B&S 18.5 HP runs good for a minute then runs rough and backfires thru carb

Hello everyone! I have a B&S 18.5 HP single cylinder OHV model 31Q777 type 0305-E1 on a Cub Cadet LT1018. My problem began with the mower trying to stall when I would turn uphill or get into thicker grass. I figured a little maintenance would fix it up so I changed the oil and filter, air filter, fuel filter, and spark plug (properly gapped) but was surprised to find that it was no better and has actually gotten worse. I have since removed the carb and did a clean and rebuild, removed the head and replaced the gasket, adjusted valves, replaced the fuel pump and fuel lines, replaced the coil, removed the flywheel bolt and checked the flywheel key, and finally gave up and replaced the carb with a new one....no change! What could it be? Any ideas would be appreciated!

Check the fuel line from the gas tank to the carb for any signs of damage or rot. Also look inside the tank to see if there might be some crap floating around the intake tube. Not sure how the tank to carb tube is installed in your mower but I have seen in-tank strainers that were full of crap cutting off the fuel flow. Also when you rebuilt the carb you did get all the linkages back in the correct place and holes? Just asking as I hook one up wrong and the engine did about the same as your's till I corrected my mistake.
Tom

Thanks Pumper54, I did actually change all the fuel lines when I put the new carb on. I can't really see into the tank all that well but I took a clear hose and ran it directly from my gas can to the carb, bypassing the fuel pump, and the fuel was immediately pulled from the tank and I never saw any interruption in fuel flow. I'm wondering if I could have a valve sticking or something? Everything seems ok when turning by hand but not sure how to check it while it's running.

I don't see where you mention checking valve adjustment. Could be you have a loose valve seat. I can send you detailed instructions on adjusting valves and or Service Manual IF you like. Address below, put in proper format and remind me engine model number and what you want.

Walt Conner
wconner5 at frontier dot com

Seeing that you have done just about every suggested, next time you test run the end try this. Start the engine and clamp off the fuel line. If the engine starts to run smoother before quitting, we know that it is a fuel problem. Getting to much gas. Next step would be to reset the governor, and if that does not work, it will be back into the carb. If the engine fails to run smoother, then we will have to check the ignition and compression systems.