Crankcase vent blocked
 

I just bought a lightly used 03 EC250 and have been having trouble fouling plugs. Changed reeds, went through carb, etc and nothing. Someone told me to check the left cylinder/crank/power valve vent to make sure it wasn't plugged and found it had been intentionally plugged off. Anyone know why someone would do this? Could this indeed foul a plug (very erratic fouling, fine for a couple of rides then bam, fouls 2 or 3)? I am of course going to fix this, but it takes a while sometimes for it to show it, so it may be a couple of rides to rear its ugly head. I thought about a crank seal, but the drain plug for the cylinder is clean, and it doesn't seem to use any oil.

From what I remember some bikes came with the left side power valve cover drain either blocked off or not drilled completely. Other years they actually zip-tied the drain hose to make a less permanent blockage. Seems it had to do with California emissions (oil dripping on the ground). Your power valve drain shouldn't be causing your plug fouling issues though. I had my bike for a year and probably went through 25 spark plugs before I found online help that led me to the LTR jet kit. I don't think it is being made any more but check the jetting sticky-post and start there. If the previous owner never opened up the power valve drain he probably never did any jetting corrections either.
btw, there is no "crank case" vent. There is a transmission vent coming off of the right side pv cover and also a hole under the engine case that vents/drains the flywheel housing. The left side pv cover vent only drains exhaust spooge.

They did have the LTR needle in there, so I am guessing they had the entire kit. It (vent hose) was zipped tied off, I guess a CA thing like you said. I don't know whats causing the fouling, its very erratic running, will do fine for a while, then foul. I don't think that would be jetting unless its the float sticking maybe. I guess I may have to break down and check the seal, but at gofaster they didn't seem to think that was it. They are thinking float needle, so I will be turning off the gas when I stop to see.