Volvo: 92 740 wagon non turbo stalls when raining, air mixture, distributor cap

Question
Since my lovely mechanic washed the engine(against my specification) the engine seems to have trouble running if its raining or very humid. I have changed spark plugs, spark plug cables, air filter nothing seems to work. Feels like rain causes gas to air mixture to lean out too much...
any thoughts ?

Answer
Husain,
Check the distributor cap and rotor, if they look old and dirty change them. There  could be water or corrosion in some of the electrical connections, so take apart the connections and clean them using a spray contact cleaner, making sure the contacts are tight and all in order before putting the connection back together.