A stumble or flat spot during acceleration indicates that the mixture is too lean. If a conventional (non-wick) accelerator pump is fitted, blipping the throttle should send a stream of fuel into the carburetor bore. Carburetors without accelerator pumps need a rich low-speed mixture, even at the expense of a smooth idle. Back out the [...]
The most common cause of erratic idle is a low-speed mixture needle adjusted to the point of either rich or lean drop-off. Other possible causes are a metering-diaphragm lever set too high, a distorted or incorrectly sized metering spring, or a leaking nozzle check valve. Many idle-related problems come about because fuel vapor drops out [...]
Refusal to idle can mean that the engine dies when the throttle is closed or that a closed throttle does not bring rpm down. If the latter is the case, look for • Maladjusted idle stop screw, also known as the idle rpm screw. Refer to the “External Adjustments” section below. • Maladjusted throttle cable. [...]
Refusal to start can nearly always be laid to insufficient fuel delivery. The spark plug tip will remain dry after extended cranking. A pressure tester will show if the inlet needle unseats, but its good practice to spray a small amount of carburetor cleaner into the spark-plug port. If the engine runs and quits after [...]
If the carburetor fails to hold pressure, immerse it in clean solvent. Bring pressure back up to 7 psi and trace any bubbles back to their sources. If, for example, bubbles stream out the vent hole in the metering-chamber cover, the diaphragm leaks. Remove the cover and repeat the test. There should be no bubbles [...]
It can be helpful to quantify inlet-needle response. With the tester connected to the carburetor inlet fitting, slowly increase pressure. At some point not always well defined by the literature, pressure will abruptly drop as the inlet needle unseats, or pops off. The reading should then stabilize as the needle returns to its seat. Ultralight [...]
The first priority is to determine if the metering system, the source of most problems, functions. Connect the tester to the carburetor inlet fitting and pressurize the circuit to 6 or 7 psi, the equivalent of fuel-pump pressure. The gauge should hold steady (Fig 4-26). Close the choke or insert a rag into the carburetor [...]
Readers who intend to do much carburetor work should purchase a Walbro PN 500-500 tool kit, available from factory distributors. The kit includes cape chisels and punches for dealing with Welch plugs, a slide hammer, and, most importantly, a PN 57-11 pressure tester. Any of these tools can be purchased separately. Other sources of pressure [...]
Carburetors cause a disproportionate amount of trouble, but by no means are the only source of problems. Before rushing to judgment • Drain the fuel tank and add fresh premix with the correct amount of lubricant. • Replace the fuel filter. • Clean or replace the tank vent valve. • Visually inspect fuel lines for [...]
Most Zama and some Walbro carburetors regulate fuel delivery mechanically with a barrel throttle and tapered needle that extends into the main nozzle. These instruments dispense with lowspeed circuit: all fuel enters through the nozzle. The Walbro range consists of the WY (Fig. 4-23) and the WZ series (Fig. 4-24), with the former distinguished by [...]
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