I have an 86 as well and just replaced my tank this past winter. Unfortunately, I did not get the original tank size, but the replacement tank was almost identical in size and is 61 gallons. So I would think maybe I had a 70 gallon tank originally. I think they only had 2 options 47 and 70 that year. And yes I had issues with the pick up, bad gas, whatever you want to call it. The solution, just replace the tank and be done with it. 86 was a long time ago. My tank could have lasted. Couple more years, but it was getting close to needing repair.