The Main Principle is Not to Touch it Needlessly. Throttle Position Sensor for Carbureted Vehicles to Optimize Ignition Timing

01-1-1.jpg How to

A throttle position sensor is essential for fuel-injected vehicles, which inject a pre-programmed amount of gasoline according to the throttle opening. On the other hand, some carburetor models are equipped with a throttle position sensor even though the amount of gasoline automatically sucked up is determined by the negative pressure and flow rate of the air being sucked in. Here we introduce the role of the throttle position sensor in carburetor models and how to adjust it.

Throttle position sensor on carbureted vehicles optimizes ignition timing according to throttle opening and load

無用に触れないのが大原則。点火時期を最適化するキャブレター車のスロットルポジションセンサー

The throttle position sensor, which was not available on the 2-valve Zephyr-era CVK carburetor, was installed in the Zephyr X. In the Zephyr era, ignition timing was determined solely by engine speed, but a major evolution was the ability to advance or retard ignition timing from the base curve by detecting engine load through throttle opening.

無用に触れないのが大原則。点火時期を最適化するキャブレター車のスロットルポジションセンサー

The main harness on the cmotorcycle side and the TPS harness in the carburetor are connected by a 3-pole coupler.
By connecting a special harness between these two harnesses, the TPS voltage can be measured at each throttle opening. The voltage applied here is the voltage flowing from the igniter.

The carburetor, which continues to create a gasoline mixture as the gasoline flow path switches autonomously from the slow jet to the jet needle to the main jet according to the amount of air the engine draws in, is simple but actually has a very sophisticated function.

Although digital fuel injection may seem like a smart and intelligent mechanism in contrast to analog carburetors, all gasoline injection amounts in relation to throttle opening and engine speed must be manually programmed to function. The throttle position sensor (TPS) in an injected vehicle is an essential device for detecting the butterfly opening of the throttle body from fully closed to fully open.

The injection is not a system that supplies gasoline according to the air flowing through the venturi like a carburetor, so in this respect, it can be said that carburetors are more nostalgic, as they can run even if they deviate slightly from the ideal setting.

However, even among carburetor motorcycles, there were some sports models and some models equipped with TPS for emission control purposes. The purpose of detecting the throttle opening here is to control the engine speed and, more specifically, the ignition timing.

The general ignition timing is to ignite near the top dead center in the starting or idling range and to change to an earlier position than before the top dead center as the engine speed increases. This is because if the ignition timing is too early in the starting range when the engine speed is slow, starting performance is poor, and if the ignition timing remains slow even when the engine speed is high, the pressure obtained from the combustion of the air mixture cannot be used effectively.

In contrast to the ignition curve, which is determined by two factors, engine speed, and ignition timing, the ignition map is determined by three factors, including the throttle opening detected by the TPS. When climbing a hill without downshifting, the vehicle speed decreases, and the throttle is opened wide. This increases the load and causes knocking, which is a premature ignition.

An effective way to mitigate this is to delay the ignition timing, but this is not possible with a two-dimensional ignition curve. However, when the throttle is opened further to maintain the same engine speed, the TPS detects this and determines that the load has increased, allowing the ignition timing to be delayed from the base curve while maintaining the engine speed.

Conversely, when the engine speed is high but the load is low = riding can be maintained with a small throttle opening, and combustion efficiency can be improved by accelerating the ignition timing earlier than the base curve.

In the case of the Kawasaki Zephyr X carburetor with TPS shown in the image, the ignition timing at idle is 17.5° before the top dead center, but from there, when the throttle is opened slightly, the ignition timing is set to delay to further approach top dead center to produce torque. On the other hand, the ignition timing is advanced at 1/4 throttle opening more than at full throttle opening to improve the power feeling.

For this vehicle, the ignition system is equipped with the SP II Full Power Kit from AS Uotani, and the stock igniter is not used. However, the SPII control unit has a built-in program for TPS-equipped vehicles, so the ignition can be controlled by a map using the TPS.

POINT

Point 1 - A throttle position sensor that detects throttle openings is essential for fuel injection, but some carburetor models are equipped with a TPS.

Point 2 - TPS for carbureted vehicles can detect the load on the engine at throttle opening and set the optimum ignition timing at the same rpm.

Calibration of the throttle position sensor at fully closed and fully open is extremely important.

無用に触れないのが大原則。点火時期を最適化するキャブレター車のスロットルポジションセンサー

TPS mounted on the right end of the throttle shaft. Kawasaki called this system K-TRIC. This carburetor can be installed on models without a TPS if the mounting pitch is correct, but the TPS will not function if the igniter does not have a control function. The Zephyr X's digital igniter has that function, of course, but the SP II control unit made by AS Uotani also has a built-in ignition map that is compatible with TPS.

無用に触れないのが大原則。点火時期を最適化するキャブレター車のスロットルポジションセンサー

The TPS is held in place by a torque bill with tamper-resistant threads so that it cannot be easily touched. The flange portion is machined with a long hole and can be rotated slightly around the throttle shaft to adjust its position.

無用に触れないのが大原則。点火時期を最適化するキャブレター車のスロットルポジションセンサー

When the output voltage of the TPS is measured with the engine started and idling, the standard value is 0.9 to 1.1V. The actual measured value is 1.114V, which is indicated only slightly higher, but this value is the best when combined with the fully open side.

無用に触れないのが大原則。点火時期を最適化するキャブレター車のスロットルポジションセンサー

The standard value for full open circuit voltage, measured with the engine stopped, is 4.06 to 4.26V. The actual value is also 4.06V, so this is also OK. This adjustment will maximize the effect of the three-dimensional map that varies ignition timing according to load.

The carburetor with TPS, which can optimize ignition timing according to throttle opening that varies with the load even at the same engine speed, sends the voltage generated by the TPS attached to the end of the throttle shaft that opens and closes the throttle butterfly to the igniter (the SP II control unit since it is the SP II full power kit from AS Uotani).

Therefore, the ignition map cannot function properly if the throttle is fully closed but not open voltage, or if it is fully open but not fully open voltage. Therefore, correct detection of throttle openings is of utmost importance.

This is not a problem for new vehicles shipped from the manufacturer, as they are calibrated by the manufacturer.
Also, the operation and service manuals prohibit shifting or removing the TPS position. Even in the case of the Keihin CVK carburetor shown in the image, the TPS unit is secured with tamper-resistant Torx screws so that it cannot be adjusted with ordinary hand tools.

However, when a 4-carburetor is disassembled into pieces and soaked in carburetor cleaner, such as in the case of a full overhaul of a long-abandoned carburetor, the TPS, which is an electrical component, must be removed. In such cases, reconditioning is necessary.

In the case of a Zephyr X carburetor, the special harness is interrupted at the coupler between the main harness on the body side and the TPS harness on the carburetor side, and the terminal voltage is measured with a circuit tester.

Measurement at the fully closed throttle is made with the engine started and idling, and the standard voltage at this time is 0.9 to 1.1V. Even though the engine is idling, the engine speed can be raised or lowered with the throttle stop screw, so measurements are taken with the engine idling at around 1200 rpm, which is standard.

The measurement at full open is normal if the voltage is in the range of 4.06 to 4.26 V when the engine is stopped and the ignition is turned on and the voltage is applied to the TPS. Since both fully closed and fully open voltages are specified in detail, the circuit tester should be a digital type, not a needle type.

The screw holes of the TPS, which are secured with Torx screws with torsion stops, have long holes that allow the angle to be adjusted within a certain range, so if either one is out of the standard voltage range, the angle is changed and the measurement is performed again. The throttle shaft rotation angle and TPS detection voltage are determined by the manufacturer, so unless there is something wrong with the TPS, if the fully closed voltage is within the standard value, the fully open voltage should also naturally be within the standard value. If the voltage does not reach the standard value no matter how many times it is adjusted, it is highly likely that the TPS itself is faulty.

The TPS, which was installed at the end of the analog carburetor era, is a very important part to make the ignition map of the digitalized and highly functional igniter function. Do not underestimate the fact that adding a sensor to the carburetor will not make much difference, but perform maintenance to make the most of its functions.

POINT

Point 1 - TPS with stock carburetor should not unnecessarily change the sensor position.

Point 2 - When removing or replacing the TPS, adjust the mounting position so that the output voltage according to the throttle opening is within the standard value.

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