Question

Why does my 25 HP Craftsman Briggs Lawn Tractor keeps backfiring?

I have a Sears Craftsman Lawn tractor that has a 25 HP Briggs and Stratton engine.

The engine has started backfiring when shutting down. It runs fine but always backfires and shoots out some flame sometimes out the exhaust.

Answer 1

Sounds to me like your carburetor may be getting too much fuel and flooding the engine.
When it gets too much fuel then it backfires because there is residual fuel left in the intake or exhaust.

Another thing I just thought of is that it your flywheel might be off a little bit and be out of time.

It could also be an overhead valve if it has overhead valves. It could be one of your valve rods or two needs adjusting.

Answer 2

The most common cause of a lawn mower backfiring is a clogged carburetor.

The carburetor on the lawnmower has a tiny jet in it which can easily get clogged and when that tiny main jet gets clogged it does not allow enough or any fuel through it properly.

This can either cause the lawnmower engine to not run, run at choke or backfire and run poorly.

The fuel and air mixture get messed up when the fuel doesn't get into the engine properly.

Cleaning the carburetor by soaking it in some carburetor cleaner such as B12 chemtool for 30 minutes to an hour after taking it apart and blowing the main jet and other passages out can help fix the backfire and make your lawnmower engine run properly.

Also your spark plugs could be bad as well but it's most often the carburetor that is causing the issue with backfiring or running poorly.