Depending upon the application, model, and build date, carburetors have as many as four external adjustments: the throttle-cable anchor, idle rpm screw, and low- and high-speed mixture adjustment needles. The throttlecable anchor is found on chainsaws and other handheld tools with remote throttle triggers. Adjust to allow some free play in the cable when the [...]
As used on Mikuni carburetors and their clones, the cold-enrichment circuit works in conjunction with a lever-operated plunger and a closed throttle to deliver a rich mixture for starting (Fig. 4-35). Intake vacuum draws fuel from the bowl, which then passes through the starting jet into a small mixing chamber for aeration. Raising the brass [...]
Air flow past the nearly closed slide does not generate sufficient vacuum for the needle jet to flow at between 0 and 1/4 throttle. Fuel enters the carburetor bore through the idle, or pilot, jet and enters the main bore through the idle discharge port, located just aft of the slide valve. While single-port models [...]
At 1/4 to 3/4 throttle, the clearance between the needle and needle jet controls fuel delivery with some assistance from the air bled and the cutaway milled on the leading edge of the slide. A clip secures the grooved needle to the slide. Raising the needle richens the mixture by increasing flow through the needle [...]
Slide carburetors are used on motorcycles, water craft, and other applications where customers insist upon performance. At WOT, the slide retracts clear of the bore, offering no obstruction to air flow. The slide works in conjunction with a tapered needle to vary both air and fuel passing through the instrument, a configuration that smoothes the [...]
As the level of fuel in the float bowl drops, the hinged float falls away from the inlet needle. Fuel flows past the needle and into the bowl at a much higher rate than the engine can use. The level in the bowl rises, lifting the float and forcing the needle against its seat. The [...]
Float carburetors give more precise control over the mixture than diaphragm types and cost no more to manufacture. But float mechanisms are gravitysensitive and cease to be reliable at angles of 30° off the horizontal. Consequently, their use is limited to motorbikes, snowmobiles, outboard motors, and other applications that, hopefully, stay upright. small engine float,small [...]
As a very general rule of thumb, most metering levers are adjusted to stand flat or just below the surrounding metal. Zama, for example, calls for the metering needle to be 0 – 0.3 mm (0 – 0.012 in.) below the carburetor body. A depth gauge can be used in the absence of the proper [...]
There are times when one must remove Welch and expansion cups for access to internal circuits, but do not rush into the job. Figure 4-29 shows correct and incorrect ways to attack a Welch plug. The chisel goes in at a shallow angle to avoid scarring the carburetor body. Note the depth of the plug [...]
Once you made a thorough diagnosis, repairs are anticlimatic. But do not expect miracles. Nothing can be done for carburetors like the one in Fig. 4-27. The next priority is to obtain a repair kit. Some engine makers refuse to catalog these kits, preferring to sell new carburetors instead. But carburetors are off-the-shelf items, used [...]
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