When the fuel tank is filled in my Poulan lawn tractor, gasoline flows into the cylinder freely. The only way to start engine is remove spark plug, drain fuel from cylinder, replace plug and start. Some valve is defective somewhere. Any suggestions about finding problem? I installed an in-line fuel shut-off but this is just a "bandaid".

Sounds like your carberator bowl is flooded. Most likely you needel valve and seat are not working properly. This is just a starting point but I'd remove and dump the bowl. Some fuel will spill because the tank line is still connected. Raise and lower the bowl sveral times. Gas should stop flowing with the bowl up and start flowing with the bowl down. If that works reinstall the bowl, you may have cleared the problem. If it does shut off/on with raising/lower the float you need to fix the needle. Also check to insure the float is good.

When you remove the bowl (float chamber) the float is inside. The float is designed that when the float chamber is full the float is pushed up and forces a needle to close off the fuel inlet, when the chamber is empty the float hangs down and the needle is open allowing fuel flow. Yours is either not floating (hole in it) or there is a piece of debris in between the needle and where it seats causing a continuous flow of fuel. Sometimes the seat becomes damaged and the needle and seat need replacing.

Don