I have been working on a 3020 that is geting to much gas. It has a cast iron carh with a brass plunger i took it off after it had been runing and it looked like gas had come from the plunger deal.
I put a pressure regulater on it and turned it down to 1 psi and it is alot better. I have a buddy that worked at a JD dealer ship he said they tryed to stop working on gas JDs there.
I bet it's the needle & seat . Grab a rebuild kit and a new float . http://www.yesterdaystractors.com/John-Deere-3020_Carburetor-Kit-Basic_Z106.html http://www.yesterdaystractors.com/John-Deere-3020_Carburetor-Float_AH10620.html