Although “backpressure” is a misnomer, the term describes what happens when the plumbing clogs with carbon. The engine chokes on its own exhaust. Lower the piston and, using a soft brass or copper tool, scrape the exhaust ports clean (Fig. 2-14). Clear the cylinder of carbon fragments by spinning the flywheel rapidly with the spark [...]
Two-cycle engines develop two kinds of compression: crankcase and cylinder. Weak crankcase compression reveals itself as a refusal to ingest carburetor cleaner sprayed into the air intake. Some mechanics claim to be able to sense the crankcase pressure, which never amounts to more than 5 or 6 psi, as resistance on starter cord. Cylinder compression [...]
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 [...]
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