QuestionHi Mark,
I've just bought this 1983 Suzuki DR125 S to teach my daughter to ride,but I'm having a problem with the carb not wanting to idle and acts if it is flooding out when you give it gas.I've cleaned the carb and reinstalled it but no change.I'ts a Mikuni carb and I'm not even sure if it's the origional one for the bike or not.I can see gas splatter at the air intake side of the carb when you give it gas.I turned the gas off to let the bike burn whats in the bowl and as it runs out the bike clears up and revs fine before it dies.
I hope you can make some sense out of this.Thanks in advance for any help you can give me!
bill
AnswerHi Bill,
I suspect the fuel level is too rich. Review the information below about adjusting fuel level (float height) and carb cleaning.
For some reason everyone seems to think tuning a carb is simple. Change a jet or two and you're done. There are quite literally millions and millions of jet combinations. A rough check on Bing carbs shows there are at least 13,860,000 different possible jetting combinations.
If you look at a carburetor, you will notice a rather large hole going from front to rear. This is called the Venturi. Air passes into the engine through the Venturi. As the velocity (speed) of the air entering the carb increases, it's pressure decreases creating a low pressure or vacuum in the Venturi. This vacuum moves around in the Venturi, as the throttle is opened, and sucks gasoline through the different jets in the carb. The gas then mixes with the air going through the Venturi. The way the jets are made causes the fuel to vaporize as it goes into the Venturi. Where the jets are placed in the carb and where the jet's outlet is located in the Venturi, determines what part of the throttle opening that jet controls. The idle jet system (comprised of pilot air jet, pilot fuel jet and pilot fuel screw) controls from 0% to about 25% of the throttle opening. The throttle valve controls 0% to 35% of the throttle opening. The needle jet and jet needle control from 15% to 80% of the throttle opening and the main jet controls 60% to 100%. This means that when you open the throttle about one eighth of the way open, all of the gas/air mixture going into your engine is controlled by the idle jet. As you can see the different jets over lap the operating range of each other. That is, the jet needle starts to effect things before the effect of the idle jet ends. This is something to remember when working on carbs... everything is interconnected. Change one thing and it will affect other things.
OK, let's go over the different systems in the carb and see what they do.
Fuel Level: The fuel level is controlled by the fuel floats and the fuel float valve. The floats are hollow or made of something that will float on gasoline, such as cork, metal, or plastic. Part of the float presses against the float valve, sometimes called a needle and seat. Most times the part of the float that touches the float valve needle is bendable so you can adjust the level of the fuel in the float bowl. Some floats are not adjustable. If this level is way too high, gas can leak out the carb overflow tube or into the engine and/or air box. If fuel gets into the engine it will thin out the engine oil, ruining its ability to lubricate. This will cause serious engine failure. If a full tank of gas in the evening turns into a half tank by morning, check your oil. If it's thin and smells like gas, change it and replace your float valve and/or check your fuel level. If the oil is OK, check under the overflow tube. If it's OK, then check where you are parking your bike because someone is walking away with your gas! If your fuel level is just a bit high, the mixture will tend to be a bit rich. If it's low, the mixture will tend to be a bit lean. This is because a higher fuel level requires less vacuum to draw fuel into the Venturi. This is called Service fuel level. You can measure this level with special tools with the carbs on the motorcycle.
Pilot Circuit: The pilot jet controls the idle and its working range is from idle to ¼ throttle opening. On some carbs there is an air jet, too. In conjunction with the pilot jet there is a pilot jet air screw. This screw leans or richens the fuel mixture for a smooth idle and up to 1/4 throttle opening. From the pilot jet there are little passages cast into the carb body that leads to holes just in front of the throttle valve or plate. There can be one or more holes depending on carb design. They affect the air/fuel mixture as long as vacuum in the Venturi is over them. As the throttle opens wider vacuum moves to the needle jet and jet needle.
Throttle Valve/Slide: The throttle slide looks cylindrical & moves in a vertical pattern. The slide has a bevel angle cut on the intake side. This angle comes in several angles and helps control the fuel mixture from idle to ¼+ throttle opening. CV or Constant Vacuum (diaphragm) type carburetor slides have flat Venturi bottoms. The throttle plate acts as the slide angle in CV type carbs.
Jet Needle: The jet needle looks like a metal tooth pick. It meters fuel/air mixture from ¼ to ¾ throttle opening. This is the needle that rides in the throttle slide and goes into the needle jet. It can come in many different sized tapers. Sometimes, one needle can have several tapers on it. The top end of the needle has grooves cut in it, usually five, and you can move the little clip on the end up or down to lean (down) or richen (up) the mixture. Most late model bikes have needles with only one groove cut in them. This is so you can't richen the mixture, thereby keeping the EPA happy.
Needle Jet: The needle jet is brass or bronze and is located directly below the throttle slide. The needle protrudes up into the Venturi. The leading edge of the protrusion creates a low pressure, or choke, on the trailing edge which aids in drawing fuel/air mixture up through the emulsion tube, and into the Venturi. The main jet and emulsion tube are directly below the needle jet. The jet needle and the needle jet work together to meter fuel/air.
Main Jet: This jet controls the fuel mixture from ¾ to wide full open throttle position.
We want nice clean acceleration from idle to full throttle with no stumbling or flat spots. This can be quite a tall order if we are starting with a new carb. Actually, it can be a real challenge to get the air/fuel mixture right after something as simple as an exhaust pipe change. If you are working at home, you may not have the jet selections the local dealer's service department has on hand. You may make several trips to the dealer before the job is done.
Read the spark plugs to determine changes to be in jetting. The following is an actual means of performing a plug reading test: Find an open highway or roadway that you can lawfully and safely perform this test. Run the engine at the throttle opening of the jetting range you want to test, for the distance of about a mile. While riding, pull in the clutch lever and kill the engine, then find a safe location to park. Remove and examine the spark plugs. Is the spark plug reading lean or rich? When you get back to your shop or garage, you can look for the jet that controls that particular throttle opening and exchange it for a richer or leaner one.
The throttle transition from one jet to the next must be smooth. Each fuel circuit overlaps another. Make sure you have a good selection of jets. Most carburetor manufacturers have tables of specifications on the jet needles and needle jets, and other jets. You will find these charts very useful. With these specs you can make a better guess as to what jet will work best to start tuning. Some places use motorcycle dynamometers for testing. These can be a big help to get real close to the best jet setting. Working out the best main jet for a 170 MPH bike can be quite unhealthy if you only have a freeway to perform tests.
Carb Cleaning 101
By M. Shively
The elements of internal combustion engines are: correct fuel/air ratio, spark at right time, adequate cylinder compression.
There are many passageways and openings to check and clean. All are important in function and when obstructed or not working properly, have subtle to radical effects on engine performance. Vacuum leaks and carburetor synchronization also have effects on performance and should be inspected and adjusted following the below procedures.
Carb Cleaning 101
Warning: Remove all rubber parts before you begin. These parts usually include vacuum diaphragms, needle valves, orings, hoses, and other parts. Spray cleaners will damage these parts. Do not disassemble individual carbs from the carb bracket.
Air & Fuel Passageways: Trace and learn individual fuel and air circuits from beginning to end. Machines can only drill straight through the cast passageways. To change direction, another angled passageway must be drilled. The union is plugged with a brass or bronze bead. Inspect and clean each passageway with spray cleaner, brushes/pipe cleaners/etc, and compressed air. Remove any discoloration and debris. Look for spray cleaner to exit from one or more passageways.
Jet Cleaning: Inspect jets by holding to light and look through them. You should see an unobstructed round hole. Clean the jets with one or more of the following: jet cleaning wires, soak solutions, carb spray cleaners and compressed air. Re-inspect jets after cleaning and install when clear of obstructions. Some main jets have paper-like gaskets. Most have metal spacers between the jet and the emulsion tube. Some screw directly into a brass emulsion tube which is machined for a 7mm wrench at its float chamber exposed base.
Inlet Fuel Valve: Inspect the needle valve & spring. Press down the tiny metal rod that protrudes from the butt or float end of the needle valve. The spring should move freely and return the rod to its location. Check the needle valve's seat area for a groove or other wear. It should appear highly polished. Some needle valve seats are rubber and wear may not be visible. Inspect the needle valve jet seat. You can clean the jet seat with Q-tips and semi-chrome polish if necessary.
Carb Body Castings: Blow air through the atmospheric vent holes located on the dome of each float bowl chamber. Air should exit via hoses or brass nipples. Inspect the emulsion tubes and passageways (cast towers that jets thread into) for discoloration and debris. Clean interior emulsion towers with a soft bristle gun cleaning brush. Clean the Venturi's (main carb bore).
Needle Jets & Jet Needles: Clean the needle jets, jet needles, and passageway or tower that needle jet screws into. Clean the emulsion tube (pipe between needle jet and main jet) (Main Jet may screw into emulsion tube). Jet needles are part of the throttle slides. See below…
Throttle Slides: There are several types of throttle slides: Mechanical linkage, vacuum, diaphragm, and cable. Disassembling the jet needle from the slide is not always required for cleaning. If you have vacuum piston type throttle slides (large diameter solid metal slide), avoid cleaning the lubrication from sides and caps. If piston type check cap vents and passageways with air. Clean if necessary and re-lube. If you have rubber vacuum throttle diaphragms, inspect for dry-rot, defects, and tears by gently stretching rubber away from center. Do this until all areas around diaphragm have been inspected. Replace any defective part as described above. Clean carb body areas around diaphragm including air passageways and air jets. Diaphragms have a locator loop or tab fabricated into their sealing edge. Observe this locator upon reassembly. Avoid pinching the diaphragm when reinstalling caps.
Fuel Screws: Fuel screws have sharp tapered ends. Carefully turn one fuel screw in while counting the turns until it seats lightly. Warning: These screws are very easily damaged if over tightened into their seats. Record amount of "turns-in" and remove the fuel screw, spring, washer, and oring. The fuel screw is part of the enrichment (choke) circuit...clean passageways as described above. When carbs are assembled, spray low PSI compressed air into diaphragm air vents located at intake side of carbs. Throttle slides should rise, then fall when air is removed. Lightly lube external moving linkages. Reinstall carbs and follow through with carburetor synchronization.
Throttle Cables: Lubricate cables periodically. If cables are disconnected from carbs or removed for replacement, etc . . . remember cable routing and ensure proper reinstallation routing. Avoid bread-tying, sharp bends, and pinching cables. Adjust cables so throttle grip has about 5mm of play or throttle slides or butterfly valves may not open completely (full throttle)(wide full open).
Float Bowls: Inspect float bowls for sediment, gum or varnish, crystallization, and defects. Clean all pipes, tubes, passageways, and embedded jets with cleaners and compressed air. Remove and clean the drain screw and area. Inspect bowl gasket and replace if necessary. Clean and inspect overflow pipes and tubes, look for vertical cracks.
Floats: There are several types of float materials: plastic, brass, black composite, tin, and others. Handle floats carefully. Avoid bending, twisting, denting, or other means of mishandling. Most floats are adjustable by bending a small metal tab near the float axle end. Do not change the float adjuster tab unless tuning fuel service levels. Clean metal floats by soaking or spray cleaners, if necessary. Replace other type floats if cleaning is necessary. Clean the float axle or pin.
Synchronization: This is a fine adjustment performed usually and preferably with the carbs installed and the engine running. The unusual part is performed with gauged wire with the carbs on the work bench. Carburetor synchronizing balances Venturi vacuum at the exhaust side of each carburetor, resulting with smooth idling and optimized performance at all throttle openings. Synchronization is checked using a set of gauges which are either air vacuum type or liquid mercury type. The gauges are connected to vacuum ports on the intake manifolds via nipple tubes or if sealed with screws, sync gauge adapters will be needed. With the engine running at temperature, and with a fan or means of forced convection aimed onto the engine, the carbs fuel screws and idle are adjusted, then the synchronization is adjusted via adjustment screws on the carbs. A reserve fuel tank is recommended for convenience of accessing carbs during this procedure. See gauge instructions and repair manuals for detailed use of synchronization gauges.
Notes: While carbs are apart, record the jet sizes. Look for a very small number imprinted on the body of the jets. It should be the same number for all. If you have dial or veneer calipers, measure and record float heights. Perform measurements with floats just touching needle valves, though not depressing the needle valve rods. Replace fuel and vacuum hoses. Be sure to use fuel rated hose for fuel. Install or replace in-line fuel filters. It's a good time to remove and clean interior petcock fuel filters. Inspect carb manifolds for dry-rotting, inspect all clamps and air ducts. Inspect, clean, lube, and/or replace air filter(s).
