Small engine hard to start: Common Causes & How to Fix
1Small Engine Carburetor
Most Common Solution
Repair Instructions:
The engine might be getting too much or too little fuel. This is usually because the carburetor is not working right. If the carburetor gets clogged with dirt and debris, it can't deliver enough fuel to the engine. Without enough fuel, the engine won't run smoothly or may stall. On the other hand, if the carburetor choke stick open, too much fuel can flood the engine. The choke controls fuel flow like a valve. It should close as the engine warms up. If the choke is stuck open, it keeps adding extra fuel. This makes the fuel mixture too rich. Getting the carburetor cleaned and adjusted or replacing it if needed will restore the proper fuel mix.
How to Replace a Small Engine Carburetor
- Disconnect the spark plug wire. Remove the wire from the spark plug so the engine doesn't accidentally start when working on it.
- Drain the gas tank. Use a siphon or pump to drain the gas from the fuel tank into an approved container. This reduces fire risk.
- Remove air filter cover. Take off the air filter housing and set aside to access the carburetor.
- Detach fuel line and throttle linkage. Use pliers to disconnect the fuel line from the carburetor inlet. Unhook the governor spring and throttle cable/rod from the carburetor.
- Remove carburetor. Take off clamp screw or nuts holding carburetor to manifold. Carefully pull carburetor away from engine.
- Clean mounting surfaces. Use carburetor cleaner and rag to wipe mating surfaces on the engine block and new carburetor.
- Install new carburetor. Place new carburetor against block, aligning holes. Reinstall clamp screw or nuts to secure in place.
- Reattach fuel line and throttle linkage. Connect fuel line back to carburetor inlet fitting. Reattach governor spring and throttle cable/rod.
- Reinstall air filter housing. Put air filter cover and housing back on, securing with clamps or screws.
- Reconnect spark plug wire. Push wire back onto spark plug and tighten nut.
- Test engine. Add gas to fuel tank. Start engine and let run, adjusting carburetor components as needed.
2Small Engine Spark Plug
Most Common Solution
Repair Instructions:
Check the spark plug for wear or damage. If the porcelain is cracked, the metal electrodes are burnt or worn down, or there's heavy carbon buildup, replace the plug. Use a spark tester tool to check if the plug is bad. Crank the engine while watching for a strong blue spark across the tester's gap. No spark means the plug needs replacing. A good spark plug should fire a sparking blue flash while cranking. Weak or no spark can cause starting issues or rough running. Worn or fouled spark plugs won't ignite the fuel mixture properly. Replacing damaged, carbon fouled, or worn out spark plugs can restore easy starting and smooth performance.
How to Replace a Small Engine Spark Plug
- Make sure the engine is cool before starting. Let the engine sit for at least 30 minutes after use before attempting to replace the spark plug. A hot engine can burn you.
- Disconnect the spark plug wire. Use a spark plug wrench to loosen the nut connecting the wire to the plug. Pull the wire off the plug.
- Clean around the base of the plug. Use a wire brush or scraping tool to remove dirt, oil or debris around the spark plug base. This allows you to access the plug easier.
- Remove the spark plug. Place the spark plug wrench or socket over the spark plug base. Turn it counterclockwise to unscrew it. Pull the spark plug out of the engine block.
- Inspect the spark plug. Check for excessive wear or buildup on the electrodes. Replace it if needed or clean it with a wire brush.
- Gap the new spark plug. Use a spark plug gap tool to set the space between the electrodes to 0.6-0.8 mm for most small engines. Refer to your engine manual for the proper gap size.
- Thread in the new plug by hand. Carefully thread the new or cleaned spark plug into the engine block by hand. This prevents cross-threading.
- Tighten with the wrench. Once seated by hand, use the wrench to tighten it another 1/8 to 1/4 turn. Don't over tighten.
- Replace the spark plug wire. Push the wire back onto the spark plug and tighten the nut to secure it.
- Start the engine and test. Start the small engine and let it run to ensure the new spark plug is working properly.
3Small Engine Fuel Cap
Common Solution
Repair Instructions:
As gas gets used up, pressure builds inside the tank. The gas cap has a small vent that lets air in to relieve this pressure. If the vent gets clogged with dirt or debris, air can't enter. Pressure keeps rising in the closed tank. When it gets higher than engine pressure, this makes starting hard. To check if the clogged vent is the issue, loosen the gas cap a bit before starting. If that allows the engine to run smoothly, the vent needs unclogging. Replace the whole gas cap if the vent stays blocked. A working vent brings the tank pressure down by letting air in. This equalizes it with engine pressure for easy starting. A plugged vent seals the tank, trapping pressure that resists starting.
How to Replace a Small Engine Fuel Cap
- Check for fuel leaks. Inspect area around fuel tank for any leaks or damage. Clean up any spilled gas.
- Remove old fuel cap. Turn cap counterclockwise to unscrew it from the fuel tank opening. Pull cap and gasket off.
- Clean fuel tank opening. Use a clean rag to wipe dirt or debris from opening and threads where cap screws on.
- Inspect new fuel cap. Confirm rubber gasket is in place inside the new fuel cap. Check that vent holes are clear.
- Install new fuel cap. Place new cap over fuel tank opening, lining up threads. Hand tighten cap clockwise until snug.
- Test for leaks. Add fuel to tank. Start engine and let run for several minutes, then turn off. Inspect for any leaks at the fuel cap. Tighten more if needed.
- Check venting. Make sure cap vents properly by removing it while engine is off. You should hear a hiss of pressure releasing.
- Confirm operation. Start mower and confirm fuel flows properly to the engine without starving out. Let engine run for several minutes.
- If leaks or other issues occur, recheck cap, gasket and tank opening for problems. Make sure cap is on correctly.