The valve refers to the intake valve and exhaust valve of the cylinder head. The intake valve directly connected to the intake manifold is the inlet the engine uses to suck in the mixture (or fresh air). The exhaust valve is connected to the exhaust pipe (that is, the muffler) and is the outlet for the engine to discharge the combustion exhaust gas. The efficiency of the inlet and exhaust is an important factor to determine the performance of the engine, when the engine is running normally the reciprocating speed of the piston is very fast, at 3000 RPM the engine to complete each intake or exhaust stroke time is only 0.04 seconds, in order to suction or discharge more gas in such a short time to increase the effective area of the inlet and exhaust. Therefore, some engines have adopted multi-valve technology. There are the following valve mechanisms: 1, 2 valves per cylinder (that is, 1 intake valve, 1 exhaust valve). 3 valve structure per cylinder (2 intake valves, 1 exhaust valve), 3, 4 valve structure per cylinder (that is, 2 intake valves, 2 exhaust valves); Three, each cylinder 5 valve structure, that is, 3 intake valves, 2 exhaust valves. However, the number of valves is not the more the better, 5 valves can indeed improve the intake efficiency, but the structure is extremely complex, difficult to process, and less is used.
The valve form of the valve mechanism of the four-stroke engine, according to the different position of the valve, there are side valve (SV), overhead valve (OHV) and overhead camshaft valve (OHC) and double overhead camshaft valve (DOHC).
In terms of structure, the side valve is the simplest, but because of the use of this valve form, the anti-knock performance and high-speed performance of the engine is poor, can only be used for low compression ratio and low speed of the engine, so it is no longer used abroad. There are still two types of models such as Changjiang 750 and Shandong 750 that use this valve form in China.
From the performance point of view, the overhead camshaft valve is the most ideal, it can meet the requirements of high speed, high compression ratio high-power models, and has good economy, so it has been widely used. Such as Jialing JH70, CG125 imported from Japan and other models, all adopt this valve form.
The overhead valve structure is relatively complex, and it is still used only in the United States, Italy and other countries where the R-series motorcycle produced by BMW factory is still used due to production habits.
Second, the adjustment of valve clearance
When the vehicle is in use, the valve clearance will change due to the wear of the parts of the valve mechanism or the loosening of the adjustment screw, so it must be checked and adjusted regularly.
one Adjusting method for overhead camshaft type valve clearance
(1) Remove the inlet and exhaust valve chamber cover and magneto outer cover;
(2) Rotate the magneto rotor so that the "T" engraving on the outer circular surface is aligned with the engraving on the housing, and the piston should be at the top dead center of the compression stroke;
(3) Insert the feeler with the thickness of the specified valve clearance value carefully into the valve clearance and pull it back and forth. If you feel a little resistance, it indicates that the clearance is appropriate.
(4) If the gap is not suitable, rotate the adjusting nut, turn the adjusting screw with a small wrench, and pull the feeler to check the gap. When the gap is suitable, and the gap changes after tightening, the feeler should be re-tested. !
2. Adjustment method of side valve clearance
Remove valve chamber cover and spark plug; Plug the spark plug hole with the finger, step the starting lever, when the finger feels the air impact, it indicates that the piston is in the compression stroke; At this time, the head of the screwdriver can be extended into the spark plug hole, and then slowly step the starting lever, when the screwdriver rises to the highest point, the piston is at the top dead center; Check and adjust the valve clearance. Methods Check and adjust the valve clearance of the same overhead camshaft.
3. A few questions to be addressed
For the side-mounted valve, the valve clearance refers to the clearance between the tail end of the intake and exhaust valve rod and the adjusting screw on the tappet; For overhead valves and overhead camshaft valves, the valve clearance refers to the clearance between the end of the valve rod and the adjusting screw on the rocker arm.
The valve clearance is divided into two types: cold clearance and hot clearance. The hot clearance is slightly smaller than the cold clearance, and the gap measured in the cold state of the engine is the cold clearance. Usually, the inlet valve cold clearance is between 0.05 and 0.10 mm, and the exhaust valve cold clearance is about 0.05 to 0.10. The adjustment should be carried out in strict accordance with the gap specified in the instruction manual. Check and adjust the valve clearance must be carried out when the intake and exhaust valves are completely closed. Since the piston is located at the top dead center of the compression stroke, the intake and exhaust valves are completely closed, so when adjusting the valve clearance, the piston must be at the top dead center.
The adjustment of the valve clearance should be carried out when the engine is cold, and it is strictly forbidden to adjust the valve clearance when the engine is running. The two-cylinder engine should be checked and adjusted cylinder by cylinder.
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