roadsmith trike rear tank not feeding carbs with fuel cap off, new fuel filter\\found that if you don't top off the tank, it feeds ok. can't find the vent tube. its suppose to be on top of the fuel neck. ran the bike with the gas cap off, no help.at times i can go a mile , other times 1000 feet before it quits. other times it will be ok running off of the rear tank for a week. bike ridden daily 10-15 miles. it has a new fuel transfer valve. connections cleaned & dielectric greased.