Keihin Cr Special Carburetor Tuning Manual Lawn

• Keihin Cvk Carburetor Tuning

TipsThis is the baseguideline about tuning your Keihin.Certain parameters may vary due to air pressure and temperature, altitude, umidity.Be carefoul when jetting, a bad jetting can produce huge engine failures.Don't do it yourself if you don't feel confident but let it to a professional mechanic.WARNING!This section features 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.Zerothrottle operation - (idle circuit) idle screw: depending on your type of ridingadjust the minimum idle speed to desired rpm making sure the engine is up to operatingtemperatureIf you do not desire any idle make sure you turn in theadjusting screw just enough so the engine will not idle. This is especially important onKeihin PJ series carbs in that the idle adjust knob ( #4 inillus.) cannot be completely closed.Such an adjustment will result in a sluggish responseoff idle.Air/gasolineadjustment screw: the carburetor pictured in the exploded view uses an airadjustment screw ( #5 in illus.) in that itis located upstream of the throttle valve (slide) and meters air; turning itcounter-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. Theidle mixture screw usually has a range of one to two turns out from fully closed. If youneed to adjust above or below this range then the slow jet will probably need to bereplaced with one richer or leaner as required, consult your owners manual for thestandard setting.1/8to 1/4 throttle slow jet and throttle valve cutaway: keeping in mind that theidle adjust screw (air/fuel screw) gives a good indication of a properly sized slow jet ( #6 in illus.).The slow jet calibrates the mixturefrom both the idle bypass and idle orifice in the jet block.

If the idle screw is properlyadjusted but the engine does not have good response when the throttle is wicked open it isusually a sign of a lean mixture and the slow jet will need to be replaced with one largersize and the air/fuel screw readjusted. Consequently, if the throttle is only partiallyopened such as in a trailing throttle situation and the bike tends to load up and emits adeep tone when the throttle is returned to full open it is usually sign of a rich slowjet. If the slow jet does not clean up this part of the circuit the slide can besubstituted for one with a different cutaway. The higher the number the larger the cutawaywill be, admitting more air to the jet block/nozzle screen leaning the mixture andconsequently a smaller cutaway will richen the mixture with its greater effect up to 1/4throttle.1/4to 3/4 throttle jet needle: the jet needle ( #2 inillus.) is comprised of five major elements.1.

Straight diameter section: in keihin carbseither the last two digit or the last letter denote the diameter of the needle. The higherthe last two numbers the leaner the needle the lower the letter the richer the needle.

Bygoing to a thinner needle there is a larger area between the jet needle and needle jetsupplying a richer mixture.2. Length of the straightsection: this determines at which point the needle taper will start relative to theclip position.

If you have to run your clip in the highest position a needle with a longerstraight section will need to be used.3. Needle clip position: this works in conjunction with the lengthof the straight section. If the engine is too rich above a quarter throttle raising theneedle clip ( #1 in illus.) will lean themixture.4. Needle taper: a larger taper will result in a leaner mixture inthe first half of the taper and a richer mixture in the last half of the needle. Forexample, a 1.34 taper will be richer in the first half and leaner in the second half ofthe 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.Needle jet: the needle jet/nozzle,varies the fuel/air mixture up to 3/4 throttle. How it overlaps with the jet needledepends on the needle jet orifice i.d., air bleed holes, and type of nozzle screen. Mostmodern japanese carburetors use a fixed needle jet/nozzle assembly which cannot beremoved. If your carburetor has a removable needle jet/nozzle please contact themanufacturer in order to decipher the nozzle code. It is also not advisable to calculatehow rich or lean a needle jet is using exclusively the nozzle inside diameter to needleoutside diameter discharge area.Wide open throttle (w.o.t.) main jet:the best trackside method to determine the size of the main jet ( #7in illus.) is to fully load the engine on a long straightaway or hill.

Atthe end of the stretch chop the throttle and hit the kill button simultaneously. The plugis 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 usuallyresult in a very light tan colored insulator tip and dark colored ring around the tip ofthe electrode. The electrode itself should have fairly sharp edges. For example, if theceramic insulator has a nice tan coloring but the electrode has a white ring around thetip and the plug is of the correct heat range then you can easily run a size larger mainjet.Please keep in mind that different types of pre mixoil & ratios, along with the gasoline, will give different readings.

Keihin Cvk Carburetor Tuning

Also, that raceor av gas is more prone to oxidization and storage deterioration, along with the fact thata multitude of types are used worldwide. When jetting your main jet try to remember to jetfor the best power for a given track. An example of this is endurance where you would wantto run a main large enough to keep the engine cool, this means that you may be on the richside but the engine will fade less towards the end of the race. Another situation could bea stadium sx track where you spend much more time on the low to mid circuits. In this caseyou 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 gainexperience 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, orjust by looking at the color of the unpainted pipe and silencer.But in the mean time always remember to changeonly one calibration component at a time!