Defiantly change out the fuel. I spent two months, trying to diagnose why I was having so many problems getting a project Pontiac 350 to run properly, just would not stay om for me. Finally, I come home from work one day, and notice a leak coming from the back of the car, the darn tank had actual rusted through in one spot and fuel was running out. Pulled the tank, OMG what a nasty mess of rust, sludge and bad fuel. Messed up my new rebuild on a Edelbrock carb I had just done. Anyway, I finally pulled that engine out and will install a 400 with a quotrojet and new Fuel tank, sender etc. in place. Bet it starts right up. LOL.