My Lawnmower Makes a Big Bang or Backfire While Mowing

You are just mowing along and your engine suddenly backfires and then it starts to continue running like normal. What could be causing this and how do I resolve it?

An engine backfire can be caused by an electrical problem or a mechanical one. In most cases it can be tracked down to an electrical problem, more specifically in the ignition system. If the ignition coil becomes too hot or has an internal breakdown, the spark will become intermittent and the combustion chamber can be ignited at the wrong time and cause a backfire out of the intake or exhaust.

The ignition kill wire could also have been compromised and partially broken because of rodent damage or heat and vibration. So you want to check it and is usually a black or gray wire that is attached to the ignition coil.

There could also be a problem with the ignition switch.

If the ignition system is working normally, then the search begins on the mechanical portion of the engine. A leak down test will need to be performed initially to determine if the intake or the exhaust valve is leaking. If one of them is leaking, the burning gasses will pass through them and cause a backfire while the engine is running and particularly under a load such as cutting the grass.

If you need a lawn mower repair shop near you to fix your mower, visit our Locations page for assistance.

Riding Lawn Mower Backfires After I Turn The Key Off

Most likely the back fire is coming from the exhaust when the key is turned off and it can be pretty violent. A loud bang sure can scare the heck out of you when it happens!

But why does your lawn mower backfire when you turn off the key?

The reason it does is because there is a solenoid on the bottom of the carburetor that shuts off the supply of fuel when the key is turned off. If this valve fails in the open position, it allows raw fuel to be pulled into the carburetor and the combustion chamber and then it ignites from the heat of the exhaust pipe. Pow!

An easy way to tell if your valve is working is to turn the key to the on position and you should hear a small click from the solenoid valve. You can also hold onto the valve while you turn the key and you will feel it move the solenoid to the open position.

You can remove this fuel solenoid but you must shut off the fuel flow or you may end up having a tank full of fuel on the floor or ground. Hose pinch pliers work well for this.

A test to ensure it is getting 12VDC power to it is also something good to do. Sometimes this valve will get stuck because of the fuel gelling up as the ethanol begins to separate from the gasoline. You can clean it up and then retest it for proper operation.

If you need a professional lawn mower repair shop in your area, visit our Locations page for assistance.


My Lawn Mower Backfires and Then Dies

The first thing you must find out is if it backfires through the exhaust or the intake or carburetor side of the engine. In a lot of cases it will be on the intake side of the engine. When it backfires, it is sending out raw fuel out the carburetor and the ignition is igniting it and you may see flames come through the air filter or it blows off the air filter box completely. When it blows the fuel out, the combustion chamber has no fuel left in or very little and the engine dies. The same thing can happen in the exhaust.

So obviously we have a timing problem or a valve problem. A set of tests must be done to determine what the problem is. The first test is a leak down test. This will tell you if your valves are leaking and which one is the culprit. It could be both of them causing the problem.

Checking the engines timing is more difficult and requires that the engine be taken apart to see if the timing gear cam or chain has jumped a tooth. But before you go ripping things apart, check the flywheel key to make sure that it is not partially sheered. One some models you have to remove the flywheel to check it, but on most you remove the nut holding it in place and you will see it.

These tests are often out of reach for many home owners because they do not have the test equipment. Check our Locations page for a shop near you for help.