Carburetor Tuning

Discussion in 'Technical' started by mudboots, Aug 4, 2013.

  1. carburetor tuning for those guys that have little or no knowledge at all, And next time will go into portting and other things for those that are interested.
    As for myself Two Stroke bikes are my thing but some of this applies to Four Strokes, I read a lot on GT Rider where a lot you guys seem to worry about fuel consumption "I do NOT i consider my bikes as toys to ride for fun, my CR500 i will flat out get one hour out of a tank of fuel but by god after one hour you need to stop anyway its a beast to hang onto, my WR 400 is nearly as bad on fuel. My CRM 250 when i am done with the mods to it will be a thirsty bike as well but it will run a lot better and i should be getting close to 38 HP when she is all done.
    So here we go... [/IMG]
    Carburetor tuning has the greatest effect on engine performance. When a motorcycle Manufacturer builds a bike, they usually install jets in the carb that are too rich in two- strokes and lean in four-strokes to do with fuel emissions these days, The manufacturers sell the same model worldwide, so they couldn’t afford to install different jets in the carb to suit all the different climates and types of fuel. In addition to the climate and fuel, the manufacturer would also have to consider many other factors, such as the terrain and type of riding.
    The Ride and Feel Method
    As for myself The most basic method of determining correct carburetor jetting is "ride and feel."I’d listen for pinging or knocking noises or excessive smoke from the pipe.If the engine is slow to respond and bogs (engine makes a booooowah sound) then the carb jetting is too lean. You can verify lean jetting by engaging the carb’s choke to the halfway position. This will make the air-fuel mixture richer and the engine should respond better. This method requires you to determine if the carburetor tuning is too rich or too lean by the sound and feel of the engine.If the carb jetting is too rich, then the engine will make a crackling sound; the exhaust smoke will be excessive and the engine will run as if the choke is engaged The first step is to mark the throttle body in 1/4-throttle increments, from closed to full open. Then, this method requires that you ride the motorcycle on a flat, road or track. To check the carb jetting for throttle positions up to 1/2 throttle, ride the motorcycle in second or third gear. Roll on the throttle slowly from 1/4 to 1/2 then 1/2 to full. I do whats called a plug chop after a few min at say running 3/4 pull in your clutch and shut the motor off at the same time stop as quick as you can pull out your plug you are looking for a nice biscuit brown color on your plug if its to light in color it running lean, dark brown rich. (have 2 or 3 new plugs on hand to do a plug chop.)
    Another important tip is to just change the jets one increment at a time, either rich or lean, until the engine runs better. Most people are afraid to change a jet because they think that the engine will be in danger of seizing. Believe me, one jet size won’t make your engine seize but it could be the difference between running bad and running acceptable.
    The carb jets that affect the jetting from 1/2 to full throttle are the jet-needle, main jet, power jet (electronic carbs) and the air jet (on four-strokes).
    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
    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 this is my new Kehin [/IMG]
    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. [/IMG] [/IMG] [/IMG] [/IMG]
    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.)
    Basic Carb 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 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!
    Mechanical Problems
    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.
    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—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, 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.
    Check these for leaks.
    Coolant-system leaks—Coolant systems leaks commonly occur at the cylinder-head gasket. When the coolant leaks into the combustion chamber, it pollutes the air-fuel mixture and causes a misfire or popping sound at the exhaust pipe. Check the engine’s coolant level frequently. Hondas and Kawasaki’s have characteristic coolant leaks because they use steel head gaskets. Yamahas and Suzuki’s use O-rings to seal the head and cylinder. Coolant-system leaks lower the engine’s peak horsepower. It makes the engine run as if the air-fuel mixture is too rich.
    Carbon-seized exhaust valves—the exhaust valves sometimes become carbon-seized in the full-open position. This mechanical problem can make the engine run flat at low rpm and make the slow-speed jetting seem lean. The carbon can be removed from the exhaust valves with oven cleaner. Clean the exhaust valves whenever you replace the piston and rings.
    Blown silencer—when the fiberglass packing material blows out of the silencer, excess turbulence forms in the silencer and the turbulence causes a restriction in the exhaust system. This restriction makes the engine run flat at high rpm.
    Broken reed-valve petals—the petals of the reed-valve can crack or shatter when the engine is revved too high. This mechanical problem makes the engine difficult to start and can also have a loss of torque. Expert rider should switch to carbon fiber reed petals because they resist breaking at high rpm. Novice riders should use dual-stage fiberglass reeds (Aktive or Boyesen). These types of reed petals provide an increase in torque. This is my standard reed block and then with Boyesen fitted. [/IMG] [/IMG]
    Make sure the packing in your silencer has not been blown out. It makes a difference.
    Weak spark—when the ignition coils deteriorate, the engine performance will become erratic. Normally, the engine will develop a high-rpm misfire problem. Check the condition of the coils with a multimeter.
    Clogged carburetor vent hoses—when the carburetor vent hoses get clogged with dirt or pinched closed, the jetting will seem to be too lean, so the engine will run sluggish. Always check the condition of your carburetor vent hoses. Make sure there is no mud in the hoses and that the hoses are not pinched between the suspension linkage.
    Carburetor float level—when the float level is too low, the jetting will seem to be too lean. Engine performance will be sluggish. When the float level is too high, the jetting will seem to be too rich.
    Worn carburetor fuel-inlet needle—when the fuel-inlet needle wears out, excess fuel enters the float bowl and travels up the slow jet and into the engine. This makes the carb jetting seem to be too rich. Replace the fuel-inlet needle and seat every two years.
    Make sure you have a good selection of jets the Chinese make kits of 10 jets for around $20 to fit most Kehin and Mikuni carburetors you can get them on e-bay i suggest getting a kit with your standerd jet in the middle so you can go up or down in size and same for pilot jets. this really comes in to play if you have mod'ed your engine with portting and after market exhaust and so on. [/IMG];-)
    The Weather Makes The Biggest Difference!
    Weather can have a profound affect on the carb jetting because of the changes in air density. When the air density increases, you will need to richen the air-fuel mixture to compensate. When the air density decreases, you will need to lean-out the air-fuel mixture leaner to compensate. Use the following as a guide to correcting your jetting when the weather changes:
    Air temperature—when the air temperature increases, the air density becomes lower. This will make the air-fuel mixture richer. You must select jet sizes with a lower number to compensate for the lower air density. When the barometric pressure decreases, the opposite effect occurs.
    Humidity—when the percentage of humidity in the air increases, the engine draws in a lower percentage of oxygen during each revolution because the water molecules (humidity) take the place of oxygen molecules in a given volume of air. High humidity will make the air-fuel mixture richer, so you should change to smaller jets.
    Altitude—generallly, the higher the altitude, the lower the air density. When riding on tracks that are at high altitude, you should change to smaller jets and increase the engine’s compression ratio to compensate for the lower air density.
    Track Conditions and Load
    The conditions of the terrain and the soil have a great affect on jetting because of the load on the engine. Obstacles like big hills, sand, and mud place a greater load on the engine that requires more fuel and typically richer jetting
    Pre-mix oils are formulated for a fairly narrow range of pre-mix ratios. You should examine the oil bottle for the oil manufacturer’s suggestion on the pre-mix ratio. All production two-stroke dirt bikes have a sticker on the rear fender suggesting that you set the pre-mix ratio to 20:1 That sticker is put there for legal purposes. Always refer to the oil manufacturer’s suggestion on pre-mix ratios. In general, small-displacement engines require a richer pre-mix ratio than do large-displacement engines because smaller engines have a higher peak rpm than larger engines. The higher the engine revs, the more lubrication it requires.
    Well i hope this has helped some of you guys. ;-)
  2. You have a point in writing I forgot about the fuel especially in Thailand. Australia you can still get 98% octane fuel most places. So here I still am getting the color on my plugs but I have not heard about using a lighted magnifier mind you I could use one the old eyesight is not as good these days:rolleyes: Long live the Two Strokes.
  3. If you increase the premix over the manufacturers recommendations you are leaning out the fuel/air ratio. Also you need to look for clouds as even if the bike is perfectly tuned it will change if the sun goes behind the clouds:lolno:
  4. There is a guy on the NSR250 forum whose signatures states:

    "If I have to take the carburettors off one more time..."

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