Briggs 12.5 HP Flat head - backfiring

[darrinster]

Model 407777 0128 e1

Engine was partially frozen, didn't take much to free it up. Crankcase was full of gas.

Engine runs, but it's backfiring through the carburetor. If I hold the choke at a certain point it seems to even out. I soaked/cleaned the carburetor and also tried another good carburetor.

Any thoughts?


Update. Checked flywheel key it's not broken. Switched out ignition coil and spark plug nothing changed.

 

[bertsmobile1]

Check the valve lash.
Exhaust not opening or not opening far enough or inlet not closing fully can do that.

 

[Tinkerer200]

Check the valve lash.
Exhaust not opening or not opening far enough or inlet not closing fully can do that.

As can burned valve, warped valve OR it is fairly common for the valve seats to come loose on these engines, hop in and out which is more common. Boy, a 12.5 hp 40 series B&S is an old booger.

Walt Conner