So you have an oil burner and you cleaned the nozzle, this is the first issue. They should be replaced every year or two. Next is an under ground tank. This is going to cause you a lot of problems. If you plan on keeping the oil burner, get the oil out of that tank and get an above ground tank. With out giving legal advice, just a warning - - the under ground tanks are known to leak. If by chance it is tested and found it leaked, the cost of clean up could range in the $20,000 to $100,000 area. Thus start now - drain the tank and hope for the best. The EPA treats this as a huge problem, forcing home owners to clean it up and gives out fines, big fines. Converting to LP is not that expensive and is much cheaper to operate. Wayne makes the conversion kit and costs in the area of $1500 from a contractor plus labor. You will not be able to install it, as it will require a combustion analyzer to tune it and set the corrected flow rate. A .75 nozzle will convert to around 90,000 to 120,000 BTU rated burner. What every you do, get the old tank out and have the ground tested. Hiding leakage is frowned upon, and can make things a lot worse. Good Luck