twice now, in the past few weeks, I've lost vacuum plugs from the carb ports. I have block off plates, so only 1 of my vacuum ports is used, the other 3 are blocked off. the bike started idling roughly and I found a missing plug from carb #3. No big deal, I replaced it with a plugged piece of hose until I get to the parts store to buy some "real" plugs. Today, the bike was idling roughly again and #1 carb was missing it's plug. Is this common or is it a symptom of a problem? The bike is running well in all other respects and seems fine again after replacing the plugs, any thoughts? Thanks.