A 4-cylinder engine that has no problems with the engine itself and the jets and needles are numbered correctly, but one cylinder does not ignite at idle and the plugs are burned in different ways. If there is no significant change even after adjusting the air screw, instead of overhauling the carburetor, try removing the air screw and blowing in the carburetor cleaner first. This alone may restore the condition of the engine.
- I suspect the pilot system is thin when it starts poorly or idles with little punch.
- Clogged pilot system possible even after overhaul
I suspect the pilot system is thin when it starts poorly or idles with little punch.
The air screw, which adjusts the mixture ratio during idling, increases the amount of intake air as it is returned from the closed position, so the mixture ratio becomes thinner. On the other hand, the pilot screw increases or decreases the amount of air mixture, so the mixture becomes thicker the more the screw is turned back.
However, since the adjustment range for both is fixed, the air-fuel ratio does not change the more the screw is turned.
The burnt condition of the plugs is one indicator of the mixture ratio. In this example, the second plug from the left has a white insulator (the white part), which indicates a relatively low mixture ratio. For a carburetor car that is mainly driven in the city, and that is a large-displacement car, the three burnt plugs that appear to be smoldering may be a better indication of the engine’s condition.
Spark plug burnout is and always has been an important clue to engine condition.
Carbureted cars can burn too much or smolder if the settings are not correct, but there are also cases where carbs for multi-cylinder engines have the same settings, but the plugs can vary in burnt condition.
Here is an example of an out-of-print car with an engine that is not running well when idling.
The specific symptoms are.
1. When the choke is used when the engine is cold, all four cylinders will burn.
2. When the choke is turned back on, only one cylinder does not burn and the engine becomes a 3-cylinder
3. Running on three cylinders and opening the throttle wide will result in four cylinders
and so on. If there is nothing wrong with the engine, these symptoms can be attributed to the carburetor. If we check the plugs of the cylinder that are malfunctioning, we can determine whether the air-fuel ratio is too rich or too light and determine the appropriate action to be taken. When the choke is turned back on, a four-cylinder engine may become a three-cylinder engine because the air-fuel ratio is too high and the plugs are covered, or because the ratio is too low and the plugs fail to ignite. In the case of the present car, the plug of the cylinder that was not burning at idle was white compared to the others, so it is thought that this is the only cylinder that is too thin and has three cylinders.
If the throttle is opened wide enough, four cylinders will be formed because the flow of the mixture ratio shifts from the pilot system to the slow system and then to the main system, which is also evidence that the malfunction is limited to the pilot system. The pilot screw and air screw are more important in the idling range than one might imagine, and even a half-turn change in the pilot screw, which increases or decreases the amount of air mixture supplied, can affect starting performance and idling performance.
Clogged pilot system possible even after overhaul
When idling sluggishness is caused by a thin mixture when the throttle is fully closed, the cause is probably in the pilot system, but in the case of the air screw, increasing the return rpm from the fully closed position will increase the air ratio and thin the mixture. The plug is burning out, so tightening the screw and reducing the return rpm will not improve the symptom. The carburetor was cleaned by disassembling the four bodies and soaking them in liquid-type carburetor cleaner just prior to cleaning, but it seems that the cleaning was insufficient.
Comparing the pilot jet (slow jet) and main jet diameters, it is clear that the pilot screw is smaller, and the pilot system also has smaller machined hole diameters than the main system.
Therefore, when fuel deteriorates due to long-term storage, the pilot system begins to clog before the main system. This is the structural fate of the inside of the carburetor, which is the reason why there are many cases where the engine runs well when the throttle is opened, but the start ability and idling are poor. For example, using a Keihin carburetor with a #35 pilot jet and a #110 main jet, the main jet has a hole diameter of 1.10 mm while the pilot has a diameter of 0.35 mm, so it is obvious which one is more likely to be clogged with foreign matter.
In the pilot system, where the jet hole diameter is small and the passage inside the carb body is narrow, a partial blockage, rather than a complete blockage, can reduce fuel flow and affect the mixture ratio. The return speed of the air screw has an adjustable range, and if the return speed is less than 1/2 turn from the cutoff, it exceeds the adjustable range for slow air.
If the mixture ratio is still thin at idle, you should suspect a pilot path failure.