Gummy fuel system problems/varnished fuel: First, drain the tank using a siphon pump or do it the old Cheech and Chong way by siphoning with your mouth and a garden hose. (Varnished fuel is the worst to swallow so if you need to Yak, go Yak) Depending on how long the car has sat with fuel in it, I use this rule of thumb. Under 10 years it should not require dropping the tank. If longer or the car has a freshly rebuilt carburetor, drop the tank and either wash it out using a pressure washer or a car wash wand. No matter what, when you go to fill the car up with fresh gasoline, add 1 gallon of acetone. This will keep any dislodged varnish from re-clogging the carburetor or help immensely. If it does start to run rough, remove the hose at the pump and run the engine off of straight acetone. Most I’ve ever had to do was around 1/2 gallon but it also helps to prime the carburetor with acetone and run it until it picks up fuel from the tank.