I have a 77 XL 75. Was running great, then the fuel system went out - rust in tank, cleaned tank, cleaned carb, new lines with filter ... Now having spark issues. With the plug out, cranking over, spark will arc to engine ground up to 1/2 inch. Once the plug is grounded, no spark at all. Have tried multiple plugs, with the same results. Plug boot seems good - no visible cracks. I am guessing a grounds problem, however not sure where to look. There are is a black wire to the coil, and a green wire grounded at the coil mount. Both of these seem in good shape. These were the only wires under the tank which may have been upset. Any thoughts or suggestions?