Craftsman 917-270514 Knocking, Spitting, Sputtering, Backfiring - (photo/video)

I have a 1998 Craftsman 42in riding lawnmower (Model: 917-270514). It has been a good one! However, this week, it started to "knock" hard (only term I can think of to describe the sound) when running. It also spits & sputters and backfires loudly.

The mower will start right up, and with some coaxing of the throttle, stay running. But, the shimmying from the knocking & backfiring sound dangerous. There is NO smoke associated with the backfiring - which I find odd.

So, I need a little help. I have a video of the engine running (link below). Also, and this could be very important -- a photo of an oddball plug that doesn't seem to fit anywhere.

THANKS for your help folks!

Briggs & Stratton 14.5HP OHV
I/C GOLD
Model 287707
Type 1255-E1

A video of the engine running - knocking, spitting, sputtering - is in a shared folder on my Dropbox: http://db.tt/EXs2PHLn

 

I saw the video. Try a new spark plug.

 

Sounds to me like the carburetor is leaning out.

 

Bet if you rebuild and clean the carburetor it will stop doing it.
This type problem as been a big issue ever since they forced ethanol fuel on us.
Run non ethanol or buy some of the additive to counter act the effects of it.

 

THANKS folks. I'll clean the carb, replace the spark plug and buy the gas additive.

Just asking... It is not odd that the mower ran for more than a decade without this problem? I replace the spark plug regularly and we have always purcahsed gas from the same country store.