A lawnmower will Backfire if there is a Mechanical or Chemical issue in the running process of the engine.
Keep in mind that an Engine backfire is only likely to occur in a Petrol or Gasoline engine.
A Backfire in an engine is usually a Secondary Combustion that happens either inside or outside the Combustion Chamber caused by the imbalanced chemical mixture or mechanical failure.
To be able to fix a Backfire on a Lawnmower it is important to know what are the causes of a Backfire.
Causes of a Lawn Mower Engine Backfire
Improper Air and Fuel mixture
This happens when you have a huge amount of fuel being mix with the air, this is commonly known as a Rich Mixture.
A rich mixture is an imbalance Air to fuel ratio. The most ideal ratio for a gasoline engine is 14.7:1. For every gram of fuel, you need 14.7 grams of air.
With too much Fuel, what happens is your having incomplete combustion leaving traces of fuel in the combustion chamber which will create secondary combustions known as Backfire.
A faulty carburetor is the most common cause of an Improper Air-fuel mixture.
A quick fix that you can do is to clean the carburetor or in worst cases replace the entire carburetor.
Engine out of Timing
Most small engines have overhead camshafts which require them to have a timing belt to sequence the Crankshaft to the Camshaft.
Sometimes a belt can jump tooth causing it to be out of timing, this happens if the belt gets weak and start to stretch and when you use a recoil starter the combustion pressure has the tendency to rotate the flywheel slightly in the opposite direction that’s causing the timing belt to jump out of timing.
The worst can happen here is if the timing belt completely snap and the engine just won’t start.
To fix this you need to remove and replace the timing belt, Below is a video that you can use as a guide.
Engine valves not properly adjusted
Overtime valves in your lawn mower engine worns our and your engine valve needs adjustment, Once your valves go too far out of adjustment it will have the tendency to cause backfire.
Also, if you adjust the valves without considering the timing position this will mess with the engine timing causing the valves to open too early or too late.
If valves open too early or too late it will cause some fuel to leak back and ignite out of the combustion chamber.
Note: Always make sure that the gasket behind and front of the carburetor is in perfect condition, if it looks brittle and squeezed out of shape it’s better to replace it especially if your putting a new carburetor.