Lawn Tractor Backfires When Shut Off
Article by Mark Trotta
After I was finished cutting the lawn, I parked the tractor and shut off the key. This was immediately followed by a loud "bang" out of the muffler.
The following week, after cutting the lawn, the same thing happened.
Why Is My Lawn Tractor Backfiring?
If you see one of these switches on the bottom of your carburetor and you have a backfiring problem, this is very likely the source of the problem.
A few types of lawn tractor engines have a fuel solenoid, also called an anti-backfire switch (also called an anti-afterfire valve). Their function is to stop the flow of fuel into an engine once the key-switch is turned off.
Basically, it's a plunger extended by a spring, which retracts when supplied with 12 volts of current. When the spring gets old and weak, it allows gas to get into the carb, causing the mower to backfire when the engine is hot.
The backfire doesn't hurt your engine, but it should be repaired (and it is an easy fix).
How To Repair A Backfiring Engine
There are two common anti-backfire valves found on lawn tractor engines. If you know your tractor's model and engine number, you could look it up that way. Or you can visually identify it.
The fuel solenoid below replaces Briggs & Stratton part numbers 699915 799728 794572 499161 496592 and 498231. The carburetors they fit include 498027 495706 494502 494392 498134 499161 496592 and 498231.
This is the switch I needed for my 19.5 hp Briggs Intek engine.
Shop: Fuel Solenoid for Briggs & Stratton Carb (699915 and others)
The switch below replaces OE part #694393 and others.
Shop: Fuel Solenoid for Briggs & Stratton Carb (694393 and others)
How To Replace Anti-Backfire Switch
The switch is easy to install - providing the old one comes off easily! If it does, it's a five-minute repair. As a precaution, pinch the fuel line to the carb before removing the switch.
There should be a brass or copper washer on the old switch. If the new switch didn't come with one, it's OK to re-use the old washer.
After I replaced the original switch on my lawn tractor, the backfire came back the following season. Apparently, ethanol-blended gas damages anti-backfire solenoids. So, after replacing the switch a second time, I switched to non-Ethanol gas in all my lawn equipment.
NOTE: The replacement anti-backfire switch I installed was a little shorter and fatter than the one I took off, but it threaded in and worked fine.