The engine appears to develop compression and the carburetor is not obviously loose on its mountings. The spark-plug tip remains dry after prolonged cranking. When mixture screws are present, backing them out has no effect. An injection of carburetor cleaner through the spark-plug port brings the engine back to life, but only for a few [...]
As a rule of thumb, engines should start when the mixture-control screws are backed out 11/4 to 11/2 turns from lightly seated. With a clean air filter in place, fully open choke, and fresh fuel in the tank, run the engine under moderate throttle for about five minutes to reach operating temperature. The tank should [...]
Figure 4-4A illustrates a Flo-Jet carburetor with adjustable main and lowspeed jets. Backing out the needle-tipped adjustment screws opens the jet orifices and enriches the mixture (Fig. 4-4B). Tightening the screws makes the mixture leaner by restricting the fuel flow through the jet. The idle-speed adjusting screw bears against the throttle stop to regulate idle [...]
With one or two exceptions, Briggs carburetors employ conventional choke valves to enrich the mixture during cold starts. The Walbro carburetor shown in Fig. 4-1A uses a pivoted choke disk. The Vacu-Jet in Fig. 4-2 has an old-fashioned plug choke. In either case, closing the choke seals off the carburetor bore. The engine, in effect, [...]
Carburetors always include a mechanism for regulating the internal fuel level, independent of delivery pressure. Most Briggs type carburetors employ a float-actuated inlet valve, known as a needle and seat (Fig. 4-1D). When the reservoir is full, the float forces the needle against its seat, which cuts off fuel delivery. As gasoline is consumed, the [...]
The throttle blade, or butterfly, controls engine speed by regulating the amount of fuel and air leaving the carburetor. It functions like a gate valve, opening for the engine to develop full power and almost completely blocking the bore at idle. The restriction generates a low-pressure zone downstream of the throttle blade, exactly as if [...]
If you look through a carburetor, you’ll see that the bore has an hourglass shape, with the necked-down portion located just upstream of the throttle plate. This area is known as the venturi. As much air leaves the carburetor as enters. Consequently, air velocity through the venturi must be greater than through the straight sections [...]
Interlocks vary, since the original equipment manufacturer is responsible for their design. Some include logic modules to defeat tampering. From a mechanics point of view, interlocks fall into two categories: those that are normally open (NO) and those that are normally closed (NC). NO interlocks shunt primary-side ignition current to ground when closed. NC contacts [...]
When there is no spark and the usual suspects—the spark plug, flywheel key and, when fitted, the contact points—have been exonerated, check for a shorted kill-switch circuit. With the control lever in the “Run” position, disconnect the ground wire at the coil and test for continuity between the wire and a good paint- and grease-free [...]
This malfunction is usually caused by an improperly adjusted Bowen cable (Fig. 3-25). Place the throttle lever or deadman’s switch in the “Stop” position and verify that the part labeled “Control lever” or the equivalent part on the brake mechanism makes contact with the copper grounding tang. If the switch is loose, replace it. Models [...]
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