i drove my 81 diesel caddy 6000 miles after a complete rebuild-it ran great the whole the whole time. It had sat for 4 years prior to purchase. Then all of a sudden it started hard and ran poor after full throttle acceleration. I changed the fuel filter and it helped for 50! miles. Turns out the in-tank strainer (very fine mesh filter) had been coated in varnish. i pulled the tank, cleaned the strainer with carb cleaner- replaced and it runs great. good luck.