An air leak upstream of the carburetor will not affect the idle speed. The crankcase seals I was talking about are the ones between the crankshaft and the crankcase - one at the bottom above the blade plate, and one at the top under the flywheel and magneto. The seals are just plastic, pushed into bored recesses in the crankcase halves. Because the pressure in the crankcase increases sharply once each engine rotation ("crankcase compression"), there is a pulsating pressure 50 times per second, trying to push the seals out. Sometimes it succeeds in pushing them permanently out of their recesses, and air can then be sucked around the outside of the seals, into the crankcase in between pressure pulses. Also, the plastic seals rub against the crankshaft at high speed. The bottom seal, above the blade plate, is prone to wearing out due to dirt and grit, or bits of grass and string getting wrapped around the crankshaft and cutting into the seal so it leaks.
You can check the bottom crankshaft seal by disconnecting the spark plug lead then looking at the bottom of the crankcase by peering in between the blade plate and the mower base. If the seal has been pushed out you should be able to see that: there will be a round piece of plastic visible below the bottom of the crankcase. If the seal seems to be still in its recess, you can drain the fuel from the tank and turn the mower upside down, then you can squirt a little bit of oil onto the crankshaft just above the seal, and see if it disappears into the crankcase. If it does, the seal is not sealing. To make a similar check on the upper crankshaft seal you would have to remove the flywheel and magneto, which I don't recommend as long as the mower is working properly. If you have an air leak into the crankcase, unless it is rather a small leak, the mower will lose power. If that happens, the leak has to be fixed, but until then, it may be best to leave it unless you are comfortable with pulling the flywheel off (it is on a taper, and will require some special persuasion - we can talk about how to do it if you wish, or you can read about it in Outdoorking archives).
As Mr Davis said, if there is nothing wrong with your mower and you are just objecting to Victa's chosen idle speed, you can modify the governor (it is inside the carburetor, behind the diaphragm) to reduce the idle speed, by adding an extra washer where he has shown on the diagram. I don't recommend that, since you would be modifying Victa's intended minimum speed, but it is sometimes done by Outdoorking members.
