I'm at wits end, started having a problem with engine running, thought it was carb first, spark looked good so did a carb rebuild and no help. Then replaced all spark parts, full tuneup with coil. After this it ran at first, then the problem was that tractor would intermittently run at idle fine and SOMETIMES run up to high RPM or sometimes sputter on a couple of cylinders over idle, and sometimes take off and run fine again. So spark was an issue at first. Replaced all fuel line, filters, took of carb again, everything looks fine, but still only running at idle, occasionally pick up a bit of speed but mostly just sputters at high er throttle open, Checked air cleaner, all good. Replaced all electrical wires suspecting an intermittent short, no help. Pulled valve cover, no stuck valves. Everything looks good, so I just ordered a new carb and an electronic ignition converter but not real hopeful thats the problem. When I go above idle throttle and is sputters, spraying starting fluid into the carb has no effect on it, good or bad, so its not starving for fuel. Plugs look like burning rich but cutting back power screw less than 1-1/4 turns doesn't help either.