Don’t Forget to Check the Jet Needle When You Overhaul the Carburetor of a Motorcycle that has Been Left for a Long Time!

Whether you have a carbureted or injected motorcycle, if you don't run it for a long time, the gasoline will deteriorate and have a negative effect on the fuel system. If you are fortunate enough to be able to start your motorcycle without the jets being clogged, you may be able to get back on track by replacing the gasoline in the float chamber, but if your motorcycle is somewhat unstable, check the jet needle for dirt.

If the jets are clogged and it won't start, it's a full overhaul. 

The CS is a single-cylinder motorcycle with a single carburetor, but with a quadruple carburetor, not only the carburetor but also the joints need to be cleaned, and sometimes the O-rings in the joints need to be replaced. So the overhaul work becomes a big deal.

Gasoline deteriorates with changes in temperature and humidity. Since the effect on performance varies depending on the storage conditions of each user, petroleum product manufacturers do not seem to have set a quality retention period, but "in general, if stored in a cool, dark place with few temperature changes, there should be no problem in using gasoline, kerosene, and diesel oil for about six months, and heavy oil A for about three months. (from ENEOS homepage) (from the ENEOS homepage) can also be seen.

If you use your motorcycle on a daily basis for commuting to work or school, you probably don't stop to refuel for six months. However, if you own more than one motorcycle and have few opportunities to ride, or if you have rented a container garage in a place other than your home for safety and security, and you are busy with your normal life, your motorcycle will be neglected and is a special rare thing It is not unusual.

If the engine is difficult to start or the power seems to have decreased, fresh gasoline can be added to get it back on track, but if the deterioration has progressed to the point of clogging the jets, drastic measures are required. If the gasoline has deteriorated to the point where it is clogging the jets, drastic action is required. When gasoline deteriorates, it gives off a distinctive odor, and any rider who has smelled it will immediately understand what needs to be done. If your motorcycle is from the carburetor era, you can visually see what's going on by removing the float chamber.

If the gasoline dries up and becomes varnish and clogs the slow jets and main jets, the gasoline will naturally stop flowing and the engine will not start even if new gasoline is added. If you know you won't be riding for a while, you can avoid clogging the jets by draining the float chamber and emptying it beforehand. In this case, however, the gasoline in the float chamber may deteriorate and become varnish and clog. If the float chamber is empty, it cannot be said to be perfect.

On the contrary, depending on the environment, the engine may start without any problem even if it is left for a reasonably long time. The pilot and slow systems, which are important for starting the engine, are more likely to be clogged than the main system because the passages are narrower, but you are lucky if the deterioration is slow and the varnish has not turned. However, even in such a case, it is not always possible to say whether the gasoline flow is smooth throughout the entire area.

Gasoline in the gap between the needle jet and the jet needle deteriorates.

The float chamber is not dried up, but the deteriorated gasoline has solidified and accumulated in the chamber and jets. At first glance, the jets don't look clogged, so I'm tempted to spray some carburetor cleaner to restore them, but the dirtier the carburetor, the more you need to be careful.

If you want to restore the proper function, it is better to remove all the parts that can be removed and soak them in a solution type cleaner and clean them slowly, so that you will not suffer from malfunction later on. Soaking cleaning is more effective than spraying for the dirt in the slow system and air passage.

It is half common knowledge that deteriorated gasoline clogs up from the pilot or slow system where the passages are narrow. However, apart from that, gasoline can also deteriorate in the area called the main nozzle, where the jet needle, which moves with the carburetor piston, enters and exits. The jet needle is shaped like a needle with a thin tip, and the gap between the needle and the main nozzle changes depending on the height of the needle, which in turn changes the amount of gasoline sucked out depending on the negative pressure of the engine.

The main nozzle consists of two pieces, the needle jet and the needle jet holder, and the main jet is attached to the bottom of the needle jet holder. In addition to the vertical hole through which gasoline passes, the needle jet holder has a horizontal hole called a bleed hole that takes in air to promote atomization of gasoline. And when there is gasoline in the float chamber, the needle jet holder is submerged in the gasoline to some extent.

When deteriorated gasoline becomes a varnish, it is a natural situation to block the slow jet and the main jet which are the gasoline intake. At the same time, we have to pay attention to the deterioration of the gasoline remaining in the main nozzle. In general, we are aware of the existence of the jet needle when the piston rises and the needle is exposed in the venturi, but when the throttle opening and the negative pressure of the venturi are small and the piston is blocking the venturi, the needle is in the main nozzle. Of course, when the engine is immobilized for a long time, it sits in the main nozzle and is completely immersed in the gasoline in the float chamber.

Therefore, when the gasoline in the main nozzle deteriorates, there is a good possibility that dirt will adhere to the inner wall of the main nozzle as well as to the Jet Needle itself. If the slow system is clogged with varnish, it is easy to see that it will not start, so the slow jet is removed and soaked in carburetor cleaner to clean it, but at the same time you also need to check the condition of the jet needle

It's no wonder the gasoline flow rate is inconsistent if the jet needles are of different thicknesses.

It's not surprising that gasoline would also stick to the jet needle, which sits inside the main nozzle in a deteriorated state. Varnish is sticky or hard, so even if fresh gasoline touches it, it won't melt and flow down. The taper angle of the jet needle is important, but the thickness of the jet needle varies depending on the amount of varnish on it, so the flow of gasoline sucked out of the main nozzle is not stable.

The needle and main nozzle, which have been cleaned to remove varnish, will stabilize the main system mixture and provide a smooth blow-off. When maintaining the carburetor after a long period of neglect, it is important to clean not only the throws and main jet, but also the jet needle and needle jet holder.

If the slow or main jets are blocked and not carrying gasoline to the venturi, you would not hesitate to remove the carburetor and do a full overhaul, and you would probably clean the jet needle and main nozzle at that time. However, it is also possible that the gasoline volatilization is halfway through and the oil level in the float chamber has not dropped enough to clog the slow and main jets.

In these cases, dirt on the needle can be a bad thing. As explained earlier, at normal oil level, there is gasoline in the main nozzle and the jet needle is submerged in it. When the oil level drops due to volatilization, varnish may form on the surface of the needle or on the exposed area in the main nozzle. If the oil level continues to drop until the jet is exposed as it is and the hole in the jet is clogged, the engine will not start even if fresh gasoline enters the float chamber, so it must be disassembled and cleaned, but if the oil level drop is halfway, the engine may start when the gasoline is fresh.

Generally speaking, this is lucky, but if the varnish adheres to the jet needle, it may cause a malfunction at the end of opening the throttle. The needle in the image shows this pattern, with the varnish making the surface uneven and the thickness varying from place to place. With such a needle, the gasoline metering that takes place in the gap between the main nozzle and the needle will be completely useless.

It is thought that as the throttle opening increases, the gasoline metered by the main jet is sucked out of the main nozzle. That's correct, but the gasoline that passes through the main jet is finally metered at the gap between the main nozzle and the jet needle before being sucked out. Therefore, if the surface of the jet needle is uneven due to varnish, the gasoline will be thinner where there is more varnish and thicker, and thicker where there is less varnish and the needle is at its original thickness. If the bleed hole in the needle jet holder is clogged, the amount of air that promotes atomization will also change, further adding to the instability factor.

Even if you are lucky enough to start your engine after replacing the gasoline that smells deteriorated in the float chamber after leaving it for a long time, the jet needle may not be functioning properly for this reason. If this is the case, do not hesitate to do a full overhaul and clean the jet needle as well as the jets.