i have a 1985 vulcan 700. run good when the gas tank is full and down to a 1/4 of a tank, at this point the motor boggs down. the motor will run but it acts like it is not geting a enough gas. know matter how much you open the throttle. the bike will only run at 50 to 55 mph. if you take and fill the gas tank back up the motor run fine. untill it get back down to a 1/4 tank and start bogging down again.this happens after i ride it for about 10 miles, turn it off,let it setfor a while and then take off again. i have check the gas tank by cleaning it out. check the pitcock filter for blockage. check the vent in the gas cap. which is clear.i put a whole can of seaform in the gas tank 8oz at each fullup. the gas fiters are good. it has me stump. need help!!!!!!!!!!!!!!!!