Introduction:

The following is a copy of Keihincarbs.com (owned by Orma Divisione Motori, the official Italian Keihin distributor).

Hildo copied this section, the Tuning Guide, from KeihinCarbs.com, as a reference aid only, to tune Keihin PE and PKW 28 carburetor as commonly used on 4-stroke mini bikes. This guide will give you a the basic understanding on how the Keihin carburetor fuel circuits work. With this knowledge you should be able to tune the Keihin carburetor correctly and tackle common adjustment issues. Note that Hildo can not be held responsible for the contents of the 'Tuning Guide' below. All tuning is done at your own risk.

This is the base guideline about tuning your Keihin.
Certain parameters may vary due to air pressure and temperature, altitude and humidity.
Be careful when jetting, a bad jetting can produce huge engine failures.
Don't do it yourself if you don't feel confident but let it be done by a professional mechanic.

Warning:

Featured are the PWK exploded views, just to let you understand where the tuning components are located. The guidelines are the same for both 2 and 4 stroke. On FCR carbs the main jet, slow jet, jet needle, air adjustment, slow jet air adjustment and gas valve are different and located elsewhere.

Zero throttle operation - (idle circuit) idle screw

Depending on your type of riding adjust the minimum idle speed to desired rpm making sure the engine is up to operating temperature.
If you do not desire any idle make sure you turn in the adjusting screw just enough so the engine will not idle. This is especially important on Keihin PJ series carbs in that the idle adjust knob (#4 in illus.) cannot be completely closed.
Such an adjustment will result in a sluggish response off idle.

Air/gasoline adjustment screw

The carburetor pictured in the exploded view uses an air adjustment screw (#5 in illus.) in that it is located upstream of the throttle valve (slide) and meters air; turning it counter-clockwise leans the mixture off idle. Some carburetors have this screw located downstream of the throttle valve, in this case the screw meters fuel and opening this screw results in a richer mixture. The idle mixture screw usually has a range of one to two turns out from fully closed. If you need to adjust above or below this range then the slow jet will probably need to be replaced with one richer or leaner as required, consult your owners manual for the standard setting.

1/8 to 1/4 throttle: Slow jet and throttle valve cutaway

Keeping in mind that the idle adjust screw (air/fuel screw) gives a good indication of a properly sized slow jet (#6 in illus.).The slow jet calibrates the mixture from both the idle bypass and idle orifice in the jet block. If the idle screw is properly adjusted but the engine does not have good response when the throttle is wicked open it is usually a sign of a lean mixture and the slow jet will need to be replaced with one larger size and the air/fuel screw re-adjusted. consequently, if the throttle is only partially opened such as in a trailing throttle situation and the bike tends to load up and emits a deep tone when the throttle is returned to full open it is usually sign of a rich slow jet. If the slow jet does not clean up this part of the circuit the slide can be substituted for one with a different cutaway. The higher the number the larger the cutaway will be, admitting more air to the jet block/nozzle screen leaning the mixture and consequently a smaller cutaway will richen the mixture with its greater effect up to 1/4 throttle.

1/4 to 3/4 throttle: Jet needle

The jet needle (#2 in illus.) is comprised of five major elements.

1) Straight diameter section

In Keihin carbs either the last two digit or the last letter denote the diameter of the needle. The higher the last two numbers the leaner the needle the lower the letter the richer the needle. By going to a thinner needle there is a larger area between the jet needle and needle jet supplying a richer mixture.

2) Length of the straight section

This determines at which point the needle taper will start relative to the clip position. If you have to run your clip in the highest position a needle with a longer straight section will need to be used.

3) Needle clip position

This works in conjunction with the length of the straight section. If the engine is too rich above a quarter throttle raising the needle clip (#1 in illus.) will lean the mixture.

4) Needle taper

A larger taper will result in a leaner mixture in the first half of the taper and a richer mixture in the last half of the needle. For example, a 1.34 taper will be richer in the first half and leaner in the second half of the taper than a 1.45 taper needle.

5) Number of tapers

The needle can have one or more tapers. The number of tapers is not usually changed.

Up to 3/4 throttle: Needle jet

The needle jet/nozzle, varies the fuel/air mixture up to 3/4 throttle. How it overlaps with the jet needle depends on the needle jet orifice i.d., air bleed holes, and type of nozzle screen. Most modern japanese carburetors use a fixed needle jet/nozzle assembly which cannot be removed. If your carburetor has a removable needle jet/nozzle please contact the manufacturer in order to decipher the nozzle code. It is also not advisable to calculate how rich or lean a needle jet is using exclusively the nozzle inside diameter to needle outside diameter discharge area.

Wide Open Throttle (WOT): Main jet

The best trackside method to determine the size of the main jet (#7 in illus.) is to fully load the engine on a long straightaway or hill. At the end of the stretch chop the throttle and hit the kill button simultaneously. The plug is then pulled for a "reading". The parts of the plug you should be looking are: the positive electrode and the last 1/4 of the ceramic insulator. Best power will usually result in a very light tan colored insulator tip and dark colored ring around the tip of the electrode. The electrode itself should have fairly sharp edges. For example, if the ceramic insulator has a nice tan coloring but the electrode has a white ring around the tip and the plug is of the correct heat range then you can easily run a size larger main jet.

Please keep in mind

Different types of pre mix oil & ratios, along with the gasoline, will give different readings. Also, that race or av gas is more prone to oxidization and storage deterioration, along with the fact that a multitude of types are used worldwide. When jetting your main jet try to remember to jet for the best power for a given track. An example of this is endurance where you would want to run a main large enough to keep the engine cool, this means that you may be on the rich side but the engine will fade less towards the end of the race. Another situation could be a stadium sx track where you spend much more time on the low to mid circuits. In this case you will probably be running a main jet that is much smaller than your usual "outdoor" jetting along with a hotter/extended electrode plug. As you gain experience and knowledge, you will be able to use other methods to determine your jetting. A good tuner can "feel" most of the circuits by slowly revving a parked bike, or just by looking at the color of the unpainted pipe and silencer.

But in the mean time always remember to change only one calibration component at a time