sounds like your carb is too small.the cfm rating or cubic feet per minute is 650 which means 650 cubic feet of air per minute,not fuel.if youve tried jetting down to no avail and had to lower float levels.it might be that your not getting enough air flow.the burning of the eyes,is an indication of that. bet you probably running a small air filter of somekind as well.take the air filter off and see if it gets any better.if you have 10 " of vacume you do not need a 2.5 power valve,you should be running a 6.5. have you set your float levels?do you have sight glasses for your floats or just screws.with the vehical running remove the flat head screws on the side or look at the the glass if you have windows to see if you can see your fuel level.if the level is to high starting with the rear first, loosen the screw while holding the nut and then turn the nut clockwise untill you see the level go down, if you have windows it should be in the center of the window if you have just scews lower the level untill the gas stops coming out of the hole.you should shake the vehical and just have gas dripping out of the hole .do the same for the front and try again.if jetting is correct and float levels are correct and your air filter is off,and your still having the same problem,you either have a leaking carb or the carb is too small.try a 750 vacume secondary,also check your accelerator pumps to see if you have any play in the arms before they squeeze.if they are too tight they will cause leaking. does a friend have a 750 you could take off and try on yours to see if its more suitable? it doesnt cost anything and you will definatley have your answer