Black, furry deposits that you can wipe off with your fingers are evidence of rich combustion (Fig. 2-10). The engine is drowning in gasoline. Suspect a maladjusted carburetor and/or a dirty air filter. Unlike lean mixtures, gasoline-rich mixtures do no permanent damage, at least on engines without catalytic converters. It is also true that chronic [...]
Lean mixtures are a cause for alarm. The operator might notice flat spots during acceleration or that the engine runs better with a partially closed choke. A mechanic will see the bleached, bone-white spark plug insulator (Fig. 2-7) and immediately suspect piston damage of the sort pictured in Fig. 2-8. In extreme cases, high combustion [...]
No fuel Crank a cold engine five or six times and remove the spark plug. If fuel is entering the cylinder, the tip of the spark plug will be wetted and smell of gasoline. If the plug is dry, momentarily disconnect the fuel line to the carburetor. Unless there is an obstruction upstream—tank filter, screen, [...]
Anyone who contemplates doing much work on small engines should invest in a Briggs & Stratton PN19053 or a STIHL Zat 4 ignition tester. These tools eliminate any fire hazard by confining the spark behind a transparent window. People with pacemakers or other medical problems should not expose themselves to electroshock. Solid-state ignition systems produce [...]
Most modern two-strokes require 89 octane gasolines to prevent detonation. And the fuel should not be more than a few weeks old. As gasoline ages, the light hydrocarbons evaporate, leaving varnish and gums behind. A fuel stabilizer, such as Briggs & Stratton Fresh Start, will preserve gasoline for as long as 24 months. However, stabilizers [...]
Near the end of the eighteenth century, James Watt observed that a mine pony tethered to a turnstile could lift 550 lb one foot per second, or 33,000 lb per minute. Horsepower was a brilliant sales tool that put steam engines into a context that potential customers could understand. In metric notation, one horsepower equals [...]
The compression ratio (CR) describes the amount of “squeeze” the piston applies to the air-fuel mixture prior to combustion. It is arrived at by dividing total cylinder volume, that is, the volume with piston at BDC, by the volume that remains when the piston rises to TDC. The latter figure is the clearance volume. Normally [...]
We class ships by tonnage, houses by square footage, and engines by the volume the piston displaces as it moves between centers. All things equal, an engine should develop power in proportion to its displacement. Displacement = bore × bore × number of cylinders × stroke × 0.7858 For example, the Tanaka series TBC-2501 has [...]
Current practice, based on work carried on in Germany during the 1920s, is to use loop, or Schn_rle, scavenging. Multiple transfer ports are arranged around the cylinder periphery with their exit ramps angled to impart swirl to the charge (Figs. 1-8 and 1-9). The miniature cyclone fills the whole combustion chamber, sweeping exhaust gases out [...]
Readers with long memories may recall the deflector pistons that were once standard ware on these engines (Fig. 1-7). The fuel charge enters through a single transfer port, rebounds upward off the deflector, and drives residual exhaust gases out the exhaust port. While this design works well at moderate speeds, at high speeds, the deflector [...]
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