I have a 21 hp Briggs and Stratton model #331877, what are the valve settings?

Eugene isn't exactly correct and 1/4" isn't correct or necessary, just make sure the piston is at the top of it's stroke.

.004 ~ .006 for both intake and exhaust

Then roll it over a couple of times, bring it back up and check them again. Then roll it over slowly watching for the compression release. pay close attention to the exhaust valve because they like to wear away from not being adjusted properly

Move crankshaft around so neither valve is raised and set loose at the rocker arm, set at 6mm intake and 8mm exhaust forget about where tdc is

Intake 003 - .005
Exhaust .005 - .007

Adjusted with the piston 1/4" past TDC of compression stroke for each cylinder.