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Join Date: Nov 2007
Location: Chandler, Arizona
If it still has a problem with the cap open, it is not a tank venting problem. Those carb vent hoses need to be in place. However, if it only happens when the tank is half full, that is not likely the cause either. Since the quantity of gas in the tank is what makes the difference, it almost has to be in the petcock assembly. It does not go on reserve till well below 50%, so the main strainer on the petcock could be partly plugged up, letting gas through to a certain level, till it reached the level on the filter where it is plugged up. I would completely remove the petcock and take a good look at it.
I am a motorcyclist, NOT a biker.
1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike