Priming a generator means getting fuel from the tank to the engine before you try to start it. The exact process depends on your fuel type: gasoline generators with carburetors use a primer bulb, diesel generators require bleeding air from fuel lines, electronic fuel injection systems handle most of the work automatically, and propane setups have their own quirks. If your generator has been sitting in storage or just ran out of fuel, priming is the step that gets it running again.
Gasoline Generators With a Primer Bulb
Most portable gasoline generators use a small rubber primer bulb mounted near the carburetor. Pressing it draws fuel from the tank into the carburetor’s float bowl so the engine has something to ignite on the first pull. The standard process is simple: press the bulb firmly and release it, repeating three to five times until you feel resistance in the bulb. That resistance tells you fuel has reached the carburetor.
Before you prime, make sure the fuel valve (if your model has one) is in the open position. Set the choke to the closed or “full choke” position, which enriches the air-fuel mixture for a cold start. Then press the primer bulb the recommended number of times listed in your owner’s manual, typically three presses. Pull the starter cord or turn the electric start key, and the engine should fire within a few pulls. Once it catches, gradually move the choke to the open position as the engine warms up.
One common mistake is pressing the primer bulb too many times. Over-priming floods the engine with excess fuel, which wets the spark plug and prevents ignition. If your generator cranks but won’t start and you smell gasoline, you’ve likely flooded it. To fix this, remove the spark plug and check whether the electrode is wet with fuel. Dry it off or replace it if it’s fouled, then crank the engine a few times with the plug removed to clear excess fuel from the combustion chamber. Reinstall the plug and try again with fewer primer presses.
Diesel Generators: Bleeding the Fuel System
Diesel generators don’t use primer bulbs. Instead, they rely on a pressurized fuel system that can’t tolerate air bubbles. Any time a diesel generator runs out of fuel, gets a new fuel filter, or sits long enough for fuel to drain back, air enters the lines and the engine won’t start. Priming a diesel generator means bleeding that air out, working from the fuel tank toward the engine.
Start by opening the fuel supply valve so fuel can flow from the tank. Locate the bleed plug (also called a bleed screw) on the fuel filter closest to the tank. Open it, then use the hand priming pump, a small lever or plunger usually mounted on or near the fuel filter housing. Pump it repeatedly. You’ll see fuel mixed with air bubbles escaping from the open bleed plug. Keep pumping until the fuel flows steadily with no bubbles at all.
Close that first bleed plug and move to the next one in the fuel path, typically on the injection pump. Repeat the same process: open the plug, pump until clear fuel flows, then close it. Always work through the bleed points one at a time, moving from the component closest to the tank toward the injectors. Some systems have two or three bleed points, others just one. Your owner’s manual will show their locations. Once all air is purged, the engine should start normally, though it may take a few extra cranks on the first attempt.
After Long-Term Storage
If your diesel generator has been sitting for more than six months, bleeding air alone may not be enough. Diesel fuel degrades over time and can accumulate water from condensation inside the tank. The best approach is to drain the old fuel entirely, replace the fuel filter, and fill the tank with fresh diesel before you begin the bleeding process. Stale fuel or water in the lines can damage injectors and make the engine hard to start even after a successful prime.
Generators With Electronic Fuel Injection
If your generator uses electronic fuel injection (EFI) instead of a carburetor, priming is largely automatic, but there’s a specific technique to help it along. EFI systems don’t have a float bowl to store fuel. Instead, an electric fuel pump pressurizes the fuel rail when you turn the key.
The system uses what’s called a two-second prime. When you turn the ignition to the “run” position without cranking, the fuel pump activates for about two seconds to charge the fuel rail. You’ll hear a brief buzzing sound followed by a click when the pump relay shuts off. The correct startup method is to turn the key to “run,” wait for that buzz-and-click cycle to finish, and only then turn to “crank.” This gives the fuel pump time to pressurize the system before the engine tries to fire.
If the generator has been sitting long enough that the fuel system is completely empty, one two-second prime cycle may not be enough. Turn the key to “run,” wait for the pump to shut off, then turn it back to “off” and repeat the cycle two or three more times before cranking. Each cycle pushes fuel a little further through the system.
Propane and Dual-Fuel Generators
Propane generators and dual-fuel models running on propane don’t need traditional fuel priming since propane is already a pressurized gas. But getting propane flowing correctly on the first start can still be tricky, especially with a new connection or a generator that’s been sitting.
The connection sequence matters. Plug the propane line into the generator first, then connect it to the tank. Open the propane tank valve slowly. On a dual-fuel model, make sure the fuel selector is set to propane, not gasoline. Set the choke to the closed position, pull the starter cord once, then switch the fuel selector or open the propane flow and pull again. Some owners find that alternating between a choke pull and a propane pull in sequence gets the engine running within three or four attempts.
If the engine won’t start on propane after several tries, check that the regulator is functioning. Propane regulators can stick after long periods of inactivity. Some models have a small vent on the regulator diaphragm that may need to be opened briefly to release trapped pressure.
Check the Fuel Cap Vent
One overlooked issue that mimics a priming problem is a clogged fuel cap vent. Gasoline and diesel tanks need to breathe. As fuel flows out of the tank to the engine, air needs to flow in to replace it. If the vent hole in the fuel cap is blocked by dirt or debris, a vacuum builds inside the tank and fuel stops flowing, no matter how much you prime.
If your generator starts fine but stalls after a few minutes, or if priming doesn’t seem to push fuel through, try loosening the fuel cap slightly. If you hear air rush in or the generator starts running better, the vent is the problem. Clean the cap or replace it. On some models, the vent is a separate tube rather than a hole in the cap, so check your manual for the location.
Keeping the Fuel System Prime-Ready
Regular maintenance reduces how often you’ll need to prime your generator. Replace fuel filters at the intervals your manufacturer recommends. For diesel generators, a common schedule is after the first 50 hours of operation on a new unit, then every 400 hours after that. If you’re using lower-quality fuel, shorten that interval. A partially clogged filter restricts fuel flow and can introduce air into the system, creating the exact problem priming is meant to solve.
If you’re storing a generator for more than a month, either run the carburetor dry by closing the fuel valve and letting the engine stall, or add a fuel stabilizer to the tank. For diesel generators, keep the tank as full as practical to minimize condensation. These steps keep the fuel system sealed and full, so the next time you need the generator, it starts without a lengthy priming session.