the cause the spark plugs to foul badly -- to the point where the motorcycle may not re-start even with a boost after several severe icing episodes. The fact that winter trips (and certainly carb-icing trips!) tend to be relatively short exacerbates this problem; the spark plugs will remain at least partially fouled until they are brought up to their self-cleaning temperature, which may be as high as 850°C. Running a hotter plug in the winter will help the spark plugs reach their self-cleaning temperature more quickly. The NGK CR7HSA is a hotter plug which will not fail in the Ninja 250 motor under commuting riding conditions, however we would not suggest subjecting them to track duty. It is also possible that a platinum plug would be a superior choice for an icing-prone Ninja 250, as they are designed to resist fouling.
When riding in carb-icing conditions, it is suggested you carry a set of pre-gapped spark plugs and sufficient tools to install them at the side of the road. Additionally, a pair of jumper cables which fit your motorcycle's battery are recommended, as it is not unlikely you will kill your battery trying to start the motorcycle after a severe icing episode, and it is generally not safe to try to bump-start a motorcycle in carb icing conditions (cold, wet roads, possibly with snow, probably sandy).
What can be done about carb ice?
If you're riding along and the engine starts running rough or loses power, there's a chance you have carb ice. If switching to reserve doesn't cure the problem, it's very likely your carbs are icing up. On the Ninja 250, other indications are:
The best and safest reaction is to pull over into a safe parking spot and stop the engine. Engine heat will heat up the carburetors, and they should be ice-free in 10-30 minutes, depending on build up, ambient temperature, and engine temperature. Note that in extremely severe weather (well below freezing), engine heat will not be enough to melt the ice in your carburetors, and you may need to abandon your motorcycle and wait for a warmer day to retrieve it (or have it towed to a heated garage).
Be sure to run the motor as soon possible once the ice has cleared. If you let it sit for more than an hour or two after a carb ice incident, the melting ice will rust the inside of the motor. The small amount of water caused by carb ice won't hurt your motor if it's running, although it may run rough while ingesting the water.
If you can't stop, your only choice is to try riding out the ice. Generally, carb icing is most pronounced at moderate throttle openings (1/3 to 2/3 throttle), so if you can change things so you're riding at nearly open throttle or closed throttle, that might help -- however, it could also be extremely dangerous if it causes you to go much faster or slower than the speed of traffic. Chances are pretty good that your carb will continue to ice up regardless of your throttle position, once the ice has started.
Riding an icing Ninja 250 can be particularly frightening. A mostly-closed throttle will almost certainly cause you to stall, however, you can usually re-start the engine by suddenly opening it quite wide. In order to keep the engine running, you will need to keep it at at least 3,000 RPM -- even when stopped for a red light. It will be much noisier than usual, but you must keep the engine revs up. When stopping for a light, do NOT shift into neutral, or you will not be able to shift back into 1st gear without dropping the revs dangerously low (or seriously abusing your transmission). When riding along, you may be able to help clear some of the ice by pulling in the clutch and opening the throttle up for a moment (many times); be sure to rev-match carefully before letting out the clutch or you may find your front wheel pointed skyward. Finally, you may lose/regain a single cylinder at random, potentially causing great excitement while rounding a slippery corner.
The conditions which cause carb ice (20° to 50° F and rain, snow, or fog) can be pretty miserable to ride in; you can prevent carb ice by just not riding in these conditions.
Why does carb ice form?
In order to understand carburetor ice, you have to understand a bit about how a carburetor works. One of the key principles of a carburetor is the venturi, a constriction inside the carb which causes air to flow faster. This drops the air pressure in the venturi, which allows fuel to be sucked from the float bowl through the jet system. The drop in pressure also lowers the temperature of the intake air. This temperature drop is aided by the evaporation of gasoline, which is being atomized by the carburetor to burn in the engine. (Note that fuel injection systems do not have a venturi, instead relying on some variety of pressurized system to atomize fuel. This is the main reason FI is practically ice-proof.)
This temperature drop just past the venturi can be pretty amazing. In airplanes, carburetor ice can form up to 90° F ambient temperatures, and a carburetor can theoretically drop the temperature by up to 100° F. (Fortunately, the Ninja 250 doesn't seem to be anywhere near as prone to carb ice as some airplane carburetors.) This temperature drop cools the body of the carburetor. When the carb body drops below the freezing point (32° F or 0° C), any moisture in the air which hits the carb body has a chance of freezing onto it. Unfortunately, the temperature drop also tends to condense water vapor out of the air, just like water collecting on the side of a cold glass. If there's any water in the fuel, this can contribute to the ice problem. As this occurs, more and more water can freeze to the carb body, gradually building up ice on any of the carb surfaces. The more ice there is, the less air can get through.
Middle throttle openings are the worst for carb ice because they allow a good amount of air to flow (and therefore pressure to drop and gasoline to evaporate), but also present a lot of surface area on which ice can accumulate. Idle throttle gives relatively little cooling, and full throttle presents relatively less surface area for ice, as well as producing higher temperatures from the engine.
This is necessarily a brief and superficial explanation of carb ice. If you want to learn more about carburetor ice, check out some of the information in this Google search. Keep in mind that most of the information available about carb ice is airplane-specific: specific temperatures and conditions may not apply to our motorcycles, and the consequences aren't quite as dire (but almost) for motorcycles.
How can carb ice be prevented?
The principal methods of avoiding carb ice are to heat the incoming air, or to heat the carburetor body. Wes has experimented with heating the carb body with some good success. Intake air is typically heated by routing it past the exhaust pipes in an airplane, and the same method could theoretically work on the Ninja 250. No one has reported attempting to use this method.
Carb heaters for the Ninja 250 are available, both in the form of electric heaters (there's a cavity on the Ninja's carbs for electric carb heaters), and engine-coolant-based heaters. Neither form is easy or cheap to find, and either one will require considerable work to implement on the Ninja 250.
Wes spent a considerable amount of effort designing an automatic circuit to control his electric carb heaters. More information about this circuit can be found in this thread, as well as on the project page Wes put together. Further information can be found by searching the board for carb heater. This is an advanced project involving the creation of circuit boards, and will require considerably more electronic knowledge and skill than the average rider possesses. This isn't required for electric carb heaters to work -- it just makes them turn on automatically versus having to switch them on manually.
If you are interested in implementing a carburetor heating system in your Ninja 250, acquire two Kawasaki P/N 99995-1422 kits and post on the main Ninja 250RC forum. You can then discuss implementation details with Wes. Note that as of this writing (Mar 2006), even a minimalistic solution will require decent design/hacking skills with automotive electrical systems, and the ability to remove/install your carburetors. The 99995-1422 kits are very expensive, however Kawasaki has been "dumping" them around the $10 mark for a couple of years now (it is suspected that they have excess stock and no demand -- they were built as a retro-fit for the KVF-400).