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Dellorto PHBG carburettor parts explained
A Dellorto PHBG carburettor controls the fuel and air mixture your Tomos, Puch or other two-stroke moped needs to run properly. The most important parts are the choke, manifold connection, float chamber, fuel hose connection, idle screw, air screw, float, float needle, main jet and idle jet.
In this guide, we explain the main parts of a Dellorto PHBG carburettor, what they do and what to look for when comparing original Dellorto PHBG carburettors with replica PHBG carburettors. The carburettor in the example photo is a 17.5 mm Dellorto PHBG replica.
What is a Dellorto PHBG carburettor?
A Dellorto PHBG carburettor is a popular carburettor for mopeds and small two-stroke engines. The carburettor mixes petrol with air before that mixture travels through the manifold into the cylinder. The better the carburettor is tuned, the better the engine starts, idles, accelerates and responds to throttle.
PHBG carburettors are often used on tuned Tomos, Puch and other mopeds, especially when a larger cylinder, different exhaust or sport air filter has been fitted. A correct setup is important to prevent poor running, hesitation, flooding or a lean mixture.
Back to topParts on the outside of the PHBG carburettor
On the outside of the Dellorto PHBG carburettor, you will find the parts used for operation, mounting and basic adjustment. The image below shows the most important points.
| Number | Part | Function |
|---|---|---|
| 1 | Choke | The choke helps start a cold engine by temporarily creating a richer mixture. In this example, it is a manual choke. A cable-operated choke is also available for many PHBG carburettors, so you can operate the choke with a cable, for example from the handlebar. |
| 2 | Manifold connection | This is where the carburettor mounts to the intake manifold. Some PHBG carburettors slide directly over the manifold. Other versions use a rubber connection or clamp-style mounting. Always check which connection type matches your manifold. |
| 3 | Float chamber | The float chamber fills with petrol. Inside this chamber are parts such as the main jet, idle jet, choke jet, float and float needle. These parts control how much fuel the engine receives at idle, part throttle and full throttle. |
| 4 | Fuel hose connection | This is where the fuel hose connects. Petrol flows from the tank through this connection into the float chamber of the carburettor. |
| 5 | Idle screw | The idle screw adjusts the idle speed. Turning the screw further in raises the slide slightly and increases rpm. Turning the screw out lowers the idle speed. |
| 6 | Air screw | The air screw is used for fine adjustment of the mixture at idle and low rpm. Small changes can make a clear difference in how the engine picks up from idle. |
Parts inside the float chamber
The float chamber contains the parts that control the fuel supply. If your moped idles badly, bogs, floods or does not respond properly to throttle, the float chamber is often a good place to inspect first.
| Number | Part | Function |
|---|---|---|
| 1 | Float | The float floats on the petrol inside the float chamber. When the fuel level is high enough, the float pushes the float needle closed. The fuel supply should shut off when the float is roughly horizontal. |
| 2 | Float needle | The float needle shuts off the fuel supply. At the tip, there is usually a small rubber end that seals against the seat where the petrol flows through. If this tip is worn or dirty, the carburettor can overflow. |
| 3 | Main jet | The main jet controls how much petrol the engine receives at higher throttle openings and full throttle. The main jet is mounted in the atomiser tube. The throttle needle moves up and down inside this tube when you accelerate. |
| 4 | Choke jet | The choke jet supplies extra fuel when starting with the choke. On many PHBG versions, this jet is not intended to be replaced because alternative sizes are often not available. |
| 5 | Idle jet | The idle jet controls the fuel supply at idle and low rpm. This jet is usually small because the engine only needs a limited amount of fuel at low throttle openings. |
How do the jets in a PHBG carburettor work?
The jets in a PHBG carburettor control how much petrol flows through the carburettor. The correct jet size depends on your setup, including cylinder size, air filter, exhaust, ignition and carburettor size.
| Part | When does it mainly work? | What can you notice with the wrong setup? |
|---|---|---|
| Idle jet | Idle and low throttle | The engine idles badly, stalls, reacts slowly to throttle or picks up poorly from a stop. |
| Throttle needle | Part throttle and acceleration | The engine hesitates, bogs or runs too rich or too lean while accelerating. |
| Main jet | High throttle and full throttle | The engine runs too lean, gets too hot, four-strokes at full throttle or does not pull cleanly. |
| Air screw | Fine adjustment at idle and low rpm | The engine reacts poorly to small throttle movements or does not idle smoothly. |
Original or replica Dellorto PHBG?
Original Dellorto PHBG carburettors and replica PHBG carburettors are both available. A replica carburettor can work well, but there are clear differences in finish, markings and parts.
| Feature | Original Dellorto PHBG | Replica PHBG carburettor |
|---|---|---|
| Overflow connections | Original PHBG carburettors often have extra overflow connections on both sides. Small hoses can be fitted here so excess fuel can drain away neatly. | Replica carburettors do not always have these connections, or they may be made differently. |
| Logo and codes | Original carburettors are marked with Dellorto PHBG markings. Loose parts may also have an INC code or other recognisable Dellorto coding. | Replica carburettors often only show the carburettor size, for example 17.5 mm or 19 mm. |
| Finish | The overall finish is usually cleaner and more consistent. This includes the casting, threads, fitment and internal parts. | The finish can vary between replica carburettors. Always check for dirt, burrs, thread quality and correctly fitting jets. |
| Replacing parts | With original parts, it is smart to compare codes, sizes and markings with the old part before replacing it. | Check carefully whether replacement jets, needles, gaskets and float parts really match your specific version. |
When should you check your PHBG carburettor?
Check your Dellorto PHBG carburettor when your moped is hard to start, idles badly, bogs, floods or does not pull cleanly. The problem is often dirt, the wrong jet size, a leaking float needle or an incorrectly adjusted air screw.
| Problem | Possible cause |
|---|---|
| Moped overflows or leaks petrol | Float needle does not seal properly, float is stuck or dirt is trapped in the seat. |
| Moped stalls at idle | Idle jet is blocked, idle screw is incorrectly adjusted or the engine is drawing false air. |
| Moped bogs at full throttle | Main jet is too small, fuel supply is restricted or the air filter/exhaust setup is not matched properly. |
| Moped four-strokes or smokes heavily when accelerating | Mixture is too rich, main jet is too large or the choke is sticking. |
| Moped picks up poorly from low rpm | Air screw, idle jet or throttle needle is not adjusted correctly. |
Basic order for tuning a PHBG carburettor
Tuning a PHBG carburettor works best when you follow a clear order. Do not change everything at once. Check the basics first and adjust step by step.
Work calmly and logically. Carburettor tuning is a combination of jet size, air supply, fuel supply and engine setup. Change one thing at a time, so you know which adjustment actually made the difference.
Back to topFAQ: Dellorto PHBG carburettors
What does the main jet do in a Dellorto PHBG carburettor?
The main jet controls how much petrol the engine receives at high throttle openings and full throttle. A main jet that is too small can make the engine run lean. A main jet that is too large can cause four-stroking, poor pulling power or a rich mixture.
What does the idle jet do?
The idle jet controls the fuel supply at idle and low rpm. If this jet is dirty or does not match your setup, the moped can idle badly or stall.
What is the air screw for on a PHBG carburettor?
The air screw is used for fine adjustment of the mixture at idle and low rpm. Small changes to the air screw can strongly affect how the engine picks up from idle.
What is the difference between an original and replica Dellorto PHBG?
An original Dellorto PHBG usually has better finish, original markings, Dellorto logos and sometimes extra overflow connections. A replica PHBG can work well, but always check the finish, jets and fitment carefully.
How do I know if my float closes properly?
The float should shut off the fuel supply when it is roughly horizontal. If the carburettor overflows or fuel keeps leaking, the float needle may be dirty, worn or incorrectly installed.
Can I use a cable-operated choke on a Dellorto PHBG?
Yes, many PHBG carburettors can use a cable-operated choke. This allows you to operate the choke with a cable, for example from the handlebar. Always check whether the choke fits your specific PHBG version.
