1, piston knocking sound: piston and cylinder clearance is too large, the engine work, will produce a larger sharp knocking sound, suddenly accelerate or reduce the feeling, the "click" sound. This noise is most obvious when the engine is cold and accelerating.
The inspection methods are as follows:
(1) before starting the engine, continuously step on the starting lever, generally can not hear the tapping sound. But when started, suddenly accelerated, it occurred "click" sound, that is, the sound of knocking cylinder.
(2) remove the spark plug, inject about 1/20 liter of oil into the cylinder, step on the starting rod several times, then start the engine, if the noise is reduced or eliminated, it is caused by the piston and cylinder wall gap is temporarily filled with oil. But after a few minutes, the oil gets hot dilute to leak, the sound will still appear.
(3) with a long screwdriver (or a metal rod), one end contacts the upper part of the cylinder, and the other end is pressed against the ear to detect the sound. Sometimes as a result of repair when connecting rod small head sleeve and piston pin, assembly is tighter also can produce knock cylinder sound, but this kind of sound is more depressing, and generally is after repair car start when just happening.
2, piston pin knock: piston pin and connecting rod small head or piston pin and piston pin hole clearance is too large, piston up and down movement causes radial percussion, produce a clear "click" sound. This noise is usually more pronounced at high speeds and with sudden acceleration. Delay the ignition time, the noise will be slightly reduced.
Check method: do not start the engine, continuously step on the starting lever, you will hear a "click, click" sound in the cylinder. After starting the engine, make the engine idle, contact the screwdriver with the upper part of the cylinder, listen with the ear side, there is a clear "click" sound.
3, knock caused by deflagration: when the engine suddenly accelerates or the motorcycle goes uphill, the cylinder sends out a continuous sharp "click" sound, which is the knock caused by deflagration.
The knock sound caused by deflagration is similar to the sound caused by excessive clearance between piston pin and bushing of small head of connecting rod. However, the knocking sound caused by deflagration will not occur at idle, and the starting lever cannot be heard when stepping before starting. After starting, the piston pin sound is continuous and disorderly, and the knock sound caused by deflation is the most obvious when increasing the load and uphill, and the sound is more clear.
The causes of deflagration are :(1) the use of low octane gasoline; (2) premature ignition time; (3) overheating of the engine; (4) excessive accumulation of carbon in combustion chamber; (5) low heat value of spark plug.
4, leakage sound: cylinder and cylinder head, cylinder and crankshaft box or left and right crankshaft box, if the fastening bolts loose or gasket damage, will produce a "haw" leakage sound. After starting, the sound is not easy to hear clearly when running at a constant speed; When stopping for inspection, obvious oil stains can be found from the leak. If this phenomenon is not serious, just tighten the fastening bolt. If it is serious, the gasket should be replaced.
5. Valve tapping: if the valve clearance is enlarged due to improper adjustment or wear of parts, the valve mechanism will make a short and sharp tapping sound. The tapping does not vary with the speed of the machine. It can be eliminated by adjusting valve clearance or replacing worn parts.
6, piston ring noise: if the piston ring torture, piston ring shell is insufficient, piston ring ring locked or piston ring gap gap, side gap, back gap is too large, when the engine is running, the explosion of gas will be rapidly down, rushed into the crankcase surface to emit leakage knocking sound. Touch the spark plug with the screwdriver and listen for a clear "thump, thump" sound.
7, the noise of connecting rod bearing: connecting rod bearing and bearing hole excessive wear or assembly clearance is too large, crankshaft rotation will produce radial impact and a heavier "du-dong" sound. The noise is obvious when idling speed running, while the noise is disorderly and hard to distinguish when running at medium and high speed. When checking, do not start the engine first, continuously step the starting lever, will hear the sound of "clack, clack, clack"; After starting, when idling, contact the lower end of the cylinder with the screwdriver and listen to the sound with the ear. There is obvious "tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap-tap.
8, carburetor throttle knocking sound: the throttle valve in the mixing chamber body is constantly moving up and down, due to excessive wear and tear resulting in excessive clearance, in the engine intake stroke due to the impact of the airflow, the throttle produced radial swing and hit the mixing chamber body, will issue a "cluck cough" sound. This sound is almost inaudible when idle speed is twirled, when the throttle is screwed to 1/4 opening degree, the sound is the biggest, when the throttle is increased again, the sound gradually decreases, when the throttle is screwed to the end, the sound disappears.
Check, can use screwdriver live carburetor audiometer, in the throttle up to 1/4 opening, there is a significant knock sound. The air filter can also be removed, with the finger against the throttle, the tapping sound disappeared, relax the finger tapping sound again, can be determined as the throttle tapping sound.
9, carburetor tempering sound: tempering sound occurs in two situations, one is when starting, from the air filter "crackle" sound, and out of a white smoke, and the engine is difficult to start; The other is when the engine suddenly makes a "popping" noise from the air filter.
10, exhaust pipe discharge cannon sound: exhaust pipe discharge cannon sound also occurs in two cases, one is due to the engine work is not good, so that the combustible mixture combustion explosion in the exhaust pipe and issued a "crackle" sound, sound clear, loud, at the same time to emit sparks or smoke; The other is due to the rupture of the muffler itself, so that part of the high-pressure exhaust gas directly into the atmosphere and issued a "crackle" sound, the sound is weak, messy and uninterrupted. In the former case, the causes of discharge pipe firing are :(1) excessive mixture; (2) ignition time is too late; (3) gasoline deterioration or moisture; (4) capacitor damage; (5) four-stroke engine exhaust valve bite dead or valve clearance. Because the silencer is broken and produces the sound of firing, it can only be re-welded, if the silencer tail opening is too big, can also be used to clamp the opening a little with pliers.