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 Kehin 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 Carburetor Types

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, its 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: <br/>&bull; 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

Basic Carburetor Service

Nobody likes to fiddle with a carb if they don't have to. Wedged in between the engine and frame with tubes, cables, and wires sprouting out like spaghetti, carbs are a pain to work on. Carbs require cleaning just like anything else, and some careful observations can save you big money in the long run. Start by pressure washing the bike, especially around the bottom of the carb where roost from the tires and oil from the chain accumulate. Take care when removing the carb, it's easy to damage the cable. Its better to remove the sub-frame so as to enable unrestricted access to the carb. This will also make it easier to route the vent hoses in their proper positions too. When you remove the carb, look at the vent hoses. Are they melted from heat or clogged with mud? If so that can cause a vapor-locking problem in the float bowl and make the engine bog.

Remove the top of the carb and disconnect the cable from the slide. Is the cable frayed or kinked? Is the rubber dust cover missing? If so then replace the cable. Now remove the float bowl, jet baffle (white plastic shroud around main jet), float and fuel inlet needle, and the air-screw. Shake the floats and listen for fluid that may have seeped inside. If so, replace the floats. Otherwise the engine might suffer from constant fuel flooding. Check the fuel inlet needle. It has a Viton rubber tip and occasionally fuel additives and dirt damage the tip. Also check the spring-loaded plunger on the opposite end of the tip. If the spring doesn't push the plunger all the way out then replace it. Check the air-screw, there should be a spring and o-ring on the end of the needle. The spring provides tension to keep the air-screw from vibrating outward and the o-ring seals out dirt and water from entering the pilot circuit. Next check the bell mouth of the carb. Look for the two holes at the bottom of the bell mouth. The one in the center is the air passage for the needle jet and the other hole offset from center is the air passage for the pilot circuit. It's typical for those passages to get clogged with dirt and air filter oil. That would cause the engine to run rough because without a steady stream of air to mix with and atomize the fuel, raw fuel droplets make the jetting seem rich.

Once the carb is basically stripped down (pilot/slow and main jet still in place) you can flush the passages. Get an aerosol can of brake or carb cleaner from an auto parts store. Make sure you get the type with the small diameter plastic tube that attaches to the spray tip. Direct the tip into the airscrew passage. When you spray the cleaner you should see it flow out the pilot/slow jet and the air passage in the bell mouth. Next spray through the pilot/slow jet, look for flow through a tiny passage located between the venturi and the intake spigot. Spraying cleaner through these passages insures that the low speed air and fuel circuits are open and free flowing. The last area to flush with the carb cleaner is the slide bore and slide. Dirt tends to trap there, causing the mating surfaces to develop scratches that could cause the throttle to stick! <br/><br/>Just a small amount of water and dirt can get trapped in the tiny passages of the carb and cause havoc with jetting or even engine damage. How often should you service the carb? When it gets dirty! For example if you ride in muddy wet conditions you should at least check the vent hose. If the riding conditions are dusty and your air filter is covered with dirt, then it's a good idea to do a basic carb servicing.

The process of jetting

Changing air or fuel jets in order to fine-tune engines' performance is very simple. Jetting becomes complicated because mechanical problems sometimes mimic improper jetting. This causes you to waste time and money trying to correct the problem with expensive carburetor jets.

Before you ever attempt to jet a carb, make sure the engine doesn't have any of the problems in the following list. If you are in the process of jetting a carb and you are stumped with a chronic problem, use this section as a guide to enlightenment!

Crankcase air leaks

Can occur at the cylinder base, reed valve, or the magneto seal. Air leaks make the throttle response sluggish and may produce a pinging sound. That sound occurs when the air-fuel mixture is too lean.

Crankcase oil leaks

The right-side crankcase seal is submerged in the transmission oil. When this seal becomes worn out, oil can leak into the crankcase. The oil is transferred up to the combustion chamber and burned with the air-fuel mixture. The oil causes the spark plug to carbon-foul. This mechanical problem makes the jetting seem to be too rich.