Mikuni carb tuning manual free download

This marking on a jetting part is your quality assurance guarantee it is a genuine Mikuni component. For tuning procedures we have a number of manuals available to the left. This is perhaps one of the best written book on carb tuning, just ignore the accelerator pump section if using a non pumper carb and remember that the mixture screw is an air screw where as on carbs with the screw located behind the bowl it will be a fuel screw in which case winding out is to make richer not leaner.

By following the procedures in the book you will determine if jets need changing and in what direction. Order required jets as your checks tell you. Please positively identify your jet type in the jets section, as all jet sales are final with no refunds.

Remember the tuning order is — idle circuit , then main jet and then finally mid range. If at all in doubt, see your local tuning shop. Then count the turns from this point to bottoming out, if you take less than 3 and more than 1 complete turns to achieve bottom, the jet is correct and you simply wind back to the centre point.

Job done. If the jet is wrong, either having no stumble in one direction or the centre point being out of the range this will let you know that a jet change is required and weather you need to order richer or leaner jets.

Nothing else other than the pilot jet has an effect at idle. Some engines will actually have two size jets that may fall within the spec in which case see which feels better when riding, generally the richer one. At wide open throttle only the main jet is working so these are also very easy to tune with a few jets and a bit of spare time.

The performance changes can be detected at wide open throttle in low gears- we do not recommend the operation of motorcycles over the speed limit. Change and test with the next size jet both up or down to determine weather an improvement can be obtained and in what direction.

A river that suddenly narrows can be used to illustrate what happens inside a carb. The water in the river speeds up as it gets near the narrowed shores and will get faster if the river narrows even more. The same thing happens inside the carburetor. The air that is speeding up will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor. Most motorcycle carburetor circuits are governed by throttle position and not by engine speed.

There are five main metering systems inside most motorcycle carburetors. The pilot circuit has two adjustable parts, fig 2. The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture.

If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture.

If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.

The pilot jet is the part which supplies most of the fuel at low throttle openings. It has a small hole in it which restricts fuel flow though it. The slides come in various sizes and the size is determined by how much is cutaway from the backside of it, fig 3. The larger the cutaway, the leaner the mixture since more air is allowed through it and the smaller the cutaway, the richer the mixture will be. Throttle valves have numbers on them that explains how much the cutaway is.

If there is a 3 stamped into the slide, it has a 3. The jet needle is a long tapered rod that controls how much fuel can be drawn into the carburetor venturi. The thinner the taper, the richer the mixture.

The thicker the taper, the leaner the mixture since the thicker taper will not allow as much fuel into the venturi as a leaner one. The tapers are designed very precisely to give different mixtures at different throttle openings. Jet needles have grooves cut into the top. A clip goes into one of these grooves and holds it from falling or moving from the slide. The clip position can be changed to make an engine run richer or leaner, fig 4. If the engine needs to run leaner, the clip would be moved higher.

This will drop the needle farther down into the needle jet and cause less fuel to flow past it. If the clip is lowered, the jet needle is raised and the mixture will be richer. The needle jet is where the jet needle slides into. Depending on the inside diameter of the needle jet, it will affect the jet needle. Most of the tuning for this range is done to the jet needle, and not the needle jet.

Once the throttle is opened far enough, the jet needle is pulled high enough out of the needle jet and the size of the hole in the main jet begins to regulate fuel flow. Main jets have different size holes in them and the bigger the hole, the more fuel that will flow and the richer the mixture.

The higher the number on the main jet, the more fuel that can flow through it and the richer the mixture. The choke system is used to start cold engines. Since the fuel in a cold engine is sticking to the cylinder walls due to condensation, the mixture is too lean for the engine to start. The choke system will add fuel to the engine to compensate for the fuel that is stuck to the cylinder walls.

Once the engine is warmed up, condensation is not a problem, and the choke is not needed. This ideal ratio is only achieved for a very short period while the engine is running. Carburetor troubleshooting is simple once the basic principles are known.

The first step is to find where the engine is running poorly, fig 7. It must be remembered that carburetor jetting is determined by the throttle position, not engine speed. While jetting carburetors, place a piece of tape on the throttle housing. Place another piece of tape on the throttle grip and draw a line while the throttle is at idle straight across from one piece of tape to the other.

When these two lines are lined up, the engine will be idling. Now open the throttle to full throttle and draw another line directly across from it on the throttle housing. At this point, there should be two lines on the throttle housing, and one on the throttle grip. Now find the half-way point between both of the lines on the throttle housing. Make a mark and this will show when the throttle is at half throttle. These lines will be used to quickly find the exact throttle opening while jetting.

Clean the air filter and warm the bike up. Accelerate through the gears until the throttle is at full throttle a slight uphill is the best place for this.

After a few seconds of full throttle running, quickly pull in the clutch and stop the engine Do not allow the engine to idle or coast to a stop. Remove the spark plug and look at its color. It should be a light tan color for more info on reading spark plugs click here. While changing jets, change them one size at a time, test run after each change, and look at the plug color after each run. After the main jet has been set, run the bike at half throttle and check the plug color. The pilot circuit can be adjusted while the bike is idling and then test run.

If the engine is running poorly just off of idle, the pilot jet screw can be turned in or out to change the air-fuel mixture. If the screw is in the back of the carburetor, screwing it out will lean the mixture while screwing it in will richen it. If the adjustment screw is in the front of the carburetor, it will be the opposite.

If turning the screw between one and two and a half doesn't have any affect, the pilot jet will have to be replaced with either a larger or smaller one.

Adjust the pilot circuit until the motorcycle runs cleanly off of idle with no hesitations or bogs. Once the jetting is set and the bike is running good, there are many factors that will change the performane of the engine.

Altitude, air temperature, and humidity are big factors that will affect how an engine will run. Air density increases as air gets colder. This means that there are more oxygen molecules in the same space when the air is cold. When the temperature drops, the engine will run leaner and more fuel will have to be added to compensate. When the air temperature gets warmer, the engine will run richer and less fuel will be needed.

An engine that is jetted at 32deg Fahrenheit may run poorly when the temperature reaches 90deg Fahrenheit. Altitude affects jetting since there are less air molecules as altitude increases. A bike that runs good at sea level will run rich at 10, ft due to the thinner air.

Humidity is how much moister is in the air. As humidity increases, jetting will be richer. A bike that runs fins in the mornings dry air may run rich as the day goes on and the humidity increases. Correction factors are sometimes used to find the correct carburetor settings for changing temperatures and altitudes.

The chart in fig 8 , shows a typical correction factor chart. To use this chart, jet the carburetor and write down the pilot and main jet sizes. Determine the correct air temperature and follow the chart over to the right until the correct elevation is found. Move straight down from this point until the correct correction factor is found.

Using fig 8 as an example, the air temperature is 95deg Fahrenheit and the altitude is ft.