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I, too, have worked on these things for hours, days , and years. The thing that gets to me is that only 90% of the 3010-20s, 4010-20s do this 10% will run year after year with no problem. I don't know which carb you have. They used both Marvel and Zenith; one was about as bad as the other. Both brands use a vacuum actuated accelerator pump, that gave the engine a shot of gas every time the govenors moved the throttle disk toward open, and the vacuum dropped. I have seen tractors sit idleing, and start blowing black smoke from a pulsating acclerator pump. The way I checked this was take the carb apart and jam the piston with a match stick, that cured the smoke at idle, but would cough and back fire when you opened the throttle. This was on a zenith carb. There were some service bulletins and change overs, on the Marvel carbs, back in the early sixtys, to help this problem. I have tried all types and brands of spark plugs including rim fire marine plugs that cost about seven dollars each; they all last about the same amount of hours. Years ago I called Champion spark plug company, and asked for the service department. I explained the situation, told him that I wasn't running his product down just wanted to know if he could help me. He told me that it was fuel fouling, it wasn't the plugs fault. He then asked if I understood the difference between a D14 and D23 plug and how to gap them. In a subtile way, he told me that I was an idiot. I responded that I understood heat ranges, ionization tracers, the difference between a spark and an arc. What I didn't understand was why the plugs only lasted about six hours. He said it was fuel fouling, end of conversation. Electronic ignition and an high energy coil, along with the basic good wires, cap and rotor helps some. I have stretched springs,shimed springs and cut springs, trying to get the accelerator pump right. If you ever do get the carb right, they will run pretty good. By the way, John Deere is not the only old engines that have fuel fouling problems.
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