I have a rebuilt Plymouth (Canadian 25" block) that wont pass aircare. It has high hydro carbon readings. It runs so smooth the mechanic never thought it would be a bad cylinder, butI found out the #2 cylinder is low . It is under 100 while the rest are around 120 on a compression test. It gets spark, the plugs are OK and seems to run fine. Can this just be a valve adjustment? Whats the likely causes and best procedure to take to try and fix it? I cant find anyone that seems to know these engines.