You can take out the valve seat so as not to get any grinding compound ( toothpaste ) in the carb body..... what to lap it with ??..... why the valve , of course !!! The object is to 'match' the valve tip to the valve seat for no leak seal, so put the valve in the end af a short piece of plastic tube ( spring end ) sized for a snug fit, put the other end in your cordless drill and run it forward and reverse with the t. paste in the seat....
This method only works for metal tipped ( earlier ) float valves