IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:
The choke assembly, consisting of a plate mounted on a shaft, is more or less a duplicate of the carburetor throttle-plate package. A choke plate will be situated at the end of the carburetor throat nearest the air cleaner – while the throttle plate resides at either the intake-manifold end of the carburetor throat, or at some mid-point between the two throat ends.
When the choke is engaged by a machine operator: 1)the choke plate closes across the carburetor throat; 2)this closure suspends air flow into the carburetor through the air cleaner; 3)the throttle plate, at the same time, remains wide open; the combination of a wide-open throttle plate and a shut choke plate magnifies the vacuum effect produced during the intake stroke of the piston; 4)increased vacuum from the piston intake stroke entails that more fuel is sucked into the carburetor throat; 5)more fuel, along with a reduction in air flow, gives a cold motor that super-rich jolt it needs to start. WHENEVER: a small engine has cold-starting problems, especially if it tends to run just fine after it has started, a poorly-adjusted or faulty choke is the likeliest cause!
Manually-controlled chokes: just about all small-engine chokes are regulated by a cable and knob or lever assembly. When engaged by the machine operator, the lever or handle moves linkage that attaches to the choke plate in the carburetor throat.
Automatic chokes: these are found on larger, costlier pieces of equipment. They will generally be held shut by either: 1)spring tension – after the engine starts, vacuum created by the piston becomes strong enough to overwhelm the spring tension, and this forces the choke plate open; or 2)a bimetallic strip – a choke plate controlled this way relies upon the heat generated by a running engine to shrivel the metal strip, and this size change draws open the choke plate.
Primer bulbs are becoming more common in small-engine applications. Some motors, primarily of the two-stroke variety, will even employ both a choke and primer bulb. Here is how a primer bulb operates: 1)squeezing or pressing the bulb sends puffs of air through a tube into the upper carburetor float bowl; the bulb does not, itself, actually transfer fuel; 2)amassing air pressure within the float bowl drives fuel through the main and idle jets of the carburetor; this fuel ends up in the carburetor throat, where it provides the extra-rich dosage needed for a cold engine to start. WHEN: a primer-bulb equipped motor has a hard-starting problem but otherwise runs okay, look first for either a leak or clog in the air-transfer line between the bulb and carburetor float bowl.