Noticing raw fuel dripping or spraying from your muffler can be alarming. This problem often points to incomplete combustion inside the engine. Instead of burning the way it should in the combustion chamber, unburned fuel is pushed into the exhaust system—where it can leak out of the muffler.

Let’s break down why this happens and how to address it.

What Causes Incomplete Combustion?

Several issues can keep fuel from burning completely in your small engine:

Carburetor Problems

A carburetor that’s stuck open, out of adjustment, or flooding will deliver too much fuel.

Excess fuel drowns the spark, and unburned gas gets pushed into the muffler.

Weak or No Spark

If the spark plug is fouled, damaged, or poorly gapped, it won’t ignite the fuel-air mixture properly.

In this case, the mixture passes through the cylinder unburned.

Low Compression

Worn piston rings, a blown gasket, or valves that don’t seat properly can cause low compression.

Without strong compression, the fuel mixture won’t ignite completely.

Flooded Engine from Starting Attempts

Repeatedly pulling the starter rope without proper ignition can overload the cylinder with raw fuel.

That fuel has to go somewhere—often out through the muffler.

Why It’s a Problem?

 Fire Hazard – Fuel in the muffler can ignite unexpectedly.

Poor Performance – The engine will bog down, stall, or refuse to start.

Damage to Muffler – Excess fuel can cause overheating or clogging inside the exhaust.

How to Fix It?

Check the Carburetor

Clean it thoroughly and replace worn gaskets, diaphragms, or needles.

Make sure the float or metering lever isn’t sticking.

Inspect the Spark Plug

Replace it if fouled or cracked.

Verify proper gap according to your equipment manual.

Test Compression

Use a compression gauge.

If compression is low, the problem may be worn rings, a bad cylinder, or valve issues.

Start Correctly After Flooding

Open the throttle fully and crank the engine with the choke off to clear excess fuel.

Preventing Future Issues

Use fresh, clean fuel.

Keep your air filter clean so the mixture isn’t too rich.

Perform routine carburetor maintenance.

Replace spark plugs regularly.

If fuel is coming out of your muffler, it’s a sign that combustion isn’t happening properly. Start by checking your carburetor and spark plug—these are the most common culprits. With proper troubleshooting, you can fix the problem and keep your equipment running smoothly.