I have a craftsman LT1000 with a B S 20 hp twin motor. Recently it started backfiring after it runs for about 10 minutes. Only after it's warm it backfires. I changed the oil and filter, plugs,fuel filter and the gas but it still backfires after it warms up. Any ideas how to fix this?or what may be causing it to backfire? thank you

Sounds like it is letting unburned fuel thru the carberator and getting ignited by the hot exhaust. Your carb is out of adjustment or could be worn out.

A bad solenoid on the carb should not cause this. It sounds more like to me the motor is out of time due to a sheered flywheel key. Appears as though the motor is firing while the intake valve is open which is why you are getting a so called backfire through the carb. I hope this has been helpful.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/steve_f5a6ca85b5d1ffc9

If it has a plastic intake manifold there is your culprit. The flanges warp and produce vacuum leaks and all three.

Had the same problem with an almost new Craftsman w/ Intek 20.0 HP. Check all the "usual suspects" and had to have a tech come out. He had me start it. It ran for a few minutes and stopped (per usual). He immediately went to the Anti-Backfire solenoid. It is an electrically operated "plunger-style" fuel line blocker. They are famous for going bad. The part is about $65 and its sole purpose is to starve the engine of its fuel supply at shutdown so it won't backfire. If it is disabled, the user must throttle the engine down for 10 seconds or so before turning the key off in order to avoid backfires. Gee... let's see, 10 seconds of my time each time I use the mower vs $65 part ... No contest! Remove the solenoid, take a pair of electrician side cutters and snip off the tip of the plunger and reinstall it. Problem solved ! ! !

For some reason your carb is getting to warm and it is vaporizing the fuel
If you can remove the hood try running it without it to see if that problem still exists

Sounds like you coil is over heating after running a few minutes. The windings are seperating, then when it sools they make contact again. Try replacing the coil.

Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/sean_54c3a3dc48a7773c

I have a 7 year-old craftsman mower with a 22 HP Briggs engine that has the same issue. After about an hour, the mower acts as if it is starving for fuel and then stalls. Then after about 5 minutes of sitting, it will run for a little while longer.
If your engine is similar to mine, then its carb has a fuel shut off solenoid which shuts off fuel to the Main Jet when Voltage is removed to prevent backfiring when the key is shut off.
Unfortunately, this solenoid can also cause the carb to starve for fuel when not working properly. In my case, when I cleaned the solenoid and the solenoid's connections, the tractor had no problem after that.

Hope that helps,
Steve

SOUNDS LIKE A FUEL PROBLEM OR DIRT WHAT OTHER SYMPTONS DOES IT HAVE

if a camshaft came loose the engine would not run at all. IF engine still runs then i would change the head gaskets. these things **** air and cause backfiring when running. but if engine doesn't run anymore i would look on ebay or craigslist for local mower for parts at least.

Hi
Two things come to mind. First it could be not getting enough fuel when hot due to carb adjustment being too lean. That could be that the float level in the carb is set too low or the low speed jet is set too lean. When cold the choke circuit in the carb supplies the extra fuel required.
Second in some enjines that have low compression they start cold but when hot the compression drops below 60 psi and makes starting very difficult until engine cools down.
I suspect a fuel problem.