The Honda GX390 is one of those engines you’ll find everywhere, from pressure washers and generators to water pumps and compactors. It’s Honda’s largest single-cylinder OHV engine, pushing out 13 horsepower from a compact, reliable package. And if you’ve ever spent a Saturday afternoon troubleshooting a rough idle or a fuel leak, you probably already know that understanding this engine’s guts is half the battle.
One area that trips up a lot of people, even those who are fairly handy, is the carburetor. It’s the beating heart of the fuel system, and it’s made up of dozens of small, very specific parts. A single clogged jet or a worn-out gasket can turn a perfectly good engine into a stubborn, sputtering mess. Knowing your way around these parts saves you time, money, and a whole lot of frustration.
That’s exactly what this guide covers. Below, you’ll find a full breakdown of the Honda GX390 carburetor parts diagram, with each component explained so you know what it does, why it matters, and what to look for if something goes wrong.
Honda GX390 Parts Diagram & Details
The diagram shown is an exploded view of the Honda GX390 carburetor assembly, labeled E-17 in Honda’s official parts catalog. It displays every individual component pulled apart and arranged in the order they fit together, like a 3D puzzle laid flat on paper. Each part is assigned a reference number, making it easy to identify and order exact replacements.
At the center, you’ll see the main carburetor body, which connects to the float bowl assembly below it and the insulator block to its left. Spreading out from the main body are smaller parts like gaskets, jets, screws, the choke mechanism, and the fuel petcock lever assembly on the right side. Hardware like springs, pins, and pan screws are scattered throughout, each one essential to keeping the system sealed and functional.
The following sections walk you through the most critical parts in this diagram, one by one. Each entry explains what the part is, where it sits, and what role it plays in keeping your GX390 running smoothly.
1. Carburetor Assembly (Main Body)
This is the central component in the diagram and the foundation that everything else bolts onto. The carburetor body houses the venturi, the throttle butterfly valve, and all the internal fuel passages that mix air and gasoline in the correct ratio before sending it to the combustion chamber.
Made from cast aluminum, the main body is surprisingly durable. But the tiny internal passages can clog over time, especially if fuel is left sitting in the engine for weeks or months. Stale gasoline leaves behind gummy varnish deposits that restrict fuel flow. If your GX390 starts fine but bogs down under load, a thorough cleaning of these passages with carburetor cleaner and compressed air often solves the problem.
2. Float Chamber Set (Float Bowl)
Sitting directly beneath the carburetor body, the float chamber is the cup-shaped reservoir that holds a small supply of fuel, ready for the engine to draw on. It attaches to the bottom of the carburetor body and is sealed with a gasket to prevent leaks.
The float bowl is typically held in place by a single bolt or a set of screws around its perimeter. Over time, the bowl can collect sediment, water, and degraded fuel residue. A drain screw at the bottom of the bowl lets you flush out old fuel without removing the entire assembly. If you notice fuel dripping from the base of the carburetor, a worn bowl gasket or a cracked float chamber is usually the culprit, and both are inexpensive to replace.
3. Float Set
The float set is a lightweight, hinged component that sits inside the float bowl. It’s essentially a buoyancy device, a small plastic or composite float that rises and falls with the fuel level inside the chamber.
As fuel enters the bowl, the float rises. Once it reaches a predetermined height, the float pushes a needle valve into its seat, shutting off the fuel supply. When the engine burns fuel and the level drops, the float descends, the needle valve opens, and fresh fuel flows in. This cycle keeps a consistent fuel level in the bowl at all times. A waterlogged or cracked float can cause fuel to overflow and flood the engine, so if your GX390 is leaking gas from the carburetor, the float is one of the first things to inspect.
4. Float Valve Set (Needle Valve)
Working in tandem with the float, the float valve set (commonly called the needle valve) is the gatekeeper that controls fuel entry into the float bowl. It consists of a small, pointed needle and a matching brass seat.
When the float rises to the correct fuel level, it pushes the needle into the seat, creating a seal that stops fuel from entering. The tip of the needle is usually made from a rubber or Viton compound that provides a tight seal against the brass seat. Even a tiny speck of debris caught between the needle and seat will prevent a proper seal, allowing fuel to continuously flood the bowl. This is one of the most common causes of fuel overflow on the GX390. Replacing the needle valve is a quick and affordable fix.
5. Main Jet
The main jet is a small, precision-drilled brass fitting that meters the fuel flowing into the carburetor’s main fuel circuit. It controls how much gasoline mixes with the incoming air when the engine is running at medium to full throttle.
You’ll find it threaded into the main nozzle or emulsion tube inside the carburetor body. Different GX390 variants may come with different jet sizes (such as #88, #90, or #92), depending on the application and altitude the engine was configured for. A clogged main jet is the most frequent cause of an engine that idles fine but loses power under load. Cleaning it with a thin wire or replacing it entirely is a straightforward repair that takes just a few minutes.
6. Pilot Screw Set
The pilot screw set manages the air-fuel mixture at idle and low-speed operation. It’s a tapered needle screw with a spring, washer, and O-ring, typically threaded into the carburetor body near the throttle bore.
Turning the pilot screw in (clockwise) makes the idle mixture leaner by reducing fuel flow. Turning it out (counterclockwise) enriches the mixture. The standard setting for most GX390 carburetors is about 1.5 to 2 turns out from fully seated, though this can vary by model. If your engine hunts at idle, surges, or refuses to idle smoothly, the pilot screw adjustment is often the first thing to check. The tiny O-ring on the screw can also deteriorate over time, leading to air leaks that throw off the mixture.
7. Main Nozzle (Emulsion Tube)
The main nozzle, sometimes called the emulsion tube, is a cylindrical brass tube that sits vertically inside the carburetor body. It works closely with the main jet to deliver fuel into the venturi.
Fuel passes through the main jet and into the emulsion tube, where it mixes with air drawn through small bleed holes along the tube’s length. This pre-mixing emulsifies the fuel, breaking it into finer droplets that vaporize more easily when they enter the airstream. A partially clogged emulsion tube changes the fuel-air ratio unpredictably, causing hesitation or rough running. During a carburetor rebuild, it’s worth removing and cleaning this tube separately to make sure every bleed hole is clear.
8. Choke Set
The choke set includes the choke shaft, butterfly plate, spring, and associated hardware mounted in the carburetor’s air inlet. Closing the choke restricts incoming air, which creates a richer fuel mixture, exactly what a cold engine needs to start.
Once the engine warms up, you open the choke to allow full airflow. On most GX390 models, the choke is operated manually by a lever mounted on the carburetor body. A stuck or binding choke shaft is a common problem, often caused by dirt buildup around the shaft bore. If your engine starts but immediately dies when you open the choke, the issue could also point to a separate fuel delivery problem, like a clogged main jet or low fuel level in the float bowl.
9. Carburetor Insulator
Located between the carburetor body and the engine’s intake port, the carburetor insulator is a thick phenolic or composite block. In the diagram, it’s the piece to the left of the main carburetor body with a large central bore.
Its primary job is thermal isolation. The insulator prevents heat from the engine block from transferring into the carburetor body, which would cause fuel to evaporate prematurely inside the float bowl and fuel passages, a condition known as vapor lock. Beyond heat management, the insulator also provides a mounting surface and helps dampen vibration. Cracks in the insulator can create vacuum leaks that cause erratic idling and lean running conditions, so inspect it carefully during any carburetor service.
10. Gaskets (Insulator, Carburetor, and Float Bowl)
Multiple gaskets appear throughout the diagram, each one sealing the junction between two mating surfaces. The three most critical ones are the insulator gasket (between the insulator and intake port), the carburetor gasket (between the insulator and carburetor body), and the float bowl gasket (between the bowl and the carburetor body).
These gaskets are thin, flat, and made from fiber or rubber compounds designed to compress slightly and create an airtight seal. Even a small air leak past a worn or misaligned gasket will let unmetered air into the fuel mixture, making the engine run lean. Lean running at higher temperatures increases the risk of engine damage over time. The good news is that a complete carburetor gasket set for the GX390 is inexpensive, and swapping them out during any carburetor service is cheap insurance against air leaks and fuel seepage.
11. Fuel Strainer Cup
On the right side of the diagram, you’ll see the fuel strainer cup, a small sediment bowl that sits at the base of the fuel petcock (fuel valve) assembly. It catches dirt, debris, and water before they can enter the carburetor.
The cup is clear or semi-transparent on many models, making it easy to spot contamination at a glance. A small gasket seals the cup to the petcock body. Over time, sediment builds up inside the cup, especially if the fuel tank isn’t clean or if ethanol-blended fuel has been sitting for extended periods. Cleaning the strainer cup during regular maintenance helps prevent fuel delivery problems caused by restricted flow or debris reaching the carburetor jets.
12. Petcock Lever and Setting Plate
The fuel petcock lever is the manual on/off valve that controls fuel flow from the tank to the carburetor. It’s paired with a setting plate that provides defined positions, typically ON, OFF, and sometimes RESERVE.
Mounted on a bracket to the right side of the carburetor in the diagram, the petcock lever works with a spring and a sealing gasket to open or close the fuel passage. Turning the lever rotates a disc or ball inside the valve body, aligning or blocking the fuel port. If you notice fuel continuing to flow even after turning the petcock to OFF, the internal gasket or sealing disc has likely worn out. Replacing just the petcock gasket and spring is usually enough to restore a tight shutoff.
13. Drain Screw Set
At the very bottom of the float bowl, you’ll find the drain screw. It’s a small bolt with a sealing washer that lets you drain fuel from the float bowl without disassembling the carburetor.
This is one of the most underappreciated maintenance points on the entire engine. Before storing your GX390 for more than a few weeks, opening the drain screw and letting the bowl empty completely prevents stale fuel from gumming up the jets and passages. It takes about ten seconds and can save you a full carburetor teardown later. The sealing washer on the drain screw can flatten and lose its seal over time. If you see fuel weeping from the bottom of the carburetor, try tightening the drain screw slightly or replacing the washer.
