Plastic block? I don't quite get that, I have 3 Stihls, and all three adjust the same, there is a high adjust (controls how high it'll rev), a low (to keep the saw running when trigger is not depressed) and idle (keeps the chain from turning in idle).
If you don't have a screwdriver small enough to fit inside the rubber housing, just take off the air-cleaner cover and pull off the rubber grommet. Just don't lean it out too much, or you could exceed max rpm's. Wash or replace the air cleaner at the same time (could be the problem). When you adjust either the high or low, check the idle to make sure your chain isn't turning in idle. Or else, you saw is a lot newer or older than mine are.
