Mikuni BST40 slide guide.

Notice the ~0.020" (0.51 mm) deep depressions in the bottom corners/concentric to the bore (circled in green), as well as the four circular ~0.023" (.58 mm) depressions.

Contrast this with the same areas of a slide guide from a KTM 640LC4 with under 20K miles (32,187 km) (claimed)(note: wear was accelerated by slide lift hole drilling). The depressions in the bottom corners/concentric to the bore (normally found in the area bounded by the red circles) as well as the four circular depressions are worn completely away. The plastic is worn completely through adjacent to the bore at 10 and 2 o'clock (when viewed in the installed position). Maximum wear measures 0.085" (2.16 mm), which works out to .0042"/1K miles (.07mm/1K km) (!).

Emulsion tube wear sets in when the circled depressions are somewhere around .010" (.25 mm) or less deep, and causes overly rich mixtures in the lower portion of the rpm range. Measure with the tail of a caliper. Zero as shown, then find and measure the lowest spot in the immediate surrounding area. Repeat for the other side.

Preset the tang on the float setting tool to the desired height (the standard setting is 20mm to the raised rim adjacent to the gasket surface). Set the tool's width just wider than the width of the floats. Orient the carburetor so that the float tang just touches the needle's spring-loaded plunger, but does not depress it. Hold the float height tool onto the carb body, making sure it is square to the gasket surface front to back (side to side is taken care of since the tool straddles the floats) and so the tang lines up with the point on the float that is farthest away from the body. The tool's tang should just touch the float but not depress it. Flip the tool 180 degrees to check the other side of the float. If the other side is different, correct the twist by bending until they are the same. Set the height by bending the float tang with a small screwdriver.

Preset the tang on the float setting tool to the desired height (the standard setting is 14.7mm). Set the tool's inside width just wider than the width of the floats. With the carburetor in your hand with the upstream side facing your palm and your index and middle fingers holding the base of the cage tightly against the body, rotate the carburetor so that the float tang just touches the needle's spring-loaded plunger, but does not depress it. With your other hand, hold the float height tool onto the carburetor body, making sure it is square to the gasket surface front to back (side to side is taken care of since the tool straddles the floats) and so the tang lines up with the point on the float that is farthest away from the body. The tool's tang should just touch the float but not depress it. Flip the tool 180 degrees to check the other side of the float. If the other side is different, set to the higher side of the float. Set by bending the float tang with a small screwdriver.

In this example, note the gap between the tool's tang and the high spot on the float, which indicates that the float height is insufficient.

For multiple carburetor engines:

Start the engine, let it warm up and ride the bike around until the engine is at its operating temperature. Synchronize the carburetors and, using the idle speed screw, adjust the idle speed to the factory recommended rpm. Use a digital tachometer to improve accuracy or if the motorcycle didn't originally come with a tachometer. Adjust the fuel screws so the CO is ~3-3.5%. If gas analysis is not available, coming from the lean side, adjust the fuel screws so that the strongest idle is achieved. You will notice there is a threshold where the mixture becomes rich enough (enough turns out) to run strongest, but beyond which no change is noticed. Adjust the idle mixture screws ~1/8-1/4 turn out from this threshold. Leave the mixture to the leaner side of these settings if the bike will be seeing altitudes much higher than the one it was set at. Set to the richer side if you would like the engine to idle well earlier during warm up. Re-synchronize the carburetors and adjust the idle speed back to the target rpm (as they are likely to have changed). On a twin-carburetor single cylinder engine, the fuel screws should always be set equally. In other multi-carburetor applications you have a choice of setting them individually or equally. An argument for individual cylinder adjustment is that each cylinder may have slightly different requirements. On the other hand, the effect of a mixture change on one cylinder is more difficult to accurately detect the more cylinders there are. Using individual-cylinder CO readings solves the problem. Extended fuel screws simplify adjustment and permit trimming on the fly at extremes in altitude.

For single carburetor engines:

The procedure is the same except that the synchronizing steps fall away.

Note: Portions of the above procedures may only be legal for closed course use. You are responsible to check and comply with your local laws.

Idle mixture adjustment-based diagnostics:

Service manuals often contain a factory recommended number of turns from lightly seated. These settings are usually provided with emissions in mind, and are therefore leaner than ideal for best idle. In general, settings best for idle end up being somewhere in the vicinity of 1/2 to 1-1/4 turns further open than the factory recommendation (which end of this spectrum it falls on depends on the zone the motorcycle was intended for). If, when set by the procedures outlined above, a fuel screw ends up outside the factory setting plus 1/2 to 1-1/4 turns range (for example, if the manual said one turn, 1-1/2 to 2-1/4 turns), this points to the carburetor (or engine) having other issues. A fuel screw needing to be opened further points to a clogged pilot jet, a missing or larger than stock pilot air bleed jet, possibly too much float height, low compression, and/or incorrect cam timing. A fuel screw needing to be closed further points to too large of a pilot jet (larger than stock, or enlarged from something having been poked through it), a clogged or smaller than stock pilot air bleed jet, hanging cold start enrichment slide (i.e. not closing all the way when the circuit is turned off), and/or fuel level too high (from incorrect float height adjustment or faulty float system).

Off idle symptoms improving from a setting that is further open than resultant from the procedures outlined in the top paragraph points to an incorrect jet needle clip position and/or possibly too much float height. Off idle symptoms improving from a setting that is further closed than resultant from the procedures outlined in the top paragraph points to a worn out needle jet, incorrect/modified jet needle, drilled slide, cut slide spring, increased slide spring preload from jet needle shimming (in the case of many but not all Mikuni CV carburetors) and/or fuel level too high (from incorrect float height adjustment or faulty float system).

Note: CV carburetors with air screws rather than fuel screws do exist, but they are relatively rare.

KTM 640 LC4 Mikuni BST40 jet needle

Left is stock needle with under 20K miles (32,187 km) (claimed) and worn by 0.0385" (0.98mm) adjacent to the slide, which works out to 0.0019"/1K miles (0.03mm/1K km) (probably not a 100% linear relationship). The wear is due to slide guide and slide wear causing the slide to be constrained by the needle in the emulsion tube. In addition, this needle was allowed to spin (as evident by the uniform wear all the way around, rather than just on one side), likely due to the white plastic spacer not fitting tightly.

Possible symptoms, roughly in ascending order of severity:

Starting easily from cold without enrichment, poor running at idle and/or small throttle openings, needing to close the fuel screw beyond a "normal" setting in order to achieve the correct idle mixture, ability to close the fuel screw completely and not stall the engine, closing the fuel screw all the way helping but still not providing a proper idle, needing to turn in the idle speed screw in order to keep the engine running, the engine then alternating between a hanging idle and stalling, all of the aforementioned becoming worse as the engine warms, spark plug(s) black, (note: some of the previous symptoms may also be attributable to other causes or combinations of causes), spark plugs fuel fouling/wet with fuel, fuel found exiting the overflow (if the carburetor has one), fuel found exiting the airbox, air cleaner, or main air bleed jet, fuel level higher than 1.5 mm when the float height is set to 14.6 mm for BST33 and 14.7 mm for BST36, BST38 & BST40 carburetors.

Consequences, roughly in ascending order of severity:

Rich mixture, with the associated poor fuel mileage and carbon buildup, washing the oil off of the cylinder wall, filling the crankcase with fuel, and/or hydraulic locking the engine. Running the engine with gasoline thinned oil destroys engine internals, as can hydraulic locking. There is also the risk of explosion or fire from ignition of leaked fuel.

Causes, in descending order of prevalence:

Damaged float seat o-ring (look for a loose fit in the bore, hardening, deformation, shrinkage, tearing, cracking or other damage), dirt/debris lodged between the needle & seat, bad float needle (look for a witness line where it has been contacting the seat - use a bright light and magnification, sacked out plunger spring, and/or stuck plunger), bad float seat (pits in the seating area so tiny that they are hard to make out even under magnification can be enough to cause a leak - applies to brass seats, but not plastic), fuel logged float (make sure it weighs 6.1 g or fewer separated from the cage), worn float pivots (look for ovaled bores, lozenge shaped pins, and/or general looseness), wrong parts (incorrect float needle or seat), damaged body (check for corrosion in the bore the float seat pushes into).

Possible misattributions of cause:

Incorrect float height cannot alone cause an overflowing problem. On a carburetor with a float system that is otherwise in proper functioning order, the float height can be decreased as far the design will mechanically allow (to ~13.3 mm), and the fuel level will not as a result be high enough to overflow. A faulty vacuum operated petcock or a petcock that is left on cannot alone cause an overflowing problem (if it could, then the carburetor would overflow any time the fuel were turned on). The source of the fuel exiting an overflow can have its source in a cracked or pin-holed standpipe.

Fuel that gathers at/drips off of the bottom of the carburetor or end of the drain/overflow may not have its source in an overflowing problem. It could have its source in a bad float bowl gasket and/or fuel feed pipe/tee seal (if applicable) instead.

The overflow (if present) uses the same outlet as the drain, so sometimes people attribute fuel exiting here to a loose drain screw. It is therefore not uncommon to find drain screws that have been significantly over-tightened, resulting in stripped screwdriver slots and sometimes even a cracked float bowl. Mikuni specifies a tightening torque of 2.0 Nm (17.7 in-lbs), and under normal circumstances nothing is to be gained by tightening beyond this.

A choke is closed when an engine is cold and opened when it is warm, whereas an enrichment circuit is opened when the engine is cold and closed when it is warm. A choke works by restricting the air flow and lowering the pressure inside the venturi, which causes the existing jets to meter more fuel, whereas an enrichment circuit adds extra fuel without restricting the air flow. A choke requires a fast idle cam or something akin to it (or you have to hold the throttle open manually), whereas an enrichment circuit adds a little extra air simultaneously with the extra fuel, so the fast idle is already built in.

Motorcycle carburetors have generally come with cold start enrichment circuits rather than chokes since some time in the '80s (the last carburetor with chokes that comes to mind is the Keihin VB56A found on the '83 Honda CB1100F).