I notice the statement that there are no signs of fresh gasoline. Is there 
even the slightest sign of seepage on the tank ends? If so, whether it looks 
fresh or not, that is likely a sign you have a problem. Put you nose down 
there and smell. Smell like gas? You have a problem! The fact that it smells 
worse with a full tank suggests it is one of these: a) a cracked spot weld up 
high; b) an end seam leak up high.; c) a filler hose that is either cracked 
or improperly sealing  (if you top off the tank fuel is left standing in the 
filler tube/hose, giving it a good chance to escape if ithis is a problem 
area); d) a cracked or dry gasket for the fuel float mount (use a neoprene 
gasket without any permatex); e) no copper sealing gaskets under the screw 
heads for the float mount; f) old cracked vent hoses above the tank; g) 
cracked solder on  some fitting atop the tank. In my book I list 12 potential 
sources of gas smells, with fixes.

If you are going to do all of this, I would also suggest soldering a penny 
over every spot weld. The spot welds hold the vertical baffles in place 
inside the tank. The baffles are there to prevent sloshing of fuel from side 
to side when cornering, but over time they are prone to rust and can actually 
begin to crack and tear out the spot weld. A penny soldered over each spot 
weld gives an extra margin of safety. I think it was Jon Myer who told me 
this trick years ago..






Stan Gundry, Author/Publisher