While fuel and a good spark are essential for starting an engine, good compression is also important. Compression is simply the capacity of the fuel/air vapor to be compressed by the piston into the cylinder (also known as the combustion chamber) before the ignition, without spilling out somewhere due to a bad seal.
You can get a rough idea of compression by placing your thumb over the hole where the screw is threaded and pulling the starter cable or turning the handwheel if it is easier. You should be able to feel the pressure on your thumb. However, with the spark plug back in place, if the compression is severely compromised, the starter cable will be easy to pull, no resistance.
Bad compression can have several causes:
- Spark Plug loose. It could be as simple as a loose spark plug, so tighten it. Do not use excessive force when tightening.
- Defective cylinder Head gasket. The cylinder head is the part of the motor on which the plug is mounted. This is screwed into the cylinder block with the gasket inserted. You may notice burns or soot marks around the edges of the gasket.
- Bore Wear. Another cause of bad compression is the wear of the orifice, which causes the piston to seal badly in the cylinder. This can happen when an engine reaches old age, or if it has been used without air filter or with little oil.
- Clogged Open valves: Fire and spit. Another cause of poor compression is an open or poorly seated valve. During the 4-stroke cycle, an inlet valve is opened and allows the air/gas mixture to flow into the cylinder, and an exhaust valve opens to allow the burned gases to be ejected from the cylinder. During the compression stroke, both valves must be closed. Now, if valves do not settle and seal completely due to wear, the accumulation of deposits or the exhaust valve burned during many years of use of the engine, the compression can suffer. Another scenario is if an engine to one of the valves open. If the motor is stored for years without use, what can happen is clogging the valve in this position. Now, normally, a cam shaft opens each valve through rods in contact with the pushers called stems, and a spring returns the valve to the closed position. If the valve becomes clogged, the spring force is not strong enough to return the valve to the closed position. The symptoms of a stuck or worn valve may spit or shoot through the carburetor or the escape while the engine starts or that the Starter cable is actually easy to pull. So the moral of the story is "exercise" machines in storage periodically to prevent this from happening. This can be as simple as gently pull cable starting motor so all move internally. This applies to anything with moving parts, including engines, motors, tools and workshop equipment.
In fact, I experienced a compression problem with my lawn mower this morning, although it's not too old. The cable was easy to pull without resistance so I guess it jammed a valve. After leaving the engine for about an hour, the problem was solved, perhaps because the oil penetrated the lifters or valve stems.
When looking inside the cylinder through the spark plug hole or removing an inspection cap over a compartment (known as a valve hood) that provides access to the pushers, you can see if a valve is opening. Some engines have OHV or higher valves, and can be accessed again by removing an access cover. The valves can be released by spraying WD40 on the valve seat and on the stems. However, this may be beyond the ability of the hobbyist. If it is necessary to remove the cylinder head, when replacing it, the screws must be tightened in a staggered sequence and the bolts should ideally be re-tightened with a torque wrench.
The spit can also be caused by a mixture that fits too thin, which requires the mixing screw to turn to the left to enrich it (see above procedure). Another cause of spitting is a cut-off key, causing the timing to be off and the spark plug to trip at the wrong time.