Craftsman 21" 247.370370 Briggs and Stratton Engine

[peabodyn]

B&S Model No. 128M02-0952-F1

Mower starts easily, but when restarting after sitting for a minute or so, it backfires through the exhaust.

It doesn't do it if the mower sits for 10 minutes or so.

The engine runs fine otherwise and always starts easily.

The backfire happens even before the engine starts - is this a bad muffler or should i replace the shear pin on the flywheel?

Thanks

 

[slomo]

Trying to recall from the working on car engine days. Backfiring through the intake was late ignition timing and through the exhaust was like cam timing issues?? Don't quote me on that but was something like it.

You could have a carb needle and seat partially flooding the cylinder. That excess fuel lighting off inside the hot muffkin??

Sure check the woodruff key out on the flywheel. Wouldn't hurt a thing. Make sure the groove is a nice straight U shape and the key is 90 degrees all around.

Remove the oil dipstick and smell for fuel. If it smells like gas, time for a carb cleaning/rebuild.

slomo

 

[bertsmobile1]

IT is nothing more than excessive fuel in the muffler.
Mufflers now run red hot to burn off any fuel not burned inside the cylinder
When you turn the engine off fuel is still drawn into the engine & expelled into the muffler
When the concentration of fuel to air is correct for igniting at atmospheric pressure, it will burn which you hear as a backfire .
If it worries you put a tap in the fuel line & shut the fuel off a minute or two before shutting down.
Too small a valve lash will make the problem worse as it increases the time both valves are open together .

 

[peabodyn]

I just put a new carburetor on it and changed the spark plug.

While it does run better, it has not cured the back fire issue.

Will probably swap out the muffler with a spare to see if that fixes it.

There is no gas in the oil.

I just ordered the sheer pin - will look at the flywheel and replace the pin if the existing pin looks compromised.

Thanks

 

[slomo]

I just put a new carburetor on it and changed the spark plug.

While it does run better, it has not cured the back fire issue.

Will probably swap out the muffler with a spare to see if that fixes it.

There is no gas in the oil.

I just ordered the sheer pin - will look at the flywheel and replace the pin if the existing pin looks compromised.

Thanks

I would leave the muffkin alone. As Bert said, its excess fuel in it lighting off.

I would check for an intake tube crack and leaky carb gaskets. Intake tube is plastic I think on your engine. Probably good idea to replace the O-ring in it. Slather on some 100% silicone grease on the O-ring. Carb O-ring the same.

Take some pinch pliers and shut off the fuel line going to the carb. Or install a shutoff valve. See if it stops backfiring temporarily with the amount of fuel in the carb bowl only.

Still waiting on the flywheel key inspection.

slomo