I have a Lincoln welder with a 16 hp briggs Stratton engine that will only run with the choke closed. what could cause this?
This is most likely an issue with the carb having issues with ethanol clogging. You may be able to get it work by using something like SeaFoam thrown into the gas tank at a high concentrate. This may entail rebuilding the carb, but I've found it's usually worth my money to replace the carburetor directly due to the amount of time/effort it takes to get a rebuilt carb running correctly.
What is going on is the ports in the carb have become gunked up due to the ethanol. The reason it will run with the choke on is because there is a greater amount of vacuum draw on the ports, which gives the engine the gas it needs to run. Unfortunately, it usually runs too rich and doesn't run as fast as it should, which causes other issues (low generator output).