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On my gas 4020 there are several marks on the flywheel. Thing to do is to turn the engine with a sturdy screw driver prying on the starter ring gear teeth through the opening you uncovered to see the timing marks. When you find TDC and the other marks, clean them and use a toothpick to put a bit of white paint in the grooves and numbers. 20 degrees advance sounds about right for 1900 rpm. Isn't there a tachometer on the tractor to set the speed by? I thought it was standard equipment, middle of the dash. I remember the point gap being .020 or .022. Its important to be certain the centrifugal advance works properly, else the timing will only be right at one speed. You have a universal replacement pump with the primer lever used on diesel engines. Its not needed for the gas engine most of the time. The valves don't need setting often. If the valve lash is excesssive, performance might be down from the valves not opening wide enough and there will be some clatter from the rocker arms. If the valve lash is not enough, the affected valves will be held open and you will hear a spit in the exhaust for the exhaust valve held open and spits back into the intake will cause rough running. Either case will lead to burned valves, so its important to never have too little valve clearance. Black smoke comes from running rich, could be a generous acceleerator pump or a leaking float needle and seat. The leaking float needle and seat is a common problem. How you adjust the carburetor depends on which carburetor you have, either the easily adjusted cast iron Zenith or the nearly impossible to adjust die cast Marvel-Schebler. But the worn and leaking float needle and seat can happen to both of them. There's a diaphragm part of the accelerator pump in the M-S that I suspect is not replaced often enough. Blue smoke is oil, probably from worn rings, but could also be worn valve stems or guides or valve stem seals. There are other problems, but I'm too lazy to type them up again. I posted on this forum in the past week about 4020 gas tuning. Those Autolite plugs are good plugs. Be sure the plug wires are copper, not carbon. Gerald J.
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