The valve clearances should be about 0.010", TEN THOUSANDTHS of a good Christian Imperial INCH.
If you are some sort of Continental pervert, that's about a quarter of a millimetre.
Whether too tight or too slack I can think of no reason why this setting could affect crankcase pressurisation, which is almost entirely caused by gas passing the piston rings - unless you have a blown head gasket or some other big problem.
If he set the valves to 25 thou it would be noisy but hardly disastrous.
Sort of a lot of clickety-clackety noises.
Remember, the crancase WILL pressurise unless the fumes are easily vented from the breather system. FIRST, make absolutely sure the breather system is clear, hose by hose, every pipe, all clear and clean.
Then worry about doing a total engine rebuild with a re-bore, or at best fitting new piston rings after glaze-busting the bores.
CharlesY