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Dellorto SHA carburetor explained
A Dellorto SHA carburetor is one of the simplest and most common carburetors used on classic two-stroke mopeds. You often see it on Puch, Tomos, Piaggio and other small moped engines because it is compact, easy to service and has fewer tuning parts than a PHBG, CP, PWK or Mikuni-style carburetor.
In this guide, we explain how the Dellorto SHA works, which parts matter most, how the main jet affects the setup, and what to check when your Puch starts badly, four-strokes, bogs, leaks fuel or only runs with the choke on.
A Dellorto SHA carburetor is a simple, compact setup for original and lightly tuned Puch mopeds.
What is a Dellorto SHA carburetor?
The Dellorto SHA is a small slide carburetor made for simple two-stroke applications. Dellorto lists the SHA4 range for two-stroke engines with 14, 15 and 16 mm body options and diffuser sizes from 9 mm through 16 mm. In moped language, this is why you see names such as SHA 14/12, SHA 15/15 or SHA 16/16.
The first number normally refers to the carburetor body size, while the second number refers to the diffuser or effective bore size. For example, a Dellorto SHA 15/15 is commonly understood as a 15 mm SHA carburetor with a 15 mm diffuser.
The SHA is popular because it is simple. It is not the carburetor you choose when you want the most detailed tuning range. It is the carburetor you choose when you want a clean, compact and reliable setup that is easy to jet and easy to maintain.
How is an SHA different from a PHBG?
A PHBG gives you more adjustment options. The PHBG tuning structure includes separate fuel-metering parts such as the idle jet, air screw, throttle slide cutaway, throttle needle, atomizer and main jet. That makes the PHBG better for more advanced tuning, but also more sensitive to setup mistakes.
The SHA is much simpler. On most SHA setups, the main jet is the most important replaceable tuning part. Idle behavior, low-throttle response and full-throttle mixture are all strongly affected by the main jet, air filter, fuel supply and whether the engine is sealed properly.
| Feature | Dellorto SHA | Dellorto PHBG |
|---|---|---|
| Best use | Original or lightly tuned mopeds | Tuned mopeds needing more adjustment |
| Main tuning parts | Main jet, air filter, fuel level, idle screw | Main jet, idle jet, needle, atomizer, slide cutaway, air screw |
| Idle adjustment | Mainly idle screw and clean fuel supply | Idle screw, air screw and idle jet |
| Mid-throttle tuning | Limited | Needle and atomizer adjustment |
| Setup difficulty | Simple | More detailed |
| Best mechanic approach | Keep it clean, jet safely and avoid false air | Tune each circuit step by step |
Main Dellorto SHA parts explained
The Dellorto SHA has fewer tuning parts than a PHBG, but the parts it does have need to be clean, sealed and correctly matched to the engine. A dirty main jet, sticking float, loose intake connection or blocked fuel supply can make the engine behave like the carburetor is badly jetted.
| Part | Function | What to check | Replacement Parts |
|---|---|---|---|
| Choke / air shutter | Helps cold starting by restricting air and enriching the mixture. | Make sure it opens fully after starting. A stuck choke can make the engine run too rich. | Shop choke parts |
| Throttle slide | Opens and closes as you twist the throttle. | Check that it moves smoothly and returns fully. Make sure the throttle cable has a little free play. | Shop throttle slides |
| Idle screw | Sets how far the slide stays open at idle. | Use it to set idle speed only after the engine is warm and the carburetor is clean. | Shop idle screws |
| Main jet | Controls the main fuel supply and is the key SHA tuning part. | Start safely rich and jet down step by step. | Shop main jets |
| Float | Controls fuel level in the float bowl. | A stuck float can cause overflowing or poor fuel supply. | Shop float parts |
| Float needle | Opens and closes fuel flow into the float bowl. | Dirt or wear can cause leaking, flooding or inconsistent running. | Shop float needles |
| Fuel hose connection | Brings fuel from the tank into the carburetor. | Check for fuel restriction, old hose, loose hose fitment and dirt from the tank. | Shop fuel hoses |
| Manifold connection | Connects the carburetor to the intake manifold. | Check the sleeve or reducer size and inspect for air leaks. | Shop intake manifolds |
| Air filter / air intake | Controls how much air enters the carburetor. | Any air filter change can require a main jet change. | Shop air filters |
How the Dellorto SHA meters fuel
The SHA does not tune like a PHBG. On a PHBG, you can separate the tuning into idle jet, needle and atomizer, slide cutaway, air screw and main jet. On an SHA, the system is more connected. Dellorto describes the SHA idle system as being derived from the main system, so the main jet and fuel supply have a larger influence across the running range than many beginners expect.
| Component | Main effect | Wrong setup symptoms |
|---|---|---|
| Main jet | Main fuel supply through the carburetor. | Too small can run lean, hot or weak at full throttle. Too large can four-stroke, smoke or feel lazy. |
| Air filter / airbox | Controls the amount and speed of air entering the carburetor. | A more open filter usually needs more fuel. A restricted filter can make the setup rich. |
| Throttle slide | Controls airflow as the throttle opens. | Dirt, cable tension or a sticking slide can cause hanging idle or poor return to idle. |
| Float and float needle | Keeps fuel level stable in the float bowl. | Overflowing, fuel starvation, hard starting or inconsistent running. |
| Choke / air shutter | Richens the mixture for cold starting. | If it stays partly closed, the engine can run rich and four-stroke. |
| Idle screw | Sets idle speed by holding the slide slightly open. | If adjusted before the engine is warm, idle may be too high or too low later. |
When should you check your Dellorto SHA carburetor?
Check your SHA carburetor when your moped is hard to start, will not idle, bogs under throttle, four-strokes, leaks fuel or only runs with the choke on. These problems are often caused by dirt, the wrong main jet, a blocked fuel path, a leaking float needle, a sticking choke or false air at the manifold.
| Problem | Possible cause | First thing to check | Replacement Parts |
|---|---|---|---|
| Engine only runs with choke | Main jet blocked, fuel restriction or false air. | Clean the main jet, check fuel flow and inspect the manifold seal. | Shop manifold gaskets |
| Engine leaks fuel | Float stuck or float needle not sealing. | Float, float needle, float bowl and fuel hose. | Shop float needles |
| Engine four-strokes at full throttle | Main jet too large, air filter too restricted or choke not opening fully. | Main jet size, air filter and choke movement. | Shop jet boxes |
| Engine runs hot or weak at full throttle | Main jet too small, air leak or fuel starvation. | Main jet, fuel flow, intake leaks and spark plug color. | Shop tuning parts |
| Idle hangs high | Slide not closing, cable too tight or false air. | Throttle cable free play, slide movement and intake seal. | Shop throttle cables |
| Engine dies at idle | Idle speed too low, dirty carburetor or poor fuel supply. | Idle screw, main jet, float bowl and fuel flow. | Shop carb parts |
Basic order for tuning a Dellorto SHA carburetor
Use this order when you install, clean or tune a Dellorto SHA on a Puch moped. Do not start by changing random parts. First make sure the carburetor is clean, the engine is sealed and fuel can reach the float bowl properly.
Interactive SHA tuning simulator
Move the throttle slider below to see how the Dellorto SHA responds across the throttle range. Use the choke switch to show what changes when the air shutter is active during cold starting.
Dellorto SHA throttle-position visualizer
See which SHA components matter most from idle to full throttle.
Shop Dellorto SHA setup parts at Puchshop
When you tune or rebuild a Dellorto SHA carburetor, it helps to have the correct main jets, gaskets, float parts, fuel hose, air filter parts and manifold parts available before you start. That way you can tune step by step instead of guessing or reusing worn parts.
Dellorto SHA 15/15 carburetor manual choke original
Original-style SHA 15/15 option for a simple, reliable Puch carburetor setup.
View SHA 15/15 original
Dellorto SHA 14/12 P carburetor manual choke original
Useful for original and mild setups where a smaller SHA carburetor size is preferred.
View SHA 14/12 original
Dellorto SHA 15/15 carburetor manual choke replica
Replica SHA 15/15 option for rebuilding or replacing a simple Puch carburetor setup.
View SHA 15/15 replicaNeed parts for your Dellorto SHA setup?
Find SHA carburetors, main jets, air filter parts, fuel hose and setup parts for your Puch moped at Puchshop.
Shop Dellorto SHA carburetor parts Tip: change one tuning part at a time, then test before making the next adjustment.FAQ: Dellorto SHA carburetor tuning
Is a Dellorto SHA good for a Puch Maxi?
Yes. A Dellorto SHA is a good carburetor for original and lightly tuned Puch Maxi setups. It is simple, compact and easy to service.
What is the main tuning part on a Dellorto SHA?
The main jet is the most important tuning part. Air filter choice, exhaust, fuel flow and false air also strongly affect how the carburetor runs.
Does a Dellorto SHA have a needle like a PHBG?
A typical SHA does not tune like a PHBG with an adjustable throttle needle and atomizer. That is one reason the SHA is simpler but less adjustable.
Why does my SHA only run with the choke on?
That usually means the engine is not getting enough fuel or is getting too much air. Check the main jet, fuel flow, carburetor cleanliness and false air at the intake.
Should I tune the SHA by spark plug color only?
No. Spark plug color helps, but also check engine temperature, throttle response, four-stroking, fuel flow, air leaks and how the engine behaves under load.
Do I need a bigger jet after changing the air filter?
Often, yes. A more open air filter usually lets in more air, so the engine may need a larger main jet to stay safe.
