I have a tecumseh engine that is 5 years old and always ran well. The 3rd time I went to start it this year it started good but stops in 5-10 seconds. There is a new spark plug and air cleaner. I have tried to start it more than 12 times now with same result.

Refer to this: https://www.askmehelpdesk.com/tools-power-equipment/engine-stops-after-few-seconds-85441.html

Where is the answer to the question about Tecumseh engine that won't keep running?

Try running with the gas cap off. Many times the vent hole will clog up thus causing a vacuum in tank and engine will stop after a short period.

I Would Start By Removing The Carb Bowl And Cleaning, Sounds Like You May Have Water In The Fuel, If You Find Water In The Bowl, Clean Entire Fuel System And Start Over.