Carb Tuning Topics

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This thread is for techniques, and information relating to carb tuning.

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Eric Idle - The Patron saint of Idle
The only importance in the number of turns of the mixture screw is if there are no turns left to go in or out. If there is not then change the idle jet. Turns are not a decent metric to know if the mixture is set correctly, and although many people will toss out a number of turns as the holy grail of mixture setting, in general, those people are talking out of their incontinentia buttocks. They obviously have no idea on how to set a proper air mixture. - Eric Idle

The basic way to adjust the air mixture is:

1 - set the idle a bit higher than normal. This is done to keep the engine from stalling during adjustment.

2 - turn the air mixture screw all the way in. This is so that you can count the number of turns out as a reference. Usually 1.5-2.5 turns out is about perfect, if it's not then the pilot jet may need replacement.

3 - with the engine running at idle turn out the air mixture screw a half turn at a time. Sometimes it takes a moment for the carb to settle. You are seeking when the engine runs at its highest idle while making changes to the air mixture screw.

4 - once you notice the idle going down turn the screw back in a half turn at a time back to the highest idle position. You may need to go back and forth a few times.

5 - once the setting is adjusted return the idle back to a normal range—usually around 1.5K-2K RPM.

6 - if the screw is either all the way in, or close to all the way out then the idle jet should be replaced with one that allows the screw to be in the 1.5-2.5 range.

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[toratora]

How Far from Four Stroking: An alternative to plug chops

From the Moped Army Wiki

Carbs rely on vacuum to pull in fuel, vacuum is created by the venturi effect and by the vacuum of the engine. Slide carbs need certain amounts of vacuum for the size of jet it has, emulsion tube, and area of fuel inlet exposed around the needle. This vacuum is changed by the amount the throttle is pulled and hence how high the slide is and also by increasing or decreasing rpm. In this tuning method the vacuum will increase because of the rapid closure of the slide. This however drops the needle and therefore decreases the area of the jet exposed to the venturi which, if the vacuum where to remain the same, would decrease the fuel let into the engine. As it were this drop of the slide and it's affect on vacuum causes more fuel to be drawn into the engine than the decreasing area of the jet causes less fuel to be drawn into the engine. In simplest terms, a vacuum increase will pull more fuel.

Simply run up to top speed at WOT and then quickly cut the throttle down to 80% of WOT. The bike should start to four stroke. This is because the vacuum has increased causing more fuel to be pulled into the engine. If it doesn't four stroke you are running lean and if it will four stroke by only cutting it down to 90% of WOT, while at top speed, you are running rich. Once you get the hang of this you can substitute the 80% rule for a higher number if you want to run richer or a lower number if you want to run leaner. You can notch your throttle grip and use a marker or center punch on your hand control bracket to correspond to 1/4, 1/2, 3/4, and WOT so you can visually estimate the correct percentage of throttle cut down you desire.

This same exercise can also be preformed at 1/4, 1/2, 3/4 or anywhere in between to tune your carb across the entire range and therefore all speeds. This is very effective because slide carbs demand that you "follow" the rpms with your throttle hand. Meaning, as the rpms increase you must increase how much the carb is open. Opening the throttle too much at too low of a rpm will lessen your vacuum and the engine wont pull in enough fuel. This is why you never go WOT except at close to top speed. To tune at other than WOT you must travel at any maximum speed any one throttle opening produces and then cut the throttle down to 80% of that amount of throttle opening. Simply put if tuning at 1/2 throttle, while at the top speed 1/2 throttle will give you, you would quickly cut the throttle down to 40% of WOT and you should four stroke.

[toratora]

Found this on Power Sport Tuning

There are lots of ways to do this, but a borescope is kind of nice. This one is pretty inexpensive.

I've never read or heard a good explanation... so I can only share what I've learned over the years. If you understand what "piston wash" is, then it's pretty easy to know what to look for and what to look out for.

The first thing you need to realize is that the top of the piston is not a uniform temperature. The area in front of the exhaust port that is exposed to all of the hot gases exiting the cylinder is naturally the hottest. The area in front of the transfer ports that is cooled by the fresh air and fuel entering the cylinder is obviously going to be the coolest. And, the center of the piston will be somewhere in between.

The second important thing is that there is some small range of temperatures where the piston is hot enough to char the oil that comes in contact with it, but cool enough not to melt the piston.

Putting those together, you have "piston wash".

When the engine is jetted very rich, most of the piston (except near the exhaust port) is too cool to char the oil; and you will have large areas spreading out from the transfer ports that appear to have been "washed" clean of carbon by the air and fuel entering the cylinder... that is "piston wash".

If you jet down leaner, the temperature of the whole piston will increase... and more areas will be hot enough to char the oil. The "washed" areas will be large circular areas, located just in front of the transfer ports. At this point, the center of the piston is quite hot; and is actually charring the oil on the underside of the dome, right in the center.

Jetting down further will cause the whole piston to get still hotter. Now, nearly the entire piston is hot enough to char the oil... and the "wash" will be just a small area about the size of a half of a dime, right in front of each transfer port. The bottom side of the piston will be charring the oil over a large area by now also. At this point, the area near the exhaust port is getting almost hot enough to melt. This would be considered (at least by me) to be jetted correctly... but near the "edge".

If you go leaner yet, the whole piston will be hot enough to char the oil on top of it and there will be no "washed" areas left. The entire piston will be covered with carbon, and the aluminum at the edge of the piston, right in front of the exhaust port will actually be slowly melting away and smearing onto the rings... more than a few seconds of this and it's time for new pistons.

That, for whatever it's worth, is my own twisted look at piston wash.

Some things to keep in mind include the fact that the size of the "washed" area depends somewhat on the upward angle of the transfer port. If the port is angled flat across the top of the piston, there will be more cooling and more wash... in spite of the fact that the area in front of the exhaust port might be just as hot as it would be on a motor with upward angled transfers that shows much less wash. The upward angled transfers don't cool as much of the piston top.

Also, the size of the "washed" area has to be somewhat proportional to the size of the piston. A "half a dime" sized wash area on a 600 triple (very small piston) means the piston is a lot cooler than a "half a dime" sized wash area on an 800 twin (with coffee cans for pistons)! What you're really interested in is how much of the piston isn't clean, because that tells you how much of the piston is hot enough to char the oil... and you know the exhaust area is hotter yet.

All this really means is that you can't just say, "Every motor should have a half-dime sized wash area." You have to correlate the size of the wash area to other indicators and engine specs... and use it as just one of your tools for jetting.

[toratora]

I’ve been posting PWK info in some of the tech threads, but I thought it would be nice to have it in this thread too.

Here’s a link to Duncan Racing’s PWK tuning guide.

Keihin Carburetion Jetting

The following information is offered as a guide for a practical interpretation of the workings of Keihin PE, PJ, PWM and PWK carburetors. Many people are intimidated by carburetion jetting. One of the easiest ways to work through your jetting frustration is to not view jetting as one big mystery. Jetting should be broken down into many small questions, which when thought through with some technical instruction and common sense can be deciphered into to the appropriate carburetor settings.

Figure 1: Needle Dimensions

Air Screw: The air screw is a small (5mm in diameter) slotted brass adjustment screw located on the inlet side (air cleaner) of the carburetor. The airscrew is a fine-tuning adjustment designed to allow the carburetor to be slightly adjusted for variances in atmospheric conditions. The airscrew works with the pilot/slow speed system of the carburetor, mainly affecting the engines initial starting, idling and initial power delivery. Proper adjustment of the airscrew can offer direct feed back on the necessary setting required for the pilot jet. The airscrew is adjusted in a rather straightforward manor. See Figure 1 for application chart

The ideal procedure for setting the screw in the correct position is to warm up your ATV engine to the proper operating temperature. Then turn the idle up so it is idling about 500 RPM’s higher than normal. Next turn the airscrew all the way in until it bottoms out, once bottomed out slowly back the screw out a 1⁄4 turn at a time (give the engine 10-15 seconds between each 1⁄4 turn of the screw, to allow the engine to catch up with the adjustments). Continue backing the airscrew out until the engine idles at its highest RPM. The preferred setting window is between 1 and 2 turns. If the engine idles at its highest RPM from 0-1 turns out this means the pilot setting is on the Lean side and a larger pilot jet should be installed. If the engine idles at its highest RPM at over 2 turns out, this means the pilot setting is on the Rich side and a smaller pilot jet should be installed.

If you get no RPM fluctuation when adjusting the air screw there is a very realistic chance that there is something clogging the pilot/slow speed system. Clean the system thoroughly with contact cleaner and blow out with compressed air. Carburetor must be disassembled. If the airscrew adjustment process is unsuccessful and leaves you confused. Set the screw at 1 1⁄2 turns out and consult a professional for further assistance.

Pilot Jet: The pilot jet is a medium size (3⁄4-1") brass jet located inside the float bowl next to the needle jet/main jet location. The pilot jet meters the fuel required for engine starting, idling and the initial throttle opening 0-1/8. A lean pilot jet setting will cause your engine to surge at very low RPM’s, bog or cut-out when the throttle is opened quickly and have trouble idling down.

A rich pilot setting will result in hard starting, plug fouling at low RPM’s, sputtering as the throttle is cracked opened. The pilot jet is not difficult to set. With proper air screw adjustment and a close initial setting from your engine tuner, fine-tuning should be painless. Once set the pilot jet is not terribly sensitive. You should only be required to adjust the setting when confronted with large weather changes or altitude swings of over 2000 ft.

If adjusting the pilot jet gives inconsistent feed back, or does unexplainable things. Check and clean out the pilot/slow speed system thoroughly with contact cleaner and blow out with compressed air.

Pilot jet sizes are numbered in the following pattern; #42, #45, #48, #50, #52, #55, #58, #60 etc. repeating the pattern. Sizes available on most models are from #35 to #80.

Slide: The slide not only monitors how much airflow goes into your engine (its main job). But it has various angles cut on the bottom of the slide to monitor airflow at low RPM’s. This is referred to as slide cut away. The slide cut away is measured in 4.0, 5.0, 6.0 etc. (see attached chart). The higher the number, the larger the cut away the leaner the slide setting is. The slide cut away generally effects the jetting in the 1⁄4 throttle range at almost the same throttle position as the needle diameter effects. The slide cut away is usually predetermined by the engine manufacture or engine tuner. As a general rule do not change the slide cut away unless instructed to do so by a skilled engine tuner. For 1⁄4 throttle jetting adjustments it is easier to adjust the needle diameter.

Needle: The jet needle is the most important component in determining your carburetors jetting. The needle is broken into 3 main functions; Diameter, Length, Taper. These needle functions have a large effect on the carburetors jetting from 1⁄4 to 3⁄4 throttle. In the following paragraphs we will explain the needles functions and how to adjust them.

Diameter: The needle diameter controls the jetting just above the pilot jet, right as the engine begins to pull. On most engine combinations the needle diameter is felt in the 1⁄4 throttle range. The setting of the needle diameter is crucial to both the engines low RPM power and reliability.

The jetting at 1⁄4 throttle is adjusted by changing the diameter of the needle. On gold colored needles identified with the 3 stamped in letter I.D. system the last letter refers to the needle diameter size. By referencing the enclosed jetting chart you can verify your needles size, and be able to determine what needle size may be required for your specific situation. In many instances you can leave the taper and length settings the same (if they are correct) and adjust only the diameter. EXAMPLE: If you have a needle marked DGJ and change it for a needle marked DGK, you have effectively Leaned the jetting at the 1⁄4 throttle position. Reference the enclosed jetting chart to clearly understand this adjustment.

When the needle diameter is Lean the machine will have a loss of low-end power. The engine will feel very zingy (like a 125cc engine). When an engine is in this condition and then put under a heavy load the engine becomes very susceptible to seizing.

When the needle diameter is Rich the machine will sputter at 1⁄4 throttle and be hesitant to take the throttle. In extreme cases the engine can feel like the choke is on or the plug is fouling.

When the needle diameter jetting is set correctly the engine will accelerate evenly thru the first part of the power band. The proper diameter setting will provide maximum low RPM power and very ride able throttle response.

It is important to remember that even though the needle diameter is mainly responsible for the jetting at 1⁄4 throttle there is some bleed effect. With experience this can easily be deciphered. An excellent way to pin point the feel of the needle diameter is test needles in your machine that have both the same taper and length but richer and leaner diameter settings. Try a needle of each setting in your machine for 10-15 minutes of riding and you will begin to understand specifically what throttle position your dealing with.

Length: The needle length is determined by the clip position (grooves at top of needle) setting on the upper portion of the needle. On most needles there are 5 clip positions. The top clip position is referred to as #1 and is the Leanest setting. The clips are referred to in numerical order with the bottom position being #5, the Richest (refer to attached jetting chart illustration). The clip/length setting covers the largest percentage of jetting in your carburetor. With an emphasis at 1⁄2 throttle, the clip (length) setting will bleed both up and down to some degree to cover a wide portion of the midrange jetting.

When the clip/length setting is Lean the machine will be very zingy sounding and feel kind of similar to an 80cc or 125cc machine. Lean in the midrange will also rob power and cause the machine to run hot and seize easily.

When the clip/length setting is Rich the machine will have a lazy feeling in the midrange. Exhaust note will be a little flat sounding. In extreme cases of richness the engine will even sputter or kind of crap out in the midrange.

The safest way to set the clip position is to richen up the clip position setting until the machine loses a little power (feels lazy/unresponsive) then lean it back one position. Ideally you like to run the needle setting in either the 3rd or 4th clip position, if possible. The needle clip jetting is especially critical to your machines reliability because on average more time is spent in the midrange than any other part of the throttle. Most machines pull very hard in the midrange, putting quite a load on the engine. This makes a lean condition very detrimental to your reliability.

Taper: The needle taper is the angle of the needle at its lower half. The taper works the transition between the midrange and full throttle/main jet (3⁄4 throttle position). The taper is the least sensitive function of the needle. Changes in the taper have very mild subtle changes in the jetting. The taper also affects the main jet size your carburetor requires. A leaner needle taper will use a richer main jet than a comparable engine/carburetor combo with a richer needle taper.

As a general rule, your engine tuner or engine manufacture should preset the taper. Once set correctly by a professional the taper setting should not need to be changed except for cases of extreme temperature reduction.

Main Jet: The main jet affects the jetting in the upper quarter of the throttle position. Coming into play at 3⁄4 throttle on through to full open throttle. Even though most people relate the main jet to their carburetor in general. The main jet is only responsible for the last 1⁄4 of the jetting. The main jet does not effect the jetting for starting and idling. It plays no part on low RPM or mid RPM jetting either. The main jet is very important to your machines overall tuning, but should never be over emphasized at the expense of needle tuning or other facets of your carburetion tuning.

When the main jet is Lean the engine will experience detonation or "pinging". Exhaust note will be of a higher, tinier type note. Engine will over heat easy and can be down on horsepower. A moderately lean main jet can cause engine seizures. A severely lean main jet can cause the engine to burn a piston (whole in top).

When the main jet is Rich the engine will be a bit flat or lazy at 3⁄4 to full throttle, giving off a flat, dead sounding exhaust note. When the main jet is severely rich the engine will sputter in the high RPM’s and have a lot of trouble making power up top.

The safest way to get the main jet setting as near correct as possible is to richen the main jet setting up until the engine begins to lose power and not rev to as high of RPM as before. On a single cylinder machine this will signal that the jetting is beginning to get rich. Depending on your riding application you can lean it down a bit from there or leave it for conditions requiring extra fuel (desert racing, long high speed runs, etc.)

As a general rule, richen the jetting up as long as the engine likes it and continues to run just as well or better than the smaller size main jet previously installed. When the engine no longer continues to improve its performance you will know you have gone to far.
Main jets are offered from #90 to #230. Starting at #90, sizing cycles like the following #90, #92, #95, #98, #100, #102, #105, #108, #110, #112, etc.... up thru #200, #205, #210, #215, #220, #230.

Never use any main jets that are not genuine Keihin jets. All genuine jets have a Keihin trademark stamped in them. cheap imitation aftermarket jets are not always sized properly, which can cause poor performance or engine damage.

Tuning Tips-General

• Make sure machine is assembled properly and engine has passed a pressure test. It can be virtually impossible to tune an engine with an air leak. It is imperative that you establish a solid baseline with proper assembly and an air leak free engine. *Note: On a 2-Stroke engine there is absolutely no way to be sure your engine DOES NOT have an air leak without properly performing a pressure test.
• Establish that the engines compression is not weakened. Any loss of compression for whatever reason will give off signs that engines jetting is rich. Consult your service manual or engine tuner for proper compression reading. Any loss of compression on a 2-stroke engine should be followed by a top end disassembly and inspection of piston, rings, cylinder liner etc.
• Compression Test How To: Always use a quality name brand gauge (SNAP-ON Best Choice). Note length of threaded probe in relation to spark plug length. Perform test with engine cold, throttle wide open. Kick machine over until needle quits moving (10-15 kicks normal). Perform 3 separate tests, record results.
  It is always a wise idea to perform a compression test on a fresh engine right after break-in to use for future reference.- Spark Plug: To properly tune a 2 or 4 Stroke engine it is imperative to have a good quality spark plug that is functioning properly. This means that the plug cannot be too old or partially fouled. Spark plug gap is essential; an improperly gapped plug (or partially fouled one) can cause the engine to run poorly, sometimes appearing to be a jetting problem when in reality the problem originates at the spark plug. Also make sure you are using the correct heat range.
• Weak Spark: Weak spark is not only detrimental to your engines performance but can make tuning your engine a nightmare. Weak spark will make the engine run rich. When an engine is running rich due to weak or poor spark the machines performance will slowly continue to deteriorate. Some common causes of weak or poor spark can be a failing or faulty stator, faulty plug cap or plug wire, failing coil, improper ground etc.
• Plug Color: Ideal plug color is a chocolate brown. A rich plug will be a darkish brown or black. May be oily and sooty. A lean plug can be a light brown, or gray (some severe cases can be white). Plug color must be checked after a plug check has been run. To perform plug check run engine at specific RPM that reading is desired for at least 5 seconds, then turn off engine without letting RPM’s fall. This test is very difficult to perform at any RPM other than wide- open throttle.
  It can take years to learn how to accurately and precisely read spark plugs. For amateurs, plug color should not be the only piece of evidence used to adjust jetting. Plug reading should be evaluated with other jetting evidence to achieve a proper conclusion on tuning.
• Fuel: The fuel used in your machine is very influential in tuning your engine. Nothing jets better or more consistent than a name brand Race Fuel designed for the specific engine in question. Things to stay away from are pump fuel and aviation fuel. Both can cause inconsistent jetting feedback and make your engine run hotter than normal.
• Silencer: 2 Stroke silencers must be functioning properly. A worn out, poorly maintained, silencer can cause the engine to run funny making tuning difficult. An excessively packed silencer can also cause a tuning difficulty, making the engine lose power and run rich.
• Exhaust Pipe: Check your pipe for severe dents in head pipe section or end cone area, dents in these areas may affect tuning. Also check for carbon build up inside pipe. Any carbon build up at all is not desirable. Heavy build up will hamper engine tuning and rob power. Pipes with these problems should be replaced.
• Silencer Color: Always take note of the silencer exhaust color where the exhaust exits the silencer. Inspect the core and corresponding end cap. Colors will be similar to plug color.
• Air Filter: It is highly recommended to do your initial engine tuning/jetting with a new air filter. A used filter will never quite work as perfect as new one. Old, dirty, improperly serviced filters will severely hamper proper engine performance and tuning. When performance is critical use a new filter.
• One Change at a time: When tuning a carburetor, only make one adjustment at a time. This is a good rule to follow so the tuner will not get confused or mislead from false information. Keeping to one adjustment at a time will help ensure that you will always know what effect each adjustment had on the engine.
• Unresponsive?: If during the jetting/tuning process you have made a number of adjustments (especially main jet or needle clip) to the carburetor and the engine has not changed. There is a strong possibility that something other than carburetion jetting is causing your engine to perform incorrectly. Jetting is a constant. When adjustments are made to a machine with all components working properly the engine will respond in some way. Depending on the adjustment made the machine will either run better or worse, but there is almost always some form of change. When changes are made without any response it is a sign of other problems. Things to check out would include; low compression, weak spark, fouled plug, failing reeds, air leak, clogged air filter, clogged or over packed silencer, etc.
• Idle Adjustment: To adjust the idle on PJ and PWM model carburetors it is necessary to turn the knob on top of the choke clockwise to lower idle and counter clockwise to raise idle. PWK models are adjusted by a screw/spring approximately 3⁄4" below the carburetor cap on the left hand side of the carburetor. Screw in to raise idle out to lower. On many models screw must be screwed in most of the way for proper idle speed. PE models also have a screw/spring that manually raises the slide. On PE models idle screw is approximately 2" below carburetor cap and perpendicular to the slide.
• Free Play: It is highly recommended that at least a 1⁄4" of free play is kept in the throttle cable. Failure to run proper free play can cause the throttle to stick.
• Proper Slide Throw: It is always a good idea to confirm that the carburetor slide opens and closes all the way. This should be done with carburetor hooked up but off the machine, as to physically see slide travel thru its complete stroke.
• PWK Cable Holder: The cable holder on PWK model carburetor must be unscrewed to remove needle for adjustment. The only way to properly remove cable holder is with a 1⁄4" ratchet and a 6mm socket. Turn counter clockwise to loosen. During reassembly do not over tighten. Do not put screwdriver in slot to attempt to loosen. This will cause severe damage to cable holder and slide.

Tuning Tips-Various Weather Conditions

• The engines fuel requirements are in a large part determined by the amount of air getting into
  the engine. More air requires more fuel, less air requires less fuel.
• Temperature; Cold weather conditions require the richest carburetor jet settings. This is because cold air condenses allowing more air into the engine than warm conditions where air expands allowing less air into the engine.
• Humidity; Conditions of high humidity cause your engine to run richer and make less power than moderate to dry conditions. The moisture in the air displacing oxygen causes this rich effect from humid conditions. Allowing less air into the engine, requiring less fuel.
• Rain: Is humidity at the next level. During rainy conditions your engine will require leaner jetting because the rain displaces the air. *Always note the ambient temperature. Sometimes cold temperatures during rain sessions can off set oxygen displacement.
• Elevation: Knowing the elevation is critical in fine-tuning an engine for maximum performance. Base line elevation should always be sea level. As engines are used at altitude above sea level for example 2000 ft, 4000 ft. etc. adjustments must be made to compensate for the loss in compression. (Atmospheric pressure decrease as altitude increases causing less air to be compressed into the cylinder)
• The proper initial adjustment is to alter the cylinder head volume (consult your engine tuner for specific instructions) to try to off set the loss of air to compress.
• After this adjustment is made the engine must have the jetting checked and possibly adjusted.
• If the engine is not modified for the new altitude, then the carburetion will most definitely need to be adjusted. As a basic rule of thumb the higher the altitude, the leaner the jetting.

Here’s an exploded diagram and parts list for the PWK, and a needle chart.

Here is a nice graph illustrating the effect of the various circuits, and their interaction on the PWK.

[toratora]

CP carburetor instructions: Assembly and Calibrate
https://www.polini.com/en/technical-are ... tructions/

1. Choice of the Carburetor Diameter or Venturi
The carburetor must be chosen according to the characteristics of your engine. If your priority is to increase the performance, the choice of the venturi diameter is fundamental because it affects the resistance that the aspiration system provides to the aspirate flow, in particular:

• The larger the venturi diameter is and the lower the pressure drop will be than those with a lower venturi diameter and with equal characteristics with consequently power increase.
• The smaller the diameter is, the more the flow speed will increase improving the power output.

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2. Fitting and Precautions
The Polini CP carburetor has slightly different dimensions than the more common carburetors. At the time of purchase it is therefore necessary to consider what adjustments are necessary for a proper assembly. The most important adjustments are the flanges to fit the filter and the curves to connect the throttle cable and choke. Once you have assembled the carburetor it must not hit or get in touch with other engine parts or bikes parts (except for the intake manifold or the filter to which must be connected).

The Polini CP carburetor is sold with standard calibration that has been set-up during the tests on the most famous standard motorcycles and scooter or equipped with power unit but whose engines do not exceed 12.000 rpm. For different settings see the proper section. If your engine exceeds 12.000 rpm and it is equipped with racing products we advise you to carefully read section 11. Competition. .

3. Fuel System
The Polini CP carburetor is of a needle type. The fuel arriving from the tank is held inside at a constant level thanks to the floating needle mechanism. The fuel level in the bowl that arrives to the jets during its working remains constant and therefore so it is the gap that the fuel must pass to get into the spray nozzle.

The fuel level in the bowl is therefore a calibration element of the carburetor, since when it changes consequently the signal on the carburetor intake nozzles changes and therefore affects the mixture ratio.

The adjustment of the level in the bowl depends on the float lever arm/needle and the weight of the float itself: .

The floating needle lever arm is not adjustable in the Polini CP carburetor. It is calibrated during its assembly and it can only be controlled. (See photos 1 to check the float measure). By installing a heavier float, the result will be a richer mixture; on the contrary, with a lighter float the carburetion will be poorer.

The floats are classified according to their weight and they should be placed inside the bowl according to precise provisions in order to achieve the best working. If you use special fuels make sure that the synthetic rubber at the ends of the needle is compatible with it. The diameter of the needle housing is another important element during the tests for the carburetor setting because it fixes the fuel quantity and its speed when entering in the tank, in particular:

• if the diameter is too small you risk to get a poor carburation because the bowl may empty very fast than the needle can make the fuel pass through, with the serious risk of damaging the engine.
• The diameter of the POLINI CP carburetor needle housing has been studied to avoid this problem as the hole diameter is oversized.

4. Throttle Valve
It is an adjusting element that determines the area actually available for the flow passage. It’s a cylindrical valve that slides across the venturi in an appropriate housing obtained in the carburetor. To eliminate the phenomena of wear or sticking of the throttle valve, the same and the elements that surround it are subjected to material hardening treatment. .

5. Idle Circuit
The working principles of all the carburetors refers to the same physical principle. The principle is that the fuel responds to a vacuum signal generated by the induction action of the engine. When the gas valve is closed, or nearly so, the depression that insists on the main spray nozzle is not sufficient to draw fuel from the bowl. It is for this reason that the carburetor has a second supply circuit that operates with this kind of conditions (at idle) allowing the engine to operate normally that otherwise would stop running.

The idle circuit is equipped with a supply port placed immediately downstream of the throttle valve, at a point such that once the valve is closed, it experiences strong vacuum conditions and therefore is in the best condition to supply fuel from the bowl. It is equipped with its own jet (idling) whose choice is very important not only to work in idle conditions but also to get the best engine response during the first opening of the throttle valve, which subsequently will characterize the next progression. If the selected idle jet is too big, the engine may tend to stall and responds slowly to the throttle. If, on the contrary, the jet is too small, the engine responds better to the throttle but when the throttle is closed, the speed remains high for few seconds before settling down to idle. .

6. The Emulsion Air Circuit
The fuel supplied by the idle circuit is mixed with a small quantity of air. The quantity of air is minimal when the idle circuit is working but it becomes more relevant for the progression circuit when the throttle valve is lifted. The idle set up is made by acting on the air screw.

• To get an enrichment you need to screw the air screw (close the air intake); otherwise to lean the carburation you need to unscrew it.
• The best setting is achieved with the air screw 1-1.5 turn open from completely closed position.

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7. The Conical Idle
The Polini CP carburetor is a needle type because by using the conical needle you get a mechanical configuration that ensures the correct mixture ratio for all operating conditions of the engine. This needle is placed in a section of the atomizer and works like this:

• When the throttle valve is slightly raised the needle reduces the space for the fuel passage and so the depression increases but the output remains low and consequently the mixture ratio remains fixed.
• When you have a large throttle opening the needle is in the atomizer and it increases the fuel supply space; then, while depression is decreasing, the mixture ratio is optimal thanks to the increase of fuel passage area. The engine is able to operate at all throttle openings. The needle is attached to the throttle valve with an elastic clip which binds to one of the notches on the end of the stem. The adjustment of the needle system can be made by acting on the needle itself. In particular, if you need to lean the mixture you must lower the needle (by moving the clip above) while if you have a fuel mixture which is too rich you must lift the needle (by moving the clip below). Even the conical degree and the length of the conical part of the needle are important parameters for the carburetion adjustment because they directly affect the overall engine response. When it is not possible to correctly adjust the carburetor acting on these parameters you will need to replace the needle with another of different features.

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8. Needle-Bushing Atomizer Coupling
The adjustment of the carburetion can be done by modifying the jets and/or changing the position and the type of the conical needle but you can also act on the atomizer.

The atomizer end parts which is very close to the venturi has a bush of calibrated diameter, available in different sizes. By increasing the atomizer’s diameter, the mixture is enriched. .

9. The Atomozier and Air Brake
The atomizer is a tube that connects the main jet to the venturi and it is called “two-stroke type” when it doesn’t have any hole. Otherwise if it has a series of holes along its entire length it is called “four-stroke type”.(Attention: to call the atomizers “2 or 4 stroke” is popular but technically incorrect because you can safely use atomizers with holes for the 2 stroke too. I.e the Minibikes have atomizers with “4 stroke” type holes). Its working is very easy: due to the depression in the venturi then, from the atomizer tube the liquid fuel is drawn, metered by the main jet and by the conical needle, while a certain airflow comes from the channel.

Air and fuel mix together and the result is a pulverized spray sucked by the engine. To intervene in this process, in addition to the atomizer hole, there are the air channel diameter, the height of the part of the atomizer protruding into the chamber and the height of the step located in the nozzle. In addition to the atomizer’s hole diameter, the variables are therefore the diameter of the air channel, the height of the atomizer’s side that protrudes in the chamber and the “step” of the delivery nozzle.

The four stroke type produces leaner mixture and more controlled; in fact since it is equipped with a series of holes the emulsion is done inside the tube, before the mixture reaches the nozzle in the venture. Obviously the arrangement of the holes and their diameter influences the delivery. Holes machined in the lower part of the atomizer are bathed in the fuel of the bowl so, by increasing the numbers of holes the flow of fuel increases and goes to emulsify itself with the air. The holes in the upper part of the atomizer are exposed to air and by increasing them you obtain weaker carburation at low rpm.

The Polini CP carburetor can be adjusted so that it works with both the types, 2 or 4 stroke, changing the brake air jet.

• Close air brake jet: 2 stroke atomizer type working as an atomizer without holes.
• Open air brake jet: the atomizer holes work and make the engine weaker.

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10. The Main Jet
It’s one of the most important elements of the carburetor’s adjustment, because it calibrates the fuel delivered by the main system. To begin you have to try some jets, and it is good to start with a bigger jet to prevent the engine damage with a too poor fuel mixture that would lead to a seizure or piston drilling. Proceed by trial and error: after having driven a straight at full throttle at the highest rpm check the spark plug color (see spark plug color table).

When you find the proper jet it is important to increase the size of a few points as a precaution because a mixture weakening could have passed caused by temperature lowering or by an increase of the environmental pressure.

Spark Plug Color Indications

1. Normal: From dark brown to light brown with dry electrode
2. Overheated: Light grey or white ⇒ Poor mixture
3. Wet: Wet or sooty ⇒ Reach mixture

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11. Competition
If the carburetor is fitted on racing engines that exceed 12000 rpm, please carefully read the following instructions. The Polini CP carburetor is lighter than other carburetors available on the market and under certain conditions it may cause fuel emulsion problems. Air bubbles are sucked into the main jet casing very serious problems to the engine that becomes poor despite having a big or correct jet; the engine begins to detonate up to its break.

First of all check the assembly instructions in the Calibrations sectionto ensure that the Polini CP carburetor has been tested on your bike or scooter model by the Polini technicians and follow the instructions. If in the Calibrations section our motorcycle or scooter model is not listed, proceed as follows:
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• Fit the transparent bowl (cod.343.0029) that will allow you to detect the problem and, if necessary, soften the rubber manifold until the problem disappears.
• If the problem persists, increase the mass of the carburetor mounting the filter coupling iron flanges (cod.343.0028) produced by POLINI. Once the problem has been solved fit the standard bowl again.

. • CP Carburetor Calibrations • Needle Measures

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Polini CP Needle Specs

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Dellorto PHVA Diagram 1


Dellorto PHVA Diagram 2


Dellorto PHBG Diagram