They are usually sealed with a block off plug for emission reasons. Just another reason every engine should be fuel injected by now. Seems like the tractor manufacturers are holding onto the cheaper to build mentality the auto industry had.
On old cars, I've drilled and pulled out those plugs to get to the mixture screws. I don't even see plugs on the carb for the JD!
The engine keeps making a loud pop when not under load. I'd think a mixture adjustment would cure it, if I could only get to the screws.