There are "rebuild" kits you can purchase but they won't include everything you need to restore the carb to its former glory. One of the first things that need to be checked is throttle shaft wear. You've heard
@ssmewing mention this already. Here's the throttle shaft of this particular carburetor. It's most likely to be have wear in the area circled in yellow, where the shaft is brighter from touching the housing. There is no bronze bushing set in the carb housing, so the housing, which is probably a aluminium or magnesium based alloy can incur wear more readily than the shaft which is steel.
For the record, the shaft measures 1/4" at the area of interest. It is supposed to be 1/4" too.
If either the shaft or the housing has excessive wear in that area, air will be sucked in and the air/fuel mix ratio will no longer be right. One way to test without disassembling the carburetor is to spray some starting fluid where the throttle shaft meets the housing while idling the engine. Safety first! Don't anything like that if you don't know how to handle very combustible gases.
In my case the shaft doesn't seem to have any significant wear. But what if the housing is worn out significantly where it meets the shaft? Here I try to move the shaft sideways to check for play.
https://vimeo.com/659102443
As you can see, there's very little play. Even so, if I want to improve the situation, what is the solution? The answer is bronze bushing.
Here's a view of where the shaft fits in the carb housing. Observe the recess area around the 1/4" hole where the shaft goes in. It is about 9/64" deep. Well, we can call it 1/8" though
The inner diameter of that recess area is 7/16"
One can find something called bronze thrust washers, made by Hillman. They are 1/16" thick, with 1/4" ID and 7/16" OD. Two of these inserted in the recess will give you the 1/8 thickness to fill the recess and provide a really nice bronze bushing bearing. Even more amazing is the fact that I found the item available in a pack of 10 on amazon and will be ordering it. I'm sure in the US it will be much cheaper and more readily available.
But I found a bronze sleeve bushing at a "local" store and will try to get it from them.
deck's on, blades sharp, let's go it's time to mow 
