Kevin Cameron Explains How To Tune Carburetors

[Hugh Janus]

From left to right, we have an Amal Concentric, a Keihin PWK flat-slide copy, a Mikuni VM28 from a Yamaha RD350, and a Mikuni TM34 flat-slide. They all share important basic features: 1) a horizontal air passage, flowing from right to left, 2) a vertically movable air slide which throttles the airflow, and 3) so-called center-float design, in which the main jet is central to a fuel bowl with floats that surround or flank the central main fuel jet. (Mark Hoyer/)

Carburetion has to be right because only a narrow range of air-fuel mixtures can be ignited in a warmed-up engine, and only a mixture of roughly 12.5-to-1 gives best power.

Range Of Ignitable Air-Fuel Mixtures

Unless the mixture is leaner than 10 parts air to 1 part fuel, and richer than 18 parts air to 1 of fuel, it can’t even be ignited by a conventional spark ignition system. To give complete combustion of the fuel and best fuel economy, the mixture must be almost 15 to 1, while for best power a mixture of 12.5 to 1 is needed. Because carburetors have a number of overlapping systems, each with its own adjustment range, tuning them to give the desired mixture over the engine’s operating rpm is a step-by-step process.

In a cold engine there are no hot surfaces to vaporize the fuel spraying from the carburetors, so the cylinders receive a very lean mixture, plus a combination of wet fuel droplets and a sliding film of fuel wetting the interior walls of the intake duct. Because the ignition spark cannot ignite droplets or wall film, for cold starting the mixture must be radically enriched (or “choked”) so that the small percentage of the fuel that does evaporate forms a mixture rich enough for a spark to ignite. For cold starting the air-fuel mixture may have to be enriched all the way to 1-to-1, and this very rich mixture is provided by using the choke or other carburetor starting system.

When the engine fires and runs, its parts warm rapidly from combustion heat, and become more and more able to evaporate the fuel coming from the carburetors, making the mixture grow richer. This is why the engine will emit black smoke and eventually stall if the chokes/starting carb system are left on too long.

Carbureted engines of the 1980s and ’90s had to meet exhaust emissions standards so their carburetors are set very lean, leaving almost no safe margin of richness. If the engine is cold and/or the weather cool, this lean mixture can easily be too lean to fire steadily, leading to the familiar “stutter-and-stall” syndrome, and to poor throttle response and surging. These are the usual reasons why people decide to install either high-performance carburetors or jet kits, the latter of which restores smooth operation with the existing carburetors.

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Before setting up your carburetors, read through this tuning procedure. Then perform the steps carefully, evaluating each aspect of carburetion. It’s tempting to slap on the new carbs and head for the hills, but this can result in a bike that runs far less well than it could if it were systematically tuned. You can avoid such problems, learn a lot about carburetion, and get results that please you.

It’s wrong to think that you don’t need smooth carburetion on a high-performance motorcycle. You can’t get off corners quickly or launch strongly from the start line if your machine lacks throttle response or has a lumpy powerband. You can solve many problems by careful carburetion work.

There may exist a standard recommended setting for the carburetors you have bought, for the machine you plan to put them on. This can make a good starting point but remember this: Carburetion changes with every alteration in air filters, exhaust system, camshaft, cam timing, and compression ratio. The carburetors don’t magically know what parts you are using on your particular engine, so to get the best results you must fine-tune them to the components you are using.

Carburetor Parts Aren’t Cheap

In what follows you will see changes of throttle slides, needle jets, and other parts referred to as if they were free. They are not. Having a range of parts suitable to this work will cost some money, and such stopgap measures as trying to “drill out” jets or file throttle slides just create unpleasant surprises for the next user or even for you, the current user.

Mikuni VM jets from a pair of carburetors. Small idle jets are on the left. In the center are needle jets. The thick end with the notch is at the bottom of the carb, the thin flanged end accepts the needle and fits neatly into the floor of the main air passage. The main jets (right) thread into the bottom of the needle jets. Jets and needles are numbered for sizing and charts are available to help you tune. (Mark Hoyer/)

Definitions

So we’ll know what we are talking about, we need definitions of the basic carburetor parts. The round bore of the carburetor, through which air flows to the engine, is called the main air passage, and is located in the main casting. Controlling the airflow through this bore is the throttle slide, a kind of gate sliding in a vertical guideway that is more or less at right angles to the main air passage. In earlier carburetors (Amals, Mikuni VMs, etc.) the throttle slide is cylindrical (commonly referred to as a round-slide carb), but in later designs from the later 1980s and ’90s it may be oval or rectangular in cross-section (known as a flat-slide carb).

Airflow through a carburetor is not smooth, but rather a series of sudden gulps, one for each engine intake stroke. In order to cause fuel to flow, such a gulp must first pull the air pressure under the slide down. Because of this, the air volume under the slide affects the speed of the fuel flow response. This flat-slide has a much lower air volume beneath it than do the round slides of earlier carb designs, giving it more immediate response. (Mark Hoyer/)

The flaring entry to the main air passage is the carb’s bellmouth (gigantic equivalents are used for ground-testing of jet engines). Attached to the throttle slide to project downward beneath it, and moving up and down with it is a tapered metering rod called the jet needle (abbreviated JN). Its tapered lower part fits into a brass sleeve located below the slide in the bottom of the air passage: This is the needle jet (NJ). Fuel flow from the needle jet is controlled by the tapered needle. Screwed into the bottom of this needle jet is the main jet (MJ). Each of these metering parts—needle, needle jet, and main jet—bears identifying markings.

Attached to the bottom of the main casting (or in many pre-1960 carbs, separate from it) is the float bowl. Fuel stands inside this bowl at a level specified in the manufacturer’s instructions. A brass or plastic float, linked to a float needle, serves to maintain a constant fuel level in the bowl. As the bowl fills from the fuel tank, the float rises, pushing its float needle toward its closed position. As the engine takes fuel from the bowl, the float drops, opening the float needle and admitting fuel to restore the correct level.

Mikuni VM28 RD350 float bowl and carburetor body. In the bowl (foreground) can be seen the two black plastic floats, each with its projecting pin that lifts the float lever (seen edge-on) as fuel level rises. As engine operation takes fuel from the bowl, fuel level and floats sink, opening the float valve to admit more. Making the float bowl concentric with the main jet (the backlit brass object at 12 o’clock) allows this arrangement to be little affected by acceleration or braking. (Mark Hoyer/)

The orifice opened and closed by this float-and-needle action can be quite large in gravity-feed fuel systems, but must be much smaller if a fuel pump is used (for otherwise pump pressure could force the float needle off its seat and overfill the bowl).

The desired fuel level is just a few millimeters below the bottom of the main air passage—enough below it so there is no dribble of fuel when the engine is not running, but high enough to respond easily to the partial vacuum generated in the main air passage by engine airflow.

Between thumb and forefinger is the float needle, which slides easily in the circular housing seen just below the bright brass main jet. Float needle height is controlled by the floats in the float bowl, lifting the float lever (not seen here) whose tang bears against the float needle. In some carbs the float needle and its orifice are a unit screwed in place, allowing provision of a range of flow rates. Such float valves may be stamped with the orifice diameter in millimeters. (Mark Hoyer/)

As air rushes through the main air passage on its way into the engine, it produces a partial vacuum in the carburetor throat, which is the region directly under the throttle slide. The vacuum lifts the fuel that last few millimeters to the top of the needle jet so that it can be stripped off by the fast-moving air and carried into the engine.

Carburetor Systems

Most carburetors have three fuel-delivery systems. The first is the choke system, which on many Japanese carburetors takes the form of a small and very rich starting carburetor, built into the side of the main casting. When you flip on the choke before cold starting, you are opening the tiny cable-operated throttle of this starting carburetor.

The Amal 932 (left) was widely used on British motorcycles in 1960s and ’70s. This Keihin PWK flat-slide copy (center) has been adapted to work as a bolt-on replacement for Amals by JRC Engineering. The Mikuni VM28 (right) round-slide remains a popular replacement carburetor. Small-diameter openings on the edge of the bellmouths? One is for the air jet (bleed air to prevent the main system from enriching with engine rpm). Another supplies air via the mixture needle to the idle circuit. The big ones on the right two carbs about halfway up sends air to the rich starting carburetor ("choke"). (Mark Hoyer/)

The second fuel-delivery system is the idle circuit, which controls the mixture at idle and up to approximately 1/8 throttle. Fuel from the float bowl enters the idle tube (projecting downward into the fuel from the main casting), right behind (that is, on the engine side of) the main well in which the needle jet is installed. Engine vacuum lifts the fuel to pass through a small brass idle jet, or pilot jet (PJ), and then mixes with air flowing in from the idle air screw (a threaded brass part with a screwdriver slot at its outer end and a tapered needle at its inner end.) The resulting mixture of idle fuel and air emerges from tiny idle orifices in the floor of the main air passage, immediately downstream of the throttle slide.

This idle circuit delivers fuel only at or just above idle, and fades out as the throttle lifts because carb vacuum is no longer strong enough to lift fuel through this circuit.

The third fuel-delivery system is the main circuit, which begins to take over mixture control from the idle circuit from about 1/8 throttle upward. It has two main parts. The first is the variable-area jet consisting of the tapered jet needle riding inside the needle jet. The needle is held in the throttle slide by a circlip placed in any one of several (Mikuni VM carb needles have five) tiny grooves in the top of the jet needle. Moving this clip to a lower groove raises the needle, increasing the flow area between it and the ID of the needle jet, allowing slightly more fuel to flow (richer mixture). Conversely, raising the clip lowers the needle and reduces the fuel flow (leaner mixture).

The needle jet’s top end is almost flush with the bottom of the main air passage, while its lower end extends down to near the bottom of the fuel bowl where it is held in place by the main jet, which screws into it, plus a retaining washer.

The Amal has a choke slide integrated into its main slide, but mixture can also be made richer by pushing down the tickler button (just under the Amal logo). This forces the float down in the bowl and allows fuel to rise high and even to flow out the main jet into the carb throat, which makes the air-fuel mixture richer and can aid cold starting. On the center PWK carburetor, the black knob to the right of the slide adds fuel for cold starting. These chokes are actually a tiny, very rich starting carburetor made in unit with the carb body. (Mark Hoyer/)

Fuel enters this main system through the main jet, flows up through the restriction between needle jet and tapered needle, and sprays out into the air stream in the main air passage. As the throttle is opened, admitting more air, the fuel flow path between needle jet and tapered needle also becomes larger until finally, near full throttle, it is bigger than the area of the main jet itself. At this point the main jet becomes the controlling factor in mixture, right on up to full throttle.

Keep in mind that all carburetor systems overlap each other. The idle system doesn’t just click off at 1/8 throttle and hand over to the main system, any more than the needle/needle jet combination ceases all influence and hands over to the main jet. The idle system can influence the beginning of the mid-throttle mixture and the needle/needle jet can certainly influence full-throttle operation. In general, however, if we want to tune the mid-throttle region it is the needle height (clip position or needle dimensions) we will alter, while if top-end/full throttle running needs tuning, it is the main jet we will be changing.

Other carburetor systems may be present for special applications. When you open the throttle suddenly the mixture leans out because the air rushes into the engine faster than the heavier fuel (650 times heavier). On some emissions-control-era carburetors the settings are so lean that sudden opening of the throttle would lean out the mixture enough to stall the engine. To prevent this, an accelerator pump (usually of the flexible diaphragm type) is operated by throttle movement. As the throttle is opened, this pump delivers an extra squirt of fuel to prevent lean stumble or stalling. With the richer settings commonly used on high-performance and racing carburetor applications, accelerator pumps are usually unnecessary.

Certain racing two-stroke applications need a richer top-end mixture than can be supplied by a carburetor with normal systems, so a power-jet circuit is added. This is a secondary main jet, supplied by hose from the fuel bowl, delivering fuel to a tube whose end is just in front of the throttle slide in the upper half of its movement. When the slide nears full-open, high-speed airflow passing over the end of this power-jet delivery tube creates enough vacuum to lift extra fuel that the engine needs through this power-jet system.

As a special feature on two-stroke racing engines, there is sometimes a solenoid-operated valve in the power-jet circuit. It is there to give greater control over when the power-jet circuit operates.

Engine Condition

Before beginning carburetion work you must make sure that your engine is in condition to work with. If you have just completed modifications, be sure that any new parts are correctly installed and timed (cams, ignition). Otherwise you may find yourself blaming your new carburetors for problems which originate elsewhere.

Checkout And Installation

Inspect your carburetors by removing the float bowls to be sure all main jets, idle jets, needle jets, and float valves are all the same. On new carburetors, I have seen mistakes made in factory assembly—not often, but sometimes. Used carburetors may not have had the best care.

Set the idle air screws by first running in each one (clockwise) until it gently bottoms, then backing it out the recommended number of turns. If no starting position is given, set them at one-and-a-half turns open. While the bowls are off you should examine the float levers to be sure that all floats rest at the same height when the carbs are held upside down. The float level will be specified in any instructions.

Reinstall all parts and tighten the float bowl screws. With four-stroke carburetors operating off of a single or dual cable rack system, you must now check for synchronization of the throttles. This makes sure that each cylinder receives approximately the same amount of idle air so no one cylinder is trying to run away while others lag behind. Throttle synchronization is a key to good throttle response and overall engine smoothness. A common cause of poor throttle sync on bikes with one cable per carb is synchronizing the throttles with the cables routed one way, then arranging them another way so you can put the fuel tank back in place. The tighter you bend a cable housing, the longer it becomes.

On a high-performance carburetor like this, the fuel inlet (double-ribbed brass fitting) is quite large to ensure adequate flow. When diagnosing fuel-flow problems, start with the gas cap, ensuring its vent is not blocked, which creates a vacuum in the tank, starving the carb. Move next to the petcocks. Hold the measuring container below the outlet to measure the flow when you open the valve. Then remove the float bowl’s bottom plug, turn on fuel, and measure the flow again. Run fuel lines without kinks and to avoid rubbing on other components. (Mark Hoyer/)

Inspect the rubber coupling or manifolds that hold the carburetors on the engine. It is common for these rubber parts to harden with age, then crack during removal of the original carbs. Examine each one using strong light, both looking and feeling for cracks. Replace as necessary. Nothing is worse for carburetion than an air leak, so take care as you mount and clamp the carbs into place (made easier by a thin wipe of oil from one finger). Tighten the metal carb clamp bands snugly but not so much as to bulge the rubber. The aging of rubber is accelerated by excessive stress—I’ve seen many a carb rubber manifold cut and split by excessive clamp tightening.

Once the carbs are in place and throttle cables smoothly routed as before, test to be sure the throttles easily and promptly return to the idle position when you let go of the grip. Some older carburetors mounted by a flange and bolts can be distorted enough by bolt tightening to prevent reliable throttle return. Amal carburetors can have their bodies permanently warped by overtightening.

Hook up the fuel lines, making sure they fit closely on the carbs and that there are no splits that could leak—I’ve seen fires from this cause. Put securing clips in place. Make sure that no fuel line is kinked, possibly blocking fuel flow. If the lines are so hardened from age that installation is difficult, now is the time to replace them.

Finally, turn on the fuel petcock(s) and wait 30 seconds for the float bowls to fill. If one or more bowls overflow, give a rap with a screwdriver handle to unstick its float valve (this was a regular occurrence with TZ750s). If leakage continues, stop and find the cause, disassembling the carbs if necessary. Fuel on your rear tire is bad and fire is worse. It is best practice to have a capable fire extinguisher near at hand.

Start-Up

Flip on the chokes/starting carbs and start the engine. Unless the idle jets or idle air screw adjustments are way off, it should start right up, possibly running a little fast or slow because the idle speed adjustment has not yet been done. As soon as the engine runs evenly ease the chokes off and continue the warm-up. Remember—you can’t tell a thing about carburetion until the engine is warm enough to vaporize all of the fuel it is getting. Once the engine is warm you can set the idle speed (unless this is a racebike whose throttle slides close completely when the throttle is dropped). Idle speed is set by screw adjusters which slightly lift the throttles.

If the engine doesn’t even fire, you must answer the classic questions: Does it have fuel? Does it have spark? Does it have compression? Before starting your carburetor work, ensure the engine has properly timed spark and good compression.

Idle System Tuning

As the engine warms up it may begin to show signs of distress if the idle mixture is wrong. If the engine begins to blubber as it warms up, it is probably rich on the idle system because engines run richer as they warm up. In that case, try opening up the idle air screws a quarter turn each using a screwdriver with good fit. If this improves the idle you can try for an even better adjustment, making sure each time to turn each carb’s air screw by the same amount. If you get confused (easy to do with all those screws), stop the engine and start over by running in all the idle screws until they lightly bottom, then back out each one to the desired setting. Take notes if that helps.

If all is well, you will find that engine rpm definitely responds to these air screw adjustments. You will find a screw position that gives maximum engine idle speed such that turning the screws either in or out from that position causes engine rpm to decrease. This is the correct position. Think of it this way: The ideal air-fuel mixture makes the most horsepower at a given throttle setting—even at idle.

Here is the rule for choosing idle (pilot) jets: If the idle jet inside each carb is correct, this position of highest engine idle speed will be somewhere in the range of one turn open to three turns open. If your engine runs best with the air screws less than one turn open, your idle jets are too small and you should fit one size larger. If your engine idles best with its idle screws more than three turns open, your idle jets are too big and you must fit smaller ones. If you do have to change the idle jets, you must repeat this rpm test to get the new correct air screw setting. Once you have the correct idle jet and air screw setting you can adjust the idle speed with the speed adjuster (if any) that raises and lowers the slide to achieve whatever idle you want. Now you are ready to move on to the next step in carbureting your engine.

The Main System And Throttle Slide

Now with a warmed-up engine bring the revs up very slowly by just cracking the throttle. This is a test of how well the main system chimes in as the idle system reaches the top of its range, and it is very important to throttle response. Air flowing under the throttle slide creates a partial vacuum above the top of the needle jet, and this lifts fuel up and out of it, to be carried into the engine by the fast-moving air stream.

A variety of throttle slides. Each connects to a throttle cable, by which the rider can raise or lower it. From the left, #s 1 and 2 are cylindrical slides, and you can see their cutaway as a smooth arc. A cutaway of 3 measures 3/16 inch higher in the center than at back and sides. On the right are two varieties of flat throttle slides, but both have cutaways. The lower the cutaway, the greater the partial vacuum above the needle jet, and the richer the mixture. (Mark Hoyer/)

When the throttle is nearly closed, the strength of this vacuum is determined by what is called the throttle slide cutaway. Cutaway is the measure of how much higher the upstream bottom edge of the throttle slide is than the engine-side edge. If you set a slide on end on a flat surface, you will immediately see the difference. A low cutaway produces more vacuum under the slide to pull more fuel out of the needle jet (richer) while a high cutaway produces less vacuum and so less fuel flow (leaner). The effects of slide cutaway are most important in the range from 1/8 to 1/4 throttle. Slides are stamped with cutaway numbers—small numbers such as 1.5 being rich and large numbers such as 4.5 being leaner. Time was when slide cutaway was measured in sixteenths of an inch, giving a 2.0 slide (think 2/16 of an inch) a cutaway of 1/8 of an inch.

Three throttle slides seen from the bottom. The Amal slide on the left has a broached hole through it for a choke slide to enrich mixture for cold starting. The central hole in each slide carries the jet needle. The throttle cable resides in the off-center hole. The angle-milled slots on the left sides of the two round slides engage the idle stop screw; screwing it in raises the slide to admit idle air to set idle speed. Numbers on each indicate cutaway height: Amal is 3, Mikuni round-slide is 2.5 and flat-slide is 4.0. (Mark Hoyer/)

If your engine stumbles or seems reluctant to accelerate as you just crack the throttle, the chances are that your slides have a cutaway that is too rich or too lean. How can you tell which it is? There are two methods. The first method makes use of the fact that a richer mixture is needed for acceleration than for steady running. Therefore if the engine stumbles when you open the throttle slowly, but seems a bit better when it is leaned out by being opened that small crack quickly, you can be sure the slides are a bit rich. The second method depends upon leaning out the carburetion by shutting off the petcock. The engine will continue to run on the gas in the float bowls, but it will run leaner and leaner as the fuel level drops. If, when running leaner in this way, the engine responds better to slight opening of the throttle, you know that the slides are too rich. Whichever method you use can give you the right answer about slide cutaway. Remember through this work that this test is concerned with throttle openings from 1/8 to 1/4 throttle. In racing I noticed that while an engine may run best on the track with a slide on the lean side (say, a 4.5) it may not push-start well on that slide, requiring a richer compromise such as a 3.5 slide (push-starts were replaced in GP racing by running-engine standing starts in 1984).

Once you have decided which way to go, repeat the test with slides a half-step (for instance, replace a 2.0 with a 2.5 or a 1.5) different from the original. If the response is better but not good enough, change the slides by another half-step and test again. Be sure that you end up with both good running at steady 1/8 throttle and good engine acceleration from idle up to that point. You need both.

This is an entering air molecule’s view of a Mikuni VM34 with its slide at about 60 percent throttle. You can see the tapered jet needle hanging from its center. On the right you can see the tip and the slotted head of the idle speed adjuster. At 9 o’clock on the left is the downward-angled idle mixture screw. Just behind it is the vertical ferrule over which the rubber line from the fuel tank petcock slips. (Mark Hoyer/)

Checking Mixture On The Needle

Next, with the engine fully warmed up, repeat the throttle cracking procedure, but this time continue opening the throttle beyond the 1/8–1/4 throttle range, up toward mid-throttle positions. If the engine happily continues to accelerate smoothly, you are in luck because the needle taper, which is now taking over the control of the mixture from the slide cutaway, is correct or close to correct, and you can move on to actual road testing. You may find it helpful to mark the throttle and housing with throttle positions 1/4, 1/2, and 3/4 to ensure accuracy and repeatability. Use a flexible tape measure and divide the distance from closed throttle to wide open, and divide into quarters.

Here are four different jet needles, each with the tiny clip that determines its height in the needle jet. The top groove is position 1, which is the leanest setting because it lowers the needle, making mixture “on the needle” leaner (and vice-versa). In another era, carb needles were brass, sometimes chrome plated for wear resistance. These needles are aluminum that has been eternally surface-hardened by anodizing. Note the obvious differences in taper among the three needles on the right. The greater the taper, the faster the needle enriches the mixture as the throttle is lifted. (Mark Hoyer/)

If on the other hand this extra throttle opening makes the engine hesitate or run roughly, you are rich or lean on the needles. To find out which it is, take the tops of the carburetors off, pull the slides, and change the position of the needle retaining clips to raise or lower the needles. Raising the clip lowers the needle, making mid-throttle running leaner. Lowering the clip raises the needle, making mid-throttle running richer. Make changes by one clip position at a time because this is a sensitive adjustment. If the changes you make are correct, you will be able to smoothly accelerate the engine up into the mid-throttle positions, and you will also have good snap throttle response.

On the most right-hand of the four slide needles you can see its identifying code. A 6DH4 Mikuni needle begins at a one-degree taper, then steepens to a two-degree taper. These codes can be found in the carburetor manufacturer’s tuning information. Usually that portion of the needle that is in the needle jet during the first 1/3 of slide lift is not tapered, allowing mixture to be controlled by slide cutaway. (Mark Hoyer/)

There are those who believe there’s something magical about having the needle clip in the center position (#3 on Mikuni VMs) but there is not. The five clip grooves are there to be used, so don’t be afraid to run with the clip in either the top or bottom groove, if that is where your engine runs best.

Looking upstream from the engine side of a Mikuni TM34, you see the jet needle disappearing into the central brass needle jet (this particular needle jet has a semi-circular shroud). On the near side of the carb throat can be seen the tiny idle system orifice, from which at idle issues a mixture of fuel metered by the pilot jet and air controlled by the idle mixture screw (not seen here). (Mark Hoyer/)

Needle Jet Tuning

Suppose that performance is better with the needle clip at the #2 position than in the middle, and better still at the #1 position. You have run out of clip positions but you wonder if the engine would run better still if there were more clip grooves to try. When this happens, it is time for different needle jets. These come in different sizes and styles, and each one has its numbers stamped on its shank. A typical Mikuni number would be 159 P-2. The first three digits (159) are an arbitrary code that you can look up in their product catalog to find out the style (whether it is for two-stroke or four-stroke, its shroud height, its length). If your machine has 159 style needle jets, stay with 159s unless a good reason to change style is found.

This shows the difference in throttle slide cross-section. The Mikuni VM on the left, designed in the 1960s, has a round slide while the flat/oval slide on the right was originally developed to improve throttle response in MX competition. The left-hand carb has a spigot mouth that slips into a matching flexible rubber manifold with a clamp band. The other has a flange mount. Both carbs have centrally located brass needle jets, metered by the tapered jet needle that rises and falls with the throttle slide. (Mark Hoyer/)

The second group of symbols indicates the bore size of the jet. A P-4 needle jet is one size larger than a P-2, and the sizing goes P-2, P-4, P-6, P-8, at which point the sizing jumps one letter of the alphabet to Q-0 and the series starts over again. A jump of one needle jet size is a small change, but you shouldn’t jump around in needle jet size by more than a number or two at a time. If your tests do indicate that you must change needle jets, make the change and repeat the throttle roll-on procedure to see if your engine responds better. If it does, try different needle positions to see which is now best. In this way you will find the best needle jet and needle position.

Steady Throttle Versus Snap Response

Slow roll-on tests give you correct mixture for steady running, but on the road you will also need good snap throttle response for downshifting (throttle blipping) and for quick acceleration. Therefore before beginning road testing you should check the snap response by grabbing a handful to see if the engine accelerates smartly, or if it lags and hesitates. Acceleration requires a richer mixture than steady running, so if there is a problem in snap response, it can be cured by enriching the carburetion in the problem area. Usually a slightly richer slide or richer needle position will cure any acceleration or snap response problem you may run into.

Road Testing And The Main Jet

Once you have good slow roll-on and snap-throttle response you are ready to begin actual road testing. Ride roads that are familiar to you so you can give some attention to how the engine is running or, for higher speeds, get time on a racetrack. If there are carburetion problems, remember at what throttle position they occur so you can work with the parts that control mixture in that range.

This is a Mikuni VM from which the float bowl and floats have been removed. The flat brass object is the float lever, whose two arms are lifted by pins on the black plastic floats (not seen), controlling the fuel-admitting float valve and preventing fuel level from rising higher than desired. At the center is the hex-headed main jet, which screws into and retains the needle jet. Below it, hiding down in its recess, is the pilot or idle jet. All jets are stamped with identifying numbers. (Mark Hoyer/)

If everything seems good after your work, you are ready to check mixture at full throttle, where it is controlled by the main jet. The method used for years in racing, before the coming of the laptop and oxygen sensors, is called “making a plug chop.” For this you must run the engine under full load, full throttle, at or near peak rpm for long enough to leave mixture indications on a fresh set of spark plugs. For the sake of public safety and your operator’s license, this cannot take place on public highways!

Spark Plug Heat Range

Spark plug heat range refers to the operating temperature of the metal plug electrodes. To chug around town, a fairly hot-running plug may be necessary to prevent rapid carbon deposition (occurs below 700 degrees Fahrenheit), but for top-end testing a cooler-running plug is needed to prevent the electrodes from operating hot enough to cause pre-ignition and engine damage (1,750 degrees Fahrenheit). Take care to use plugs of heat range suitable for high-speed operation.

At left is a 2011 Harley-Davidson Sportster plug with about 1,000 miles running. Center is a traditional 1974 Norton 850 Commando plug with about 300 miles (note dark shiny surface on plug rim from oil past valve guides). At right is a new plug suitable for 1973 Yamaha RD350 two-stroke. Note shorter reach (length of threaded area). The two plugs on the left are of the projected-tip gap style that places the spark in the path of turbulent fresh mixture in the combustion chamber. (Mark Hoyer/)

After a plug chop the indications on the plugs can then be “read” to see if the engine is running rich or lean. Just giving the engine a burst of throttle in lower gears doesn’t expose the plugs to combustion heat long enough to leave readable indications on the plugs, so you will have to find a safe, legal long straightaway where you have room to reach top-end. A racetrack, a disused airport, or a dragstrip are useful for this purpose, if permission can be had. Accelerate to top speed in top gear, then chop the engine dead while pulling the clutch. The engine must not be allowed to idle or it will wipe out the indications on the spark plugs. Coast to a stop, pull the plugs (they are hot so you’ll need the traditional rag to handle them) and compare them with the pictures in any spark plug manufacturer’s color chart.

Spark plug reading is a specialized art but in general if the tip of the plug’s ceramic insulator is shiny white and the engine accelerated freely to maximum, the mixture is close to right. Plug reading is seldom practiced in racing today, but peering through an illuminated magnifier to read a plug had a specific goal—to look down the length of the ceramic insulator to see if there is a dark ring around it at some point. Most of the insulator in a well-running engine will operate too hot for carbon to stick, but toward the cooler end (where it seals to the plug body) there should be the dark ring that tells you that some free carbon is being generated by combustion. That can only happen when the mixture is on the safe, rich side. If there is no such ring, you are lean.

If the tip of the insulator shows a light brown color mixture in these test conditions is a bit rich and even darker colors indicate more and more richness. If the plugs are a dusty white and the engine misfired or accelerated sluggishly, it is lean.

Another main jet tuning method exists—one used by racers for many years. While running at top speed, full throttle, very slightly close the throttle and watch the tach. If the revs pick up slightly, the main jet mixture is rich.

Modern plugs use different materials for center electrode—platinum in this case. Years ago the center-wire was made of nickel, and high-current spark discharge rapidly eroded it, causing a recurring need for plug re-gapping. Today’s ignitions with high secondary resistance deliver only the capacitive part of the discharge, greatly reducing gap erosion. Assisting in this is the use of high-melting-point center-wires of platinum or iridium, making possible use of fine wire whose small tip radius reduces the voltage needed to jump the gap. (Mark Hoyer/)

For a street machine a slightly rich indication is good, together with steady running on top-end. Leave the bone-white plugs and the last few top-gear rpm to the racers, who are going to change their jets every time the sun goes behind a cloud anyway (modern digital engine management systems do this for you automatically). Make your jet changes and repeat your tests with fresh sets of plugs each time, always using the same heat range. Such work consumes a lot of plugs, so you may want to clean them for reuse as suggested to me years ago by “the Champion man,” Bobby Strahlman: oven cleaner, followed by a water rinse.

This is a projected-tip gap style that places the gap in the combustion chamber where it is more certain to encounter ignitable “packets” of swirling air-fuel mixture. This Norton Commando 850 plug has an old-style heavy nickel center-wire. The light color of the ceramic insulator and absence of dark deposit on the center-wire indicate close-to-correct mixture. Oil on the face of the steel shell (which runs comparatively cool) indicates this engine’s oil control is failing. (Mark Hoyer/)

Dark plugs will not “clean up” to give a good indication of mixture. Fresh plugs each time.

You may find the above confusing because you have always been told that plug insulators in well-carbureted engines should be tan or even chocolate brown. This is true of plugs that have run hundreds or thousands of miles on the street, running some of the time at idle, mostly in the midrange, and only occasionally on top-end. In carburetion testing, however, you are starting with fresh, clean plugs and are running for only a few minutes or laps of a track at most. A plug that shows close to shiny white in these tests will in a hundred miles of street running accumulate a tan deposit using the same main jet.

This RD350 plug has never been run, and has the older non-projected gap. To improve exposure of the gap to mixture some tuners used to file away the side electrode to about the midpoint of the center-wire. This plug has shorter “reach” (threaded length) than the Norton 850 plug pictured above, to bring its shell flush with the RD’s combustion chamber. When new, the center-wire has sharp sheared edges. A highly effective ignition will somewhat soften those edges, leaving an eroded area—the spark track. (Mark Hoyer/)

If this procedure seems time-consuming and laborious, you should consider that factory carburetion engineers of the past did exactly the same tests on every new design, at some point in its development. When you make changes in your machine, or if you have special carburetion requirements, you can get what you want by using such methods.

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