How To Tune PHBG Carburetors
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
- A la Harold from the performance forum (Re: tuning a dellorto phbg 21mm racing type --Date Posted: 02-05-08 14:17)
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 tightening it 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.
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-W2; 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.
- 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.
Tuning the Pilot Jet and Idle Jet
Tuning PHGB pilot jet and mix screw (idle jet) together:
To tune the pilot jet and mix screw, flip the throttle from closed to WOT a few times. It should be smooth and fast if tuned correctly.
- If there is sputtery hesitation and sluggishness right off the bat, turn the screw in.
- If there is a bog or gasping for air feeling right off the bat 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 bikes like either a 45 to 55 pilot jet. Also note that the mix screw doesn't directly effect the pilot jet. Rather, it effects a hole further down the venturi that is also supplied by the pilot jet but metered by the mix screw. This is called the progression circuit, and it is fully activated at around 1/8 throttle… just that keep it in mind.
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 it 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
REMOVE THE METAL DISK! It sits between the main jet and diffuser tube. Remove it because it often blocks fuel to the main jet and causes extreme lean conditions.
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.
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 to 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. --Elliot 15:15, 16 June 2008 (EDT)
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.)