Mikuni carburetor operation and tuningMikuni Motorcycle Carburetor Theory101Motorcycle carburetors look very complex, but with a little theory, youcan tune your bike for maximum performance. All carburetors work under the basicprinciple of atmospheric pressure.
Atmospheric pressure is a powerful forcewhich exerts pressure on everything. It varies slightly but is generallyconsidered to be 15 pounds per square inch (PSI).
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This means that atmosphericpressure is pressing on everything at 15 PSI. By varying the atmosphericpressure inside the engine and carburetor, we can change the pressure and makefuel and air flow.Atmospheric pressure will force high pressure to low pressure. As the pistonon a two stroke engine goes up (or goes down on a four stroke engine), a lowpressure is formed inside the crankcase (above the piston on a four stroke).This low pressure also causes a low pressure inside the carburetor. Since thepressure is higher outside the engine and carburetor, air will rush inside thecarburetor and engine until the pressure is equalized. The moving air goingthrough the carburetor will pick up fuel and mix with the air.Inside a carburetor is a venturi, fig 1. The venturi is a restrictioninside the carburetor that forces air to speed up to get through. A river thatsuddenly narrows can be used to illustrate what happens inside a carb.
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The waterin the river speeds up as it gets near the narrowed shores and will get fasterif 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 thecarburetor.
The faster the air moves, the lower the pressure inside thecarburetor.FIG 1Most motorcycle carburetor circuits are governed by throttle positionand not by engine speed.There are five main metering systems inside mostmotorcycle carburetors. These metering circuits overlap each other and they are:. pilot circuit. throttle valve.
needle jet and jet needle. main jet. choke circuitThe pilot circuit has two adjustable parts, fig 2. The pilot air screwand pilot jet. The air screw can be located either near the back side of thecarburetor or near the front of the carburetor. If the screw is located near theback, it regulates how much air enters the circuit. If the screw isturned in, it reduces the amount of air and richens the mixture.
If it isturned out, it opens the passage more and allows more air into the circuit whichresults in a lean mixture. If the screw is located near the front, itregulated fuel. The mixture will be leaner if it is screwed in and richer ifscrewed out. If the air screw has to be turned more than 2 turns out for bestidling, the next smaller size pilot jet will be needed.FIG 2The pilot jet is the part which supplies most of the fuel at low throttleopenings. It has a small hole in it which restricts fuel flow though it.
Boththe pilot air screw and pilot jet affects carburetion from idle to around 1/4throttle.The slide valve affects carburetion between 1/8 thru 1/2 throttle. Itespecially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2. Theslides come in various sizes and the size is determined by how much is cutawayfrom the backside of it, fig 3. The larger the cutaway, the leaner themixture (since more air is allowed through it) and the smaller the cutaway, thericher the mixture will be. Throttle valves have numbers on them that explainshow much the cutaway is. If there is a 3 stamped into the slide, it has a 3.0mmcutaway, while a 1 will have a 1.0mm cutaway (which will be richer than a 3).FIG 3The jet needle and needle jet affects carburetion from 1/4 thru 3/4 throttle.The jet needle is a long tapered rod that controls how much fuel can be drawninto the carburetor venturi.
The thinner the taper, the richer the mixture. Thethicker the taper, the leaner the mixture since the thicker taper will not allowas much fuel into the venturi as a leaner one. The tapers are designed veryprecisely to give different mixtures at different throttle openings.
Jet needleshave grooves cut into the top. A clip goes into one of these grooves and holdsit from falling or moving from the slide.
The clip position can be changed tomake an engine run richer or leaner, fig 4. If the engine needs to runleaner, the clip would be moved higher. This will drop the needle farther downinto 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 insidediameter of the needle jet, it will affect the jet needle.
The needle jet andjet needle work together to control the fuel flow between the 1/8 thru 3/4range. Most of the tuning for this range is done to the jet needle, and notthe needle jet.FIG 4 The main jet controls fuel flow from 3/4 thru full throttle, fig 5.Once the throttle is opened far enough, the jet needle is pulled high enough outof the needle jet and the size of the hole in the main jet begins to regulatefuel 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 numberon the main jet, the more fuel that can flow through it and the richer themixture.FIG 5The choke system is used to start cold engines.
Since the fuel in a coldengine is sticking to the cylinder walls due to condensation, the mixture is toolean for the engine to start. The choke system will add fuel to the engine tocompensate for the fuel that is stuck to the cylinder walls. Once the engine iswarmed up, condensation is not a problem, and the choke is not needed.The air/fuel mixture must be changes to meet the demands of the needs of theengine. The ideal air/fuel ratio is 14.7 grams of air to 1 gram of fuel.
Thisideal ratio is only achieved for a very short period while the engine isrunning. Due to the incomplete vaporization of fuel at slow speeds or theadditional fuel required at high speeds, the actual operational air/fuel ratiois usually richer. Figure 6 shows the actual air/fuel ratio for any giventhrottle opening.FIG 6Carburetor Jetting TroubleshootingCarburetor troubleshooting is simple once the basic principles are known. Thefirst step is to find where the engine is running poorly, fig 7. Itmust be remembered that carburetor jetting is determined by the throttleposition, not engine speed.
If the engine is having troubles at low rpm(idle to 1/4 throttle), the pilot system or slide valve is the likely problem.If the engine has problems between 1/4 and 3/4 throttle, the jet needle andneedle jet (most likely the jet needle) is likely the problem. If the engine isrunning poorly at 3/4 to full throttle, the main jet is the likely problem.FIG 7While 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 thethrottle is at idle) straight across from one piece of tape to the other. Whenthese two lines are lined up, the engine will be idling. Now open the throttleto full throttle and draw another line directly across from it on the throttlehousing. At this point, there should be two lines on the throttle housing, andone on the throttle grip.
Now find the half-way point between both of the lineson the throttle housing. Make a mark and this will show when the throttle is athalf throttle. Divide the spaces up even again until idle, 1/4, 1/2, 3/4, andfull throttle positions are known. These lines will be used to quickly find theexact throttle opening while jetting.Clean the air filter and warm the bike up. Accelerate through the gears untilthe 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 andstop 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 (formore info on reading spark plugs click here). If it's white, the air/fuelmixture is too lean and a bigger main jet will have to be installed. If it'sblack or dark brown, the air/fuel mixture is too rich and a smaller main jetwill have to be installed. 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 theplug color. If it's white, lower the clip on the jet needle to richen theair/fuel mixture.
If it's dark brown or black, raise the clip to lean theair/fuel mixture.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 beturned in or out to change the air-fuel mixture. If the screw is in the back ofthe carburetor, screwing it out will lean the mixture while screwing it in willrichen it. If the adjustment screw is in the front of the carburetor, it will bethe opposite. If turning the screw between one and two and a half doesn't haveany affect, the pilot jet will have to be replaced with either a larger orsmaller one.
While adjusting the pilot screw, turn it 1/4 turn at a time andtest run the bike between adjustments. Adjust the pilot circuit until themotorcycle runs cleanly off of idle with no hesitations or bogs. Altitude, Humidy, and Air TemperatureOnce the jetting is set and the bike is running good, there are many factorsthat will change the performane of the engine.
Altitude, air temperature, andhumidity are big factors that will affect how an engine will run. Air densityincreases as air gets colder. This means that there are more oxygen molecules inthe same space when the air is cold. When the temperature drops, the engine willrun leaner and more fuel will have to be added to compensate. When the airtemperature gets warmer, the engine will run richer and less fuel will beneeded. An engine that is jetted at 32deg Fahrenheit may run poorly when thetemperature reaches 90deg Fahrenheit.Altitude affects jetting since there are less air molecules as altitudeincreases. A bike that runs good at sea level will run rich at 10,000 ft due tothe thinner air.Humidity is how much moister is in the air.
As humidity increases, jetting willbe richer. A bike that runs fins in the mornings dry air may run rich as the daygoes on and the humidity increases.Correction factors are sometimes used to find the correct carburetor settingsfor changing temperatures and altitudes. The chart in fig 8, shows atypical correction factor chart. To use this chart, jet the carburetor and writedown the pilot and main jet sizes.
Determine the correct air temperature andfollow the chart over to the right until the correct elevation is found. Movestraight down from this point until the correct correction factor is found.Using fig 8 as an example, the air temperature is 95deg Fahrenheit andthe altitude is 3200 ft. The correction factor will be 0.92. To find out thecorrection main and pilot jets, multiple the correction factor and each jetsize. A main jet size of 350 would be multiplied by 0.92 and the new main jetsize would be a 322. A pilot jet size of 40 would be multiplied by 0.92 and thepilot jet size would be 36.8.FIG 8Correction factors can also be used to find the correct settings for theneedle jet, jet needle, and air screw. Use the chart from fig 9 anddetermine the correction factor.
Then use the table below to determine what to dowith the needle jet, jet needle, and air screw.Needle Jet/Jet Needle/Air Screw Correction ChartCorrection factor1.04 or above1.04-1.001.00-0.960.96-0.920.92 or belowNeedle jetTwo sizes largerOne size largerSame sizeOne size smallerTwo sizes smallerJet needle settingLower clip positionSameSameSameRaise clip one positionAir screw openingOne turn in1/2 turn inSame1/2 turn outOne turn outFIG 9 This article was written by, and has been reprinted with thepermission of Doug Jenks. All rights [email protected] to page.
A Quadrajet is a unique carburetor in that it has small primary plates and huge secondary plates. It is a good carburetor for fuel economy and still has stellar performance when required. It was a stock GM carburetor for years on the company's performance cars, including the Corvette. It is a good carburetor for up to close to 400 horsepower. Its limit in horsepower applications and ability to be modified was its downfall at the racetrack when horsepower climbed. The Quadrajet uses many moving parts, unlike most carburetors.
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Remove the air cleaner. Inspect the choke system while the engine is cold. The choke mechanism is the plate on the top of the carburetor.
It has a spring located in the manifold with a rod running up to the plate. There will be a vacuum-operated servo on the passenger side rear of the carburetor, with a rod also attached to the choke mechanism. When the engine is cold, the spring contracts and creates a pull on the choke to close. The throttle keeps it from closing as long as it is not disturbed. When the engine is started, the throttle should be tapped just a little to allow the choke to snap shut. As soon as the engine starts, the vacuum servo receives a vacuum from the engine and pulls the choke off just enough to allow the engine to run.
However, it still creates a partial choke condition. As the engine heats up, the spring in the manifold expands and puts pressure on the choke to open the choke. On all carburetors, the vacuum created as air is moved into the engine sucks the fuel out of the passages in the carburetor.
When the engine has just been started, there is obviously no vacuum-so no fuel is sucked into the engine to start. The choke, when closed, creates the suction to pull fuel into the engine without pumping the gas. Pumping the gas a couple times can also start the engine, since the carburetor has an accelerator pump. However, it's easy to flood the engine this way. Move the throttle by hand just a little and see if the choke snaps closed. It should go all the way closed, or the rod is bent or the spring is maladjusted. Start the engine and make sure the choke opens a quarter of the way.
If not, pull the hose off the servo and see if the arm extends and contracts when the hose is reinstalled. If not, the servo must be replaced because it operates with a rubber bladder inside and is leaking. This means that not only does it not work, it's leaking vacuum and interfering with the mixture in the carburetor. This causes the engine to run poorly.
If the engine runs OK except at an idle, the idle jets may be out of adjustment of dirty. Using a small screwdriver and starting on the driver's side, adjust the screw in until the RPM starts to drop. Turn the screw out just until the RPM stops rising and turn the screw one half of a turn further. Do the same to the other side and then repeat the process one more time. If the engine idles fine but will not accelerate when the pedal is floored and just falls flat, the accelerator pump is not working and the carburetor has to be overhauled.