Hi Bojan,
When the blower is right-side up, fuel is pulled from the tank through the fuel line to the carburetor. When it's upside down, gravity assists this flow, or it changes how air might be getting into the system.
Possible Causes
Fuel Lines (Especially the one in the tank):
Cracks or Pinholes: Even tiny cracks or pinholes in the fuel lines (both the one from the tank and any internal lines within the tank) can allow air to be sucked in, causing a lean condition and choking the engine, particularly under load (full throttle). When inverted, fuel might temporarily block these leaks.
Loose Connections: Check all fuel line connections to ensure they are tight and not allowing air in.
Hardened or Kinked Lines: Over time, fuel lines can become stiff and develop kinks, restricting flow. This might be more pronounced in certain orientations.
The "Weight" on the Fuel Pickup: Inside the tank, there's usually a weighted fuel filter/pickup. This ensures the filter stays submerged in fuel even when the tank is low or tilted. If the line connecting this weight to the main fuel line is damaged, kinked, or the weight isn't allowing the pickup to reach fuel properly, it could cause issues. When upside down, the fuel level shifts, potentially providing better access to fuel for the pickup.
Fuel Tank Vent: If you have replaced the cap then it shouldn't be the problem but can be checked easily.
The fuel tank needs to vent to allow air in as fuel is consumed. If the vent is clogged, a vacuum can build up in the tank, preventing fuel from flowing freely to the carburetor. When you turn it upside down, it might temporarily break that vacuum or allow air to enter from a different point. Try running it with the fuel cap slightly loose (but be careful of spills) to see if that makes a difference. If it does, the vent is the problem.
Integrated Vent in the Cap: The fuel cap isn't just a sealed lid. It has a tiny, often very intricate, one-way valve or labyrinthine passage built into it. This allows air to enter the tank as fuel is consumed (preventing a vacuum) but prevents fuel from leaking out, even when the machine is tilted or inverted.
Carburetor Diaphragm/Pump Issues (Even on a new carb):
While you replaced the carburetor, sometimes a new one can be faulty, or a small piece of debris can get into it during installation. The "upside down" behavior strongly suggests an issue with how the fuel pump diaphragm or needle valve within the carburetor is operating under normal orientation versus inverted.
Incorrect Gasket Installation:, for the GX25 engine (which the HHB25 uses), incorrect installation of the fuel pump gasket and diaphragm can cause running issues. Ensure these are correctly oriented (gasket first, then diaphragm) if you disassembled the new carburetor at all.
Low Oil Sensor (Less likely for your specific symptom, but worth checking): If it has one
Honda engines often have a low-oil shutdown sensor. If the oil level is borderline low, or if the sensor is faulty, the change in orientation could trigger or prevent it from cutting out. However, this usually results in a complete shutdown, not just choking at full throttle. Still, check your oil level.
Muffler
A clogged muffler can cause an engine to bog down under load. It's less likely to be orientation-dependent, but if the exhaust is highly restricted, turning it upside down could change the pressure dynamics slightly. Easy to check and clean.
Cheers
Max.
