1986 Mazda b2200

Electrical problem
1986 Mazda b2200 4 cyl Two Wheel Drive Manual 195000 miles

My 1986 Mazda B2000 has a problem with the gas gauge. When the tank is full it doesnt work at all (reads empty), but when I get down to less than a quarter of a tank it works correctly. I changed the gauge itself on the dash and when I had the bed off I checked the float in the tank and it seamed to be working freely. What am I missing. Please help.
July 12, 2009.

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