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How to Adjust Motorcycle Engine Valve Clearance?

Dec. 08, 2020

When a motorcycle is ridden for a long period of time, the engine will generally have a valve clearance that is either too large or too small, too large or too small, too large or too small.

Many people who know a lot about valves may not be aware of what these problems can cause, and the information they consult is often complicated. The motorcycle engine valve supplier will give you a simplified analysis, from understanding valves to drawing conclusions.

Several valve types of air distribution mechanisms

Four-stroke motorcycle engine valve distribution mechanism valve form, according to the valve position, there are side valves (SV), overhead valves (OHV), and overhead camshaft valves (OHC) three kinds.

1. Side-mounted valve

Structurally speaking, the side valve is the simplest, but due to the adoption of this valve form, the engine's anti-explosion performance and high-speed performance is poor, can only be used for low compression ratio and low-speed engines, trapped foreign countries have stopped using.

2. Overhead camshaft type valve

In terms of performance, the overhead camshaft type valve is the most ideal, as it can meet the requirements of high speed, high compression ratio, and high power vehicles, and at the same time has a good economy, so it is widely used.

3. Overhead valve

Its structure is more complex. At present, most of them continue to be used in the United States, Italy, and other countries in former West Germany (R-series motorcycles produced by BMW) due to production habits.

 Engine Valve for Motorbike

Adjustment of Valve Gap

When the vehicle is in use, the valve lash will change due to wear of the parts of the air distribution mechanism or loosening of the adjustment screws, so it must be checked and adjusted periodically.

1. Overhead camshaft type valve lash adjustment method

(1) Remove the intake and exhaust chamber cover and the magneto cover;

(2) Rotate the rotor of the magnetomotor so that the "T" mark on the outer surface of the rotor is aligned with the mark on the casing, and the piston should be at the upper stop of the compression stroke.

(3) Carefully insert the ruler with the thickness of the specified valve clearance value into the valve clearance and pull it back and forth, if you feel a slight resistance, the clearance is suitable.

(4) If the clearance is not appropriate, then turn the adjusting nut, and use a small wrench to turn the adjusting screw while pulling the stopper to check the clearance, and after the clearance is appropriate, tighten it again.

2. The adjustment method of side valve clearance

Remove the valve chamber cover and remove the spark plug; plug the spark plug hole with your finger and step on the start pedal, when your finger feels the impact of airflow, the piston is in the compression stroke; then the head of the screwdriver can be inserted into the spark plug hole, and then slowly step on the start pedal when the screwdriver rises to the highest point, the piston is at the stop point;.

Check and adjust the valve lash. The method is the same as the overhead camshaft valve lash check and adjustment method.

3. Problems to be explained

For side-mounted valves, the valve clearance is the clearance between the end of the intake and exhaust valve stem and the adjusting screw on the tappet; for overhead valves and overhead camshaft valves, the clearance between the valve stem and the adjusting screw on the rocker arm.

The hot gap is slightly smaller than the cold gap, and the gap measured in the cold state of the engine is the cold gap.

Usually, the intake valve cold gap is between 0.08~0.10mm, and the exhaust valve cold gap is around 0.10~0.12mm. The adjustment should be made strictly according to the clearance specified in the instruction manual.

The checking and adjustment of valve clearance must be done when the intake and exhaust valves are completely closed. When the piston is at the upper stop point of the compression stroke, the intake and exhaust valves are completely closed, so when adjusting the valve clearance, the valve plug must be at the upper stop point.

The adjustment of the valve lash should be done when the engine is cold, and it is forbidden to adjust the valve lash when the engine is running. For the two-cylinder engine, check and adjust each cylinder.