904 tranny problems

It started backfiring while idling after warmup. The engine and trans sat for around eight years. The reason I say its always full is after I changed out the filter on the tranny I put around eight quarts in and the day I drove it and would still have to rev high to move but ran a little better than now. The day after that it had a small leak that I thought was the pan but was coming from the shifter. Looks like a it lost a lot of fluid but started it up and checked at idle and was still full. Should I tune it up and how do I get rid of the lean. What's the best way to fix the boring on the shifter. Thank for the help fishy