I have a Craftsman DLT Riding Mower Automatic with a BS Intek 25 V-Twin engine that is suddenly running very rough. As soon as the brake is disengage and power applied, it backfires and runs real rough.

Any ideas?

suddenly running very rough

What can cause this is a bad plug or more likely dirt in the carb got in to make such a quick change.

Check plugs, If good then carb needs cleaned out.

Also check fuel filter...

The "backfire" is unburnt fuel entering the muffler and the explosion is there and not in the cylinder. That is indicative of too much fuel into cylinders from carb.

Clean needle and seat in carb also. If its also smokes when running rough that is also too much fuel to air ratio at carb level

Thanks for your help. I replaced the spark plugs, fuel filter and air filter. Still same issue. How do I get to the carb?

The engine is a B&S Intek Vtwin 26hp
Family: YBSXS.7242HK

I checked further and one cylinder is not firing. The spark plug is firing but when I take it totally out, the engine runs the same, which makes me think that the one piston is not getting fuel.

Where do I check next?

Hi,
The easiest examination of firing on a V-twin is to lock the park brake, crank the engine, with a pair of well-insulated pliers remove on spark plug cap and observe the running, replace and then try the other. Acylinder that is not firing will do nothing while a cylinder that is alone providing the power will let the engine die when the plug is no longer connected. You may have received part of the answer already i.e. "gas in the muffler" which would possibly mean that unburned fuel is passing through a cylinder that is not firing and thereby entering the muffler.
Peace,
Clarke
PS That was a EPA emission family tag number you gave; the real numbers are on a black and silver tag 1/2" by 3" rivetted to the blower housing and having "Model, Type and Code on it.

Thank you all for great advice. I finally figured out the one cylinder is not firing. I read somewhere that putting a little bit of fuel in the cylinder may work. I did that and the engine is running great! I have no idea why that is but I am not asking...

Thanks for your help