If you lack a vacuum on the crankcase vent, the progression of problems will be as follows;
1) increased crankcase pressure leading to increased oil being drawn past the rings into the combustion chamber.
2) the increased oil in the combustion chamber will cause an increase in carbon deposits on the spark plug, valves, chamber, top of the piston, and most importantly, the RINGS.
3) an increase in carbon on the rings will cause them to stick in the groove and cause a loss of seal. This loss of seal will allow more exhaust gases to escape past the rings and more oil to get into the combustion chamber, which leads back to step (1).
What you would notice: the symptoms are PROGRESSIVE and will get worse over time. With enough oil getting into the combustion chamber, you can definitely experience spark plug fouling problems. Ever notice how 2-smokes foul a lot of plugs? Thats because of the oil getting on them.
Note that even with stock crankcase ventilation, there is a set of circumstances that can lead to the same problem. These circumstances involve the breather freezing up inside the airbox, causing a loss of ventilation. If this happens enough, the rings can be compromised by carbon and lead to excessive blowby to the point that the crankcase vent is insufficient, you then can experience the same cascading progression of symptoms.
