How To Tune PHBG Carburetors

See also: PHBG tuning made easy (a step by step guide)

First Things First

Here are some resources for those of us new to the world of PHBG Tunings. . .

First, you need to install a clean air filter. Second, you need to insure the float level is properly set. If the gasoline level is set too high or too low, properly jetting your bike will be impossible. A high float level will cause it to run rich and a low level cause it to run lean. A properly set float will be parallel to the base of the carb body when it is held upside down w/o the float bowl on. PHBG's always seem to come level, but its good to check. Finally, make sure to start with a fresh spark plug when tuning.

Functions of Jets and Needles

On a PHBG carb, the main jet starts functionality about 1/3 of the way through but doesn't become truly effective until 1/2 throttle. If you are having issues at 1/3 throttle, the best thing you can do is to adjust the needle setting. “Lowering the needle” by “raising” the clip up one position makes the the gas flow leaner in the low to mid throttle range. The needle is tapered so when the it is 1/4 to 3/4 of the way out, the angle of taper is what determines how much gas can escape through your main jet. Once you reach WOT, the amount of gas is completely controlled by the main jet. Actually, there is one other element to the equation: the air/mixture screw. Look at the chart see how it is moderately effective all the way through. That is because it uses a separate jet which also sucks fuel that doesn't have a needle. The adjustable screw that is flush with the carb body controls it. By turning that counterclockwise you will let a just a tiny bit more gas flow in to the system, and by turning it clockwise you will cut that off. So you can make very small adjustments by using the screw, or you can make major adjustments by changing the jet.

Here is an Image by Bourbon Bandit MarkRyan on the placement of all the PHBG Jets.PhbgJets.jpg

Here is image originally submitted by a Dutch Tuner, and updated by I.R.E. Tom (the new one you can print at 7" Wide) of the Working Range for the PHBG jets / Needle .PHBGGraph.png

Tuning PHBG Needles

  • Also, from Brett in the same thread as mentioned earlier

Think of the clip as being stationary and the needle moving up and down along it like a window sash. The higher it is the more open and therefore rich, the lower it is the less open and therefore lean.

Top clip setting, leanest.
Bottom clip setting, richest.
Standard, second from top.

I wouldn’t mess with your needle until you’ve gotten the main jet figured out and have set your idle. Elliot seized his Polini by leaning out his needle.

Out of the Package

PHBG carbs do not always come with the same # needle stock, so it is important to check your needle # before installation of the carb. The needle # is stamped onto the needle right below the clip notches. PHBG needles come in a range of W1-W25; these numbers are not arranged in direct order like jets. You should refer to the chart below when sizing needles. The most common needle sizes that come stock in PHBG carbs are: W6 (usually in racing carbs), W7, and W9. However, they are not limited to these sizes -- I have found a W10 and W16 in stock PHBG carbs before. If your carb did not come with a W6, W7, or W9 and you are having a lot of tuning problems I would suggest starting with a W7 and work from there.

Needle Characteristics

  • Thinner needles are overall richer, and thicker needles are overall leaner.
  • Needles with longer tapers are "earlier" because they begin to meter the air fuel mix at a lower throttle position, and they tend to be richer overall.
  • Needles with a shorter taper are "later" because they begin to meter the air fuel mix at a higher throttle position, and they tend to be leaner overall.
  • Needles with shallow tapers and fat tips are leaner, while needles with pronounced tapers and small tips are richer.

As stated previously, a good place to start is with the needle clips. Raising the needle by putting the clip in a lower notch will richen the midrange mix, and lowering the needle by putting the clip on higher notches will lean it out. If you find the bike responds poorly on all notches or only runs okay on one of the very outside notches, you should look into getting a different needle.

Needle selection is somewhat of an art. One should choose a needle that delivers optimum mix at all its effective throttle positions. Unfortunately, this art is not a simple as going up or down a number like it is in jets. When tuning with needles, be very careful because an improperly tuned needle is just as dangerous as a main jet that is too lean --as I learned the hard way when I seized a Polini when experimenting by moving the clip from the perfect position to one notch leaner.

Throttle slide cutaway

This is a bit unintuitive, so it's worth mentioning. A lot of people think a larger cutaway increases airflow and therefore fuel delivery. While it allows for the passage of more air this in turn reduces vacuum pressure, resulting is less fuel being drawn through the main jet/atomizer. So a larger cutaway leans the mixture over its effective range. From the Dellorto manual:

"The deciding factor which determines airflow in this phase [1/8 - 1/2 throttle] is the throttle valve cutaway. A small cutaway creates a greater vacuum and consequently causes a larger amount of fuel to be drawn up through the atomizer; on the other hand, a larger cutaway would lower the vacuum and therefore reduce the fuel delivered. Because of this, fitting a lower slide cutaway results in enrichment and vice versa."

Tuning the Idle Jet

Tuning PHGB idle jet and mixture adjustment screw:
Start by turning the idle speed screw in such that the engine is running above idle speed. Then, turn the mixture screw in and out to determine the location that results in the smoothest operation. Then, gradually turn out the speed screw to decrease RPMs, at the same time playing with the mixture screw. By adjusting the two back and forth you should be able to arrive at settings that provide a low, stable idle speed (roughly 1,200 RPM). If upon letting out the speed screw the engine will not come down to an idle speed regardless if the mixture screw position your idle jet is likely significantly too big or small.

To test the progression circuit, flip the throttle from closed to 1/8 throttle a few times. It should be smooth and fast if tuned correctly, and should quickly return to idle.

  • If there is sputtery hesitation and sluggishness, turn the screw in.
  • If there is a bog or gasping for air feeling and loss of power, turn the screw out.
  • If the screw gets to one turn or closer to full in, lean the jet out.
  • If the screw is one or closer to falling out, richen the jet up.

Most (kitted) bikes like an idle jet somewhere between 40 and 50. Also note that the idle jet serves two purposes: to supply a minimal amount of fuel to keep the bike running at idle, and to supply fuel to transition from idle jet to needle jet (a la the progression). At idle, air flows into the progression hole (up stream), picks up fuel from the idle jet, and comes out the idle passage (down stream). What comes out of the idle passage is metered by the mixture adjustment screw in addition to the idle jet. During progression, flow through the progression hole changes direction; instead of air going in, fuel comes out. This is not affected by the adjustment screw, rather it's solely metered by the idle jet. Because of this, it's possible to achieve a decent idle that results in poor progression. For instance, if you have to turn the mixture screw all the way in to achieve a decent idle your idle jet is probably too big which is likely going to result in a boggy progression. But keep in mind, needle taper, needle clip position, and throttle slide cutaway will all have an effect on progression as well.

Side Notes

The Choke Circuit

PHBG carbs come with one of two types of choke mechanisms that can be specified when bought: cable or click. Cable chokes work similarly to the throttle cable and require a second cable to be installed and fed into a lever. The click choke works by pulling up and twisting the knob on the carb. MOST BIKES WITH PHBG CARBS WHEN PROPERLY TUNED WILL NOT START EASILY W/O THE CHOKE ON! If you find yourself pedaling for a long time before your bike kicks over from a cold start, it's because you are not using the choke. The choke jet is always at 60, and I have never needed to change it. If you are new to tuning PHBG carbs and don’t know what to feel for, one trick is to use the choke to gauge whether you are too rich or too lean . If you're not sure that sluggishness is a lean gasp bog or a rich gurgle load (because it's not really obvious if it's only off by a bit) just flip the choke on. If the bike gets better at that throttle position your tuning for its too lean and vise versa.

Little Nozzles and Black Rubber Caps

Don't remove the black rubber caps they will cause air leaks. These nozzles are for vacuum lines and are mainly used on four-stroke motors. If your carb did not come with these caps, tape them up on the brass nozzles. (I am not referring to the overflow outlets that face downward and are "L" shaped and on either side of the upper carb body. Leave those open).

The Metal Disk

THE METAL DISK! It sits between the main jet and diffuser tube. It was designed to break up air bubbles in the fuel when high vibrations cause foaming within the carburetor Bowl. This help prevent air bubbles getting trapped within the fuel metering system which would cause an extreme lean condition under high throttle power demands. Many have removed this disk because the new fuel which has been developed has different properties of tension thus it tends to trap the air bubbles underneath the apron causing extremely lean conditions under high vibration modes at full power or close to it. By enlarging the holes in the disc will allow the air bubbles to flow through this mod must be done especially if you're running fuel with any alcohol in it and most new fuels due to the difference in the chemical blending from when this device was originally designed to work with the fuels properties of tension.

I have found that running carburetors that suffered from heavy foaming that by reinstalling the disk with larger holes assisted greatly with breaking up the bubbles and causing them to move away from the main jet pocket under the desk.

The Hand Throttle

Stock moped throttles for the most part can not fully open a PHBG carb slide. For proper tuning, it is advised that you install an aftermarket throttle assembly that can fully open your new PHBG carb.

Atomizers & Slides

Note: Atomizer & Slide will dictate/control/change the range of rich and lean of the plot/idle jet. If you run a PHBG, it is recommended that you check the atomizers first before installing the carb; because Dellorto supplied shops with some PHBGs with four-stroke AN atomizers, and the 9562 AN type atomizer bushing. If the atomizer part number starts with AN, it's a four-stroke atomizer, and it may be more difficult to tune and run very rich at idle and too lean at the 3/16 to 5/16 throttle position on piston port engines, more so than set ups with reeds. All two-stroke atomizer part numbers start with AU and use the 9655 AU atomizer bushing.

Links [1] and [2].

The atomizer bushing on the left #9655 is for 2 strokes, the atomizer bushing on the right #9562 is for 4 strokes

Symptoms of having a rich/less defusing slide or to large/rich atomizer with reeds and or a AN atomizer and four stroke AN atomizer bushing on a piston port engine

  • It will be very difficult, if not impossible to tune and boggy at low RPM’s.
  • It will run very rich at idle up to the 1/8 throttle position even with a small 35 idle jet.
  • It will be lean at the 3/16 to 5/16 throttle position.
  • The WOT throttle position the main jet might be richer or leaner and make you up or down jet the main by 4 or 8 sizes higher or lower after changing the atomizer and or slide, so plug chop the the main jet in last keep it rich till you tune it in.
  • CK your PHBG float the 4 stroke float is heaver by 1gram p/n 9450.2 - 5 grams, And one is not listed on Delldrto's UK site is p/n 9450.3 - 4.5 grams.?? the 2 stroke one is the lighter one p/n 9450.1 - 4 grams. The p/n number's or on the floats.
  • WARNING: upjet your main jet by 4 or 8 to be safe when you change to leaner atomizer or your switching to or from the 4 stroke or 2 stroke atomizer parts.
  • Up jet and rich in the needle when you change to leaner atomizer lower part number. Tuning chart [[3]]

NOTE:If you have reed valves

  • If you're running the thick stiff reeds you will be fine with running the four stroke AN bushing just by running a richer or leaner AN atomizer, because reed valves reduce the intakes back flow/charge out the carb and back in. The soft reeds will reduce less of the intakes back flow/charge. It is the piston port set ups that will benefit the most by running the two stroke AU atomizer and bushing because the C shaped AU atomizer dam helps prevent the piston ports intake systems, back flow/charge out the carbs bore at low RPM’s from sucking more gas out of the atomizer when exiting out and back in the carb. So the two stroke AU bushing and atomizer will pick up less gas from the AU atomizer system at low RPM’s, on piston port engine's. But do check your PHBG float the 4 stroke float is heaver by 1gram p/n 9450.2 - 5 grams, And one is not listed on Delldrto's UK sight is p/n 9450.3 - 4.5 grams.?? the 2 stroke one is the lighter one p/n 9450.1 - 4 grams. The p/n number's or on the floats. But every set up and combination will be deferent to tune in.

How to ID your atomizer and atomizer bushing

Remove your air filter and bring the slide up to WOT. Look in the down the carb bore. The two stroke AU atomizer bushing will be sticking up 2 or 3 mm from the bottom of the carb boar and will have a C shape. The four stroke atomizer bushing will be flat. Alternatively, remove the main jet and atomizer.

The stamping on the two stroke atomizer will be:

  • AU 258 Atomiser PN#9654.258
  • AU 260 Atomiser PN#9654.260
  • AU 262 Atomiser PN#9654.262
  • AU 264 Atomiser PN#9654.264
  • AU 266 Atomiser PN#9654.266
  • AU 270 Atomiser PN#9654.270

The stamping on the four stroke atomizer will be (NOTE four stroke will have pin holes running up and down the sides):

  • AN 258 Atomiser PN#9511.258
  • AN 260 Atomiser PN#9511.260
  • AN 262 Atomiser PN#9511.262
  • AN 264 Atomiser PN#9511.264
  • AN 266 Atomiser PN#9511.266

You must replace the atomizer and atomizer bushing together to change your PHBG from a four stroke carb to a two stroke PHBG carb.

How to remove your atomizer bushing.

  1. Remove carb slide the bottom of the carb and atomizer.
  2. Take the old atomizer and grind/file it down at the hex end.
  3. See where the part # is stamped, and cut /file/grind that part down until it will fit/slide through the carb housing upside down.
  4. Now install the old atomizer right side up into the atomizer bushing 6 or 7 turns and tap on the old atomizer driving it up and out where the slide sits to remove atomizer bushing.

To install the atomizer bushing.

  1. By looking down from the top where the slide came out of the PHBG, you will see the hole where the new 2 stroke atomizer goes in.
  2. The 4 stroke atomizer has a flat spot/notch on the side at the top, That needs to be installed with the flat spot/notch facing the brass fuel inlet so the hole in the bushing's side lines up with the hole in the carb's housing.
  3. The 2 stroke atomizer has a C shape to it that needs to be installed from the top where the slide sits with the C opening facing to the engine side of the carb bore, NOT THE AIR FILTER SIDE.
  4. Tap in the new atomizer with a small tube or socket from the top, but don't hit the C shaped dam on the 2 stroke bushing.

How to adjust and select idle jet and atomizer.

  1. Remove carburetor from box or bike and write down all jet, needle, and atomizer settings and part numbers.
  2. To set your idle speed screw, look into the engine side of the of the carburetor's bore. Turn the idle screw counter clockwise until the slide is just closed. Now turn the idle screw in 1 1/2 turns.
  3. To set the brass idle mixture screw, turn it in all the way in slowly BUT not tight, just until it touches the bottom, then turn it out 2 1/2 turns. This is just a base setting and you will need to fine tune it later.
  4. Install carb: pull the choke and start it up. Count to seven and turn off the choke. Let it warm up.
  5. Now give it 3/4 throttle than let go.
  6. What you want is the motor to settle right down smoothly without it hanging on to the rev's or loading up.
  7. Now turn the mixture screw till you get a nice snap and set idle speed
  8. If the low speed idle/jet mixture screw gets lower than 1 turn out you need a smaller idle jet and if it gets higher than 3 turns out you need a bigger idle jet.
  9. Now do full throttle range plug chops to set an select slide needle and main jet.
  10. Repeat steps 5 6 7 8 9.

NOTE if the idle jet gets smaller than a #38 you might need a leaner atomizer or steeper slide with bigger vent holes. If the idle jet gets bigger than a #52 you might need a richer atomizer. And if your idle speed screw is more than 3 1/2 turns in you might need a higher number slide, like a #40 with it's steeper angel and bigger vent holes. Needles with longer tapers are earlier because they begin to meter the air fuel mix at a lower throttle position, Needles with a shorter taper are later because they begin to meter the air fuel mix at a higher throttle position, The AU 260 – AU 262 or AU 264 atomizers and AU bushing fix most PHBG. Most 19mm to 21mm carbs will need a #40 slide, W9 – W3 - W7 or ? some carb setups if you use a W7 it can be to lean with the clip in the top/leanest notch. 17mm #30 slide 18mm carbs will use a #30 or #40 slide and use W7 – W4 or ? on smaller carbs 30 or 20 Slide or ? And check your float the 2 stroke one is the lighter one p/n 9450.1 - 4 grams. The p/n number's or on the floats. Every set up and combination will be different to tune in.

Final Note

Final note on all multi-circuit (PHBG style) carbs:

It is important to note that a PHBG carb or any multi jet/circuit carb can have several differently tuned throttle positions. For example, one could have a rich pilot jet, the correct needle setting, and a lean main jet. One might think their bike is tuned correctly if they never give it full throttle for more than a few seconds and are always effectively at 3/4 throttle or "riding in the needle" while tuning. While putting around it may 4 stroke due to the rich pilot jet setting, and one might think the bike is too rich. After a couple hundred miles post-kit break-in, one may gain some confidence and go for an extended WOT run and seize due to the main jet being too lean.

As such, it is important to think back to the chart above when tuning your PHBG carb. Each fuel metering element comes into play at different throttle positions, not only different RPMS, or speeds. Remember to feel all throttle positions to accurately gauge what areas need attention.

SO when tuning the main jet make sure to be at WOT, or you are not getting an accurate main jet reading. Start big and work your way down. If the bike is slower at WOT than at 3/4 throttle, the main is still too big. When you are really close to the perfect #, the bike will be the same speed or only slightly faster at 7/8 throttle than at WOT. This is your safe zone, be very careful when down jetting past this point.


You can also change diffusers. There are a few sizes as well as slide cutaways (or throttle slides), but on a single speed bike it's not really worth getting too involved here.

Good luck, modify as needed.


Click on chart and diagram for slightly larger versions. (Clicking on them will take you to the image page where you can click on "download high resolution version" right underneath the picture.)

Dellorto slide needle

Dellorto needle size chart


PHBG diagram