Briggs And Stratton Backfires Through Carburetor? (Fix It Now!)

Dennis Howard
by Dennis Howard

It is an alarming sound and situation. You have been running your lawnmower over your yard. Suddenly, as you are shutting the Briggs and Stratton engine down, it backfires through the carburetor.

Backfire, or afterfire, occurs when you shut the engine down at high speed. Fuel continues to pump to the engine after shutting down. Hot spots in the engine’s intake system can ignite the air/fuel mixture resulting in a pop or bang at the carburetor.

Do You Need a Mower Repair Service?

Get free, zero-commitment quotes from pro contractors near you.

How to Avoid Briggs And Stratton Carburetor Backfires

In most cases, a backfire or afterfire in your Briggs and Stratton engine will not cause any damage. The occasional backfire is normal in small engines. However, if the backfire happens regularly, there is probably something else wrong. Here are some tips to help you diagnosis the reason for backfiring in your Briggs and Stratton engine.

1. Avoid Hot Shutdowns

Doing a quick shutdown when your lawnmower has been running at high can cause backfires. An engine that has been running under a heavy load can also backfire if shut down too quickly.

After running at high RPM, the carburetor continues to feed air and gasoline to the engine after shutdown. Hot spots or carbon deposits inside the engine can ignite the air/fuel mixture. The result is the distinctive pop or bang at the carburetor.

The easiest fix for this problem is to reduce the throttle on your Briggs and Stratton engine. Reducing the engine to idle speed for a few seconds before the shutdown is always a good practice. At idle speed, the engine can cool down and the carburetor is not delivering fuel at such high rates.

The reduced RPM’s, air/fuel flow, and a chance to cool should eliminate any backfires. Allowing your Briggs and Stratton engine to cool down slowly by reducing the throttle can add years to the life of your equipment.

Consult the owner’s manual that came with your Briggs and Stratton engine. The manual may have specific instructions on properly performing a shutdown to minimize backfires and wear on your small engine.

2. Use High-Quality Fuel

Almost all the gasoline sold at service stations these days contains alcohol. In general, the gasoline you buy at the pump is about 10 percent alcohol. However, in some areas of the country, this percentage can be as much as 15 percent.

Alcohol has a much lower ignition temperature than gasoline. Alcohol rich gas mixtures will often backfire easier and quicker due to the small engine’s high temperatures.

Switching to a higher grade of gasoline for your Briggs and Stratton engine may help the backfire problem. You can purchase gasoline with no alcohol content from many lawn and garden centers or your local Briggs and Stratton dealer.

You should always follow the recommendations in the owner’s manual for your Briggs and Stratton engine. Some models require different grades of gasoline or manage peak performance.

3. Keep the Carburetor Properly Tuned and Clean

The carburetor is the heart of your Briggs and Stratton engine. The air and fuel mixture that allows the engine to run is mixed and delivered through the carburetor. Like any mechanical device, the carburetor is subject to wear, tear, and maladjustment.

Proper maintenance of your carburetor, including the air filter, is essential for proper operation. A misadjusted carburetor can make the air/fuel mixture too lean or too rich. Both conditions can cause backfire problems.

4. Avoid A Wet Carburetor During Storage

Carburetors left sitting for a while can become clogged by the residue of gas that evaporates inside the carb. This build-up inside the carburetor can cause the same sorts of problems with a too lean or too rich mixture. The problem can become so severe that the carburetor will not deliver any fuel to the engine.

Never store your lawnmower with gasoline in the carburetor. Most new lawnmowers have a fuel shutoff valve between the carburetor and the fuel tank. If you intend to store your lawnmower for more than 30 days, you should run the carburetor out of gas.

With your Briggs and Stratton engine running, turn the fuel valve to the off position and allow the engine to run until it stops on its own. Shutting off the fuel with the engine running leaves the carburetor free of fuel and eliminates build-up in the carburetor.

Always consult the owner’s manual for expert advice on preparing your Briggs and Stratton engine for storage. The manual will also give you guidance on the proper methods of shutting down your engine.

5. Keep the Air Filter Clean

A dirty air filter can cause your Briggs and Stratton lawnmower to run with a too lean air/fuel mixture. Running the engine too lean can cause overheating, which can lead to backfires or more serious damage.

Many lawn professionals suggest cleaning or changing the air filter at each fuel fill-up. A clean air filter will ensure that your Briggs and Stratton engine gets the proper fuel/air.

Your owner’s manual has a maintenance schedule for your Briggs and Stratton engine. Following the recommended maintenance schedule will keep your engine running longer and at peak efficiency.

6. Look for Leaks

Larger Briggs and Stratton engines, mostly those over 20hp, have mechanical or electrical fuel pumps. These pumps deliver gasoline to the carburetor under low pressure to supply the engine. Several things can happen that will reduce the flow of gasoline to the carburetor and cause backfire issues.

Leaky Hoses and Filters: Check the fuel line from the fuel tank to the fuel pump and from the fuel pump to the carburetor. Even a pinhole-sized leak can reduce the fuel flow enough to cause a lean fuel/air ratio.

A cracked inline fuel filter can also be a problem. If your Briggs and Stratton engine has an inline fuel filter, you should routinely inspect and change the filter to ensure the best performance. Consult the owner’s manual for the proper type and schedule for fuel filter replacement.

7. Is the Fuel Pump Pumping?

Fuel pumps, both electric and mechanical, are prone to failure. In some instances, the pump will continue to operate but will not deliver enough fuel to the engine. The engine will start and idle but will not deliver full power. The lean fuel/air mixture can cause backfiring and other problems.

If the fuel pump on your Briggs and Stratton engine is faulty, replacements are available through your Briggs and Stratton dealer. Replacing a fuel pump is not difficult and may solve your backfire or lack of power problem.

8. Keep Your Mower Clean

Lawnmowers and other equipment powered by Briggs and Stratton engines work in dirty environments. The combination of oil and dirt can often build up around the engine. This build-up causes the engine to run hotter and work harder.

A periodic cleaning of your Briggs and Stratton engine’s cooling fins is a part of good maintenance. Backfiring is often a symptom of an engine that is running too hot. Insufficient airflow over the cooling fins on the engine head leads to overheating.

Using a stiff bristle brush to remove the built-up dust, debris, and oil from the cooling fins is the easiest method. A parts cleaning brush is a perfect tool for this job. If you have a compressor, give the entire engine a good blow after brushing.

Never use a water hose to clean your Briggs and Stratton engine. Newer small engines have electronic components that can be damaged if sprayed. If you spray cold water on a hot engine, you risk cracking or warping the engine’s metal parts.

Always follow the recommendations and instructions in the owner’s manual for your Briggs and Stratton engine when performing any maintenance or cleaning.

The Anti-Afterfire Solenoid

Your Briggs and Stratton engine may have an anti-afterfire solenoid. Diagnosing and repairing problems on an engine equipped with such a device is usually beyond most homeowners. You should consult a trained Briggs and Stratton engine technician for these kinds of problems.

Backfires or Afterfires in the Muffler

Your Briggs and Stratton engine may suffer from backfires or afterfires at the other end. Backfires or afterfires in the muffler are not uncommon. These noisy occurrences are usually the result of an air/fuel mixture that is too rich. The unburned air/fuel mixture ignites in the hot muffle with a loud bang.

Adjusting the carburetor to deliver the proper air/fuel ratio will usually solve this problem. Occasionally, shutting down your Briggs and Stratton engine too quickly can cause fuel to run on into the muffle with the same effect.

Bigger Problems

If a carburetor adjustment or proper shut down procedures don’t solve the muffler backfire problem, you should suspect a bigger problem internally. A bad exhaust valve in the engine can allow unburned air/fuel mixtures to leak out of the cylinder.

A bad valve requires a trip to the local Briggs and Stratton repair shop for diagnosis. Your Briggs and Stratton trained repair technician has the tools and equipment to get the job done correctly.

Do You Need a Mower Repair Service?

Get free, zero-commitment quotes from pro contractors near you.

Keep Your Briggs and Stratton Engine Running Better and Longer

Proper maintenance and care will give your Briggs and Stratton engine a longer life and provide better performance. Backfiring through your Briggs and Stratton engine’s carburetor should be a warning that something is not right and needs investigation.

We hope that these tips help you understand and eliminate problems with your Briggs and Stratton engine.

Dennis Howard
Dennis Howard

Dennis is a retired firefighter with an extensive background in construction, home improvement, and remodeling. He worked in the trades part-time while serving as an active firefighter. On his retirement, he started a remodeling and home repair business, which he ran for several years.

More by Dennis Howard