The Differences in Two-Stroke and Four-Stroke Carbs

The difference between a two-stroke and four-stroke engine is intake velocity. Two-stroke engines have lower velocity so the needle jet has a half-moon shaped hood protruding into the venturi to produce a low pressure area that aids in drawing the fuel up through the needle jet. Four-stroke carbs need to atomize the fuel more so than a two-stroke carb because so much of the fuel shears along the intake port and separates from the mixture stream. Four-stroke carbs have more jets and finer adjustment screws, plus they usually are equipped with an accelerator pump. A typical state of the art four-stroke carb is the Keihin CR.
The latest trend in two-stroke carbs features a pump that sprays fuel into the venturi from 1/4th to 3/4th throttles. In the past, carb manufacturers made jet needles that attempted to compensate for the natural lean condition of the mid-range but that compromised the jetting at full throttle. The auxiliary pumps are powered by electricity supplied by the alternator (about 5 watts) and controlled by either a throttle position or an rpm sensor.

Identification Guide to Popular Carb Types

Keihin PWK Carburetor.

On two-stroke engines, several different model carbs have been used over the years, but there are basically two big carb manufacturers. Kehin and Mikuni are two popular brands of Japanese carbs used on nearly every dirt bike.
Kehin has several different models. The most popular ones are the PJ, PWK, and PWM. The PJ is used on Honda CR125, 250, and 500 models from 1985-1997 The slide is oval shaped and there are no additional pumps, and it’s just a simple carb. In fact it’s so simple that the choke and idle screw share the same jet. The PWK was the next step up from the PJ. The PWK has a crescent shaped slide and a separate idle circuit from the choke. The PWK is used on Kawasaki KX125, 250, and 500 models from 1990-97. The latest version of the PWK features a pump to supply extra fuel in the mid-range. The PWM is similar to the older PWK (no pump) and the overall length is shorter.
Mikuni has several different model carbs too. The original model VM had a round slide. There are many different parts available including needle jets of different diameters and jet needles with different taper angles and diameters. The next model was the TMX, which became available in 1987. It was a flat-slide carb, which offered a greater peak flow rate. The TMX was revised several times, becoming smaller with fewer parts. The TMS carb introduced in 1992 had no main or pilot jet. The slide and jet needle handled all the jetting. That carb worked great on 250cc bikes but never became popular. The PM is the latest Mikuni model. It features an oval crescent shaped slide and a very short body. That carb comes standard on Yamaha YZ125 and 250 1998 and newer models.
Carburetor Parts and Function
A carburetor is a device that enables fuel to mix with air in a precise ratio while being throttled over a wide range. Jets are calibrated orifices that take the form of parts such as pilot/slow jets, pilot air screw, throttle valve/slide, jet needle, needle jet/spray-bar, air jet, and main jet. Fuel jets have matching air jets, and these jets are available in many sizes to fine-tune the air-fuel mixture to the optimum ratio for a two-stroke engine, which is 12.5: 1.
Fuel Jets, Air Jets, and Throttle Positions
Three circuits control the air: the air-screw, the throttle slide, and the air jet. Four circuits control the fuel: the pilot/slow jet, the spray-bar/needle jet, the jet needle, and the main jet. The different air and fuel circuits affect the carb jetting for the different throttle-opening positions, as follows:
Closed to 1/8 throttle—air screw and pilot/slow jet
1/8 to 1/4 throttle—air-screw, pilot/slow jet, and throttle slide
1/4 to 1/2 throttle—throttle slide and jet needle
1/2 to full open—jet needle, spray-bar/needle jet, main jet, and air jet
(Note: On many modern carbs the spray-bar/needle jet and air jets are fixed-diameter passages in the carburetor body and cannot be altered.)