PHBG tuning made easy (a step by step guide)

See also: PDF tuning guide with color photos

Here is an orderly checklist that new tuners could follow to try to tune their carbs as opposed to meandering through the process. I came up with a rough draft borrowing heavily from the previous wiki entries and Dellorto manual. Hope this works for you.

See also: How To Tune PHBG Carburetors

Make sure you do not have any air leaks before trying to tune your carb..

Overview of what order to tune

  1. Idle Screw
  2. Set Idle Jet/Mixture Screw
  3. Set Main Jet
  4. Set Needle
  5. Plug chop

Note: 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 5/16 throttle position on piston port engines. All two-stroke atomizer part numbers start with AU and use the 9655 AU atomizer bushing. For more information, please read this.


  • Zero-eth: Remove the brass disk that lives between the main jet and the atomizer (if equipped) and throw it in the trash. While this "anti-surge washer" may be useful for some applications, it doesn't seem like use in mopeds is one. Keeping this disk installed may result in an extremely lean condition, wherein a) you will not be able to keep the bike running past ~1/4 throttle regardless of main jet, atomizer, or needle size, and b) you risk serious damage to the engine.
  • First: start with choke on until it starts. Let the bike warm up and then disengage the choke
  • Second:With your engine idling, adjust idle screw until it idles how you want (back wheel moving). then turn it IN half a turn more (dunno why but it says it in the Dellorto manual).
  • Third: 1/8ths throttle is controlled by your idle jet fine tuned by your mixture screw adjustments.
  • Fourth: Do 1/8th revs rotating the mixture screw until it is fast and responsive. If it is studdering like its rich, turn it IN. If it is boggy or unresponsive, turn it out until it just starts to four stroke, then turn it in half to one full turn until it just stops four stroking. That is your safe/hot spot.
  • Fifth: If it is still four stroking when you turn the mixture screw all the way in, down jet your idle jet. If it is too boggy and you can't get it to four stroke all the way out, upjet
  • Sixth: You will know you are idle jetted right when it is fast and responsive at 1/8ths twists.
  • Seventh: Then install a rich estimate of what your main jet should be. And also set your needle at the richest or next to rich (clip on the bottom notch or second from bottom) setting.
  • Eighth: Ride bike and try to push all the way to your main jet/WOT full throttle. If you can't reach your main jet because you are too rich, raise the clip in your needle and try again.
  • Ninth: At full throttle, if your bike is four stroking, down jet and do it again. If it is boggy upjet. If it is boggy or seems fine and you want to be extra safe, upjet until it four-strokes on the top, THEN downjet 2-4 jet sizes on your main until it just stops four stroking. That should be your WOT hot spot of maximum safe power output.
  • Tenth: Next when running at 1/4ths to 3/4th throttle with your needle in a rich setting you will likely have felt four stroking somewhere within the progression. If not, leave your needle. If you do, lean your needle one notch at a time until you do not feel this four stroking.
  • Eleventh: Now depending on how drastic you have changed your main jet and adjusted your needle, your idle circuit can be slightly affected by this change. This is because your main and needle have an affect on how your bike operated at 1/8ths throttle. If you have leaned your main and needle, your idle will be slighly leaned. If you have richened your main and needle, your idle will be richer. Recheck your 1/8ths throttle response and repeat step 4 and 5 if necessary.
  • Twelft: Now do a final run. 1/8ths throttle should be responsive. Past 1/8ths should be a smooth progression as your needle and main jet are taking over. between 3/4th to 4/4th throttle, there should be only a slight increase in power for when you are full throttle.
  • Thirteenth: Next with a new plug run the bike doing plug chops, killing the bike with a kill switch for each throttle positions to see how lean/rich you are running in each point of the progression. Pull the plug and look at the porcelain tip surrounding the electrode. White is bad. Black is Bad. Light Brown to Dark Brown is good.
  • Fourteenth: Then turn out your idle screw that half turn so that it slowly moves the back wheel.



    • "step one.0": first read the dellorto manual.
    • step two.0": ive looked over this and steps two through six are wrong.
    • step three.0": to tune the idle circuit and progression circuit, the idle is jetted by the mixture screw position. the progression circuit is controlled, by the idle jet and the throttle valve. this is unofficial, but. after step one above, come down to my steps for the next steps two through six. after your bike, has been started and you've followed step one above, if the bikes running, you've accomplished the first part. use your speed screw to up the rpms to around 1200 on 4-strokes and 1400rpms on 2 strokes. you want it running a little high. now if the bikes, running, this is unofficial, the order in which this is done, up to here.

now to find the right idle jet, you rev the throttle back, to 1/4 throttle. if it cuts out or hesitates, then the idle jet is too lean. if its uneven then your idle jet is too rich. also the throttle slide here controls some of the jetting. after revving back to 1/4 and its good, to set the idle circuit and the mixture screw. get the idle running to 1400 rpm on 2 strokes, 1200 on 4 strokes, using the speed screw, and then adjust the mixture screw until the rpms are even. then lower the speed screw back ot where you want the rpms to be. that procedure adjusts the idle and progression circuit.

idle jet controls the progression circuit. along with the throttle valve. and the mixture screw adjusts the idle circuit.

this is for a mixture screw carb. if it has an airscrew, theirs a different set of variables that control the circuits.

now proceed back up and i havent checked that over but back to the other set of instructions for the rest, for step 7.