My lawn tractor has a 17.5 hp b&s and it starts right up and runs for approx. 20 sec. then stalls.I cleaned the carb, put fuel stabilizer in still the same.could it be the fuel solenoid or a faulry safety switch?

Hi,
I have seen a combination of the two do it. It is that timespan that is a bit off. A fuel solenoid causes about 3 - 5 seconds delay.
I would check the fuel entry system, filter, pump, etc.
Peace,
Clarke