* Clogged fuel filter: A clogged fuel filter restricts fuel flow. The pressure builds up behind the obstruction, forcing fuel back towards the carburetor.
* Faulty fuel pump: A weak or failing fuel pump might not create enough pressure to overcome the carburetor's restrictions, leading to fuel backing up. Conversely, a *too strong* fuel pump (though less common) could overwhelm the carburetor's ability to regulate flow.
* Blocked or restricted carburetor jets or passages: Dirt, debris, or varnish buildup within the carburetor itself can block tiny jets and passages responsible for metering fuel. This restriction creates backpressure. This is a very common problem in older vehicles.
* Float level problem: The carburetor's float mechanism controls the fuel level in the bowl. If the float is stuck, bent, or the float needle valve is faulty, the fuel bowl can overfill, causing fuel to overflow or back up.
* Vacuum leak: A significant vacuum leak in the intake manifold can affect the carburetor's operation and potentially lead to erratic fuel flow, though this is less likely to directly cause *backup* as much as erratic operation.
* Incorrect fuel pressure regulator (if equipped): Some systems have a fuel pressure regulator. If it's malfunctioning, it might allow excessive pressure to build up, forcing fuel back towards the carburetor.
* Blocked vent in the gas cap: A blocked vent in the gas cap can create a vacuum in the fuel tank, hindering fuel flow. While it won't directly cause backup *in* the carburetor, it can contribute to overall fuel delivery problems that might manifest as symptoms similar to backup.
To diagnose the problem, you'll need to systematically check these possibilities. Start with the simpler checks (fuel filter, gas cap vent) before moving on to more complex carburetor disassembly or fuel pump inspection. A mechanic's manual specific to your 1985 Chevy Blazer will be invaluable for this process.
