McCulloch Chainsaws Service & Repair Manual

Two-Stroke Engine Repair & Maintenance

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1 Fundamentals There comes a time when work stops and the mechanic becomes abstracted, dis- tant from the task at hand. Something about the machine does not conform to the picture in the mechanic’s mind. Images flash by until he or she finds one that most closely conforms to actual conditions. Once that is done, repairs can begin. Constructing visual images is what mechanics do; the other stuff is mere wrench-twisting. This chapter provides grist for these mental images. Because the material must be conveyed in words, it tends to be abstract. But once you can picture how these engines work, you will have made the first step in the journey to becoming a real mechanic. Spark-ignition engines operate in a cycle consisting of four events: intake, compression, expansion (or power), and exhaust. A fresh charge of air and fuel is inducted into the cylinder, which then is compressed by the piston and ignited by the spark plug. The pressure created by combustion reacts against the piston to generate torque on the crankshaft. The spent gases then exhaust into the atmosphere. Four-stroke-cycle engines require four up and down strokes of the piston, or two full crankshaft revolutions, to complete the cycle. Two-stroke-cycle engines tele- scope events into two strokes or one crankshaft revolution. For convenience we abbreviate the terms to four-cycle or four-stroke, and two-cycle or two-stroke. Two-cycle operation Focus on the piston. The double-acting piston works in both directions to compress the air-fuel mixture in the cylinder above it and in the crankcase below it. The piston and connecting rod convert a portion of the heat and 1

energy released by combustion into mechanical motion that turns the crankshaft. Were that not enough, the piston also acts as a slide valve to open and close exhaust, transfer and (in some applications) intake ports. Because it works so hard, the piston is the first mechanical part to fail on two-cycle engines. Third-port engines Third- or piston-ported engines have three ports cast or milled into their cylinder liners. The inlet port admits fuel to the crankcase, the transfer port conveys fuel from the crankcase into the combustion chamber, and the exhaust port opens to the atmosphere. First, let’s look at events above the piston during a full turn of the crankshaft. In Fig. 1-1A the piston approaches the upper limit of travel, or top dead center (TDC), and has compressed the air-fuel mixture above it. The piston has also uncovered the inlet port to admit fuel and air from the carburetor to the crankcase. Figure 1-1B illustrates the beginning of the power stroke under the impetus of expanding combustion gases. As the piston falls, it first uncovers the exhaust port (Fig. 1-1C) and, a few degrees of crankshaft rotation later, the transfer port (Fig.1-1D). Fuel and air pass through the transfer port and into the cylinder bore. Meanwhile, much is happening in the crankcase. As the piston falls on the power stroke, it partially fills the crankcase, reducing its volume, as shown in Fig. 1-1C. Since the piston now covers the inlet port, the pressure of the air-fuel mixture trapped in the case rises. Near bottom dead center (BDC) the piston uncovers the transfer port and the pressurized fuel mixture passes through this port to the upper cylinder (Fig. 1-1D). The piston then rounds BDC and begins to climb, an action that simultaneously compresses the mixture above the piston and creates a par- tial vacuum under it. Once the inlet port opens, atmospheric pressure forces fuel and air from the carburetor into the crankcase. A problem with third-port engines is fuel reversion. At low speeds the crankcase fills to overflowing. When the piston reverses at the top of the stroke, some of the charge can flow back through the inlet port to the carburetor. A fog of oily fuel hovers around the air cleaner, dirtying the engine and playing havoc with carburetor metering. Reed-valve engines Although third-port engines are still encountered, many manufacturers prefer to control crankcase filling with a reed valve installed between the carburetor and crankcase. The valve, similar to the reed on musical instruments, opens and closes in response to crankcase pressure (Fig. 1-2). Utility engines make do with a single reed, or pedal, athwart the intake port (Fig. 1-3). High-performance 2 Fundamentals

Two-cycle operation 3 Transfer D Intake A Power stroke B C Exhaust FIG. 1-1. Operating sequence of a third-port, loop-scavenged engine. Walbro engines employ a tent-like valve block with multiple reeds. This arrangement provides a large valve area for better crankcase filling (Fig. 1-4). For mini-two-strokes, the position of the carburetor indicates the type of inlet valve: when a reed is present, the carburetor mounts on the crankcase (Fig. 1-5). Third-port engines mount their carburetors on the cylinder barrel in line with the inlet port, as shown in Fig. 1-6. Being able to recognize the presence of a reed valve without disassembling the engine is useful, since the reed can malfunction. Should the pedal split or fail to seal, the engine will not start.

McCulloch Chainsaws Service & Repair Manual

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