Could be something as simple as bad fuel. We have a 3020 gas that my dad has owned since the 70's. Always have performed regular tuneups and it has done everything we asked of it from plowing to chopping silage. One summer it started running rough and to have any power, needed to have the choke part of the way out.
Changed the plugs, points, condenser, coil, put new plug wires on it, new fuel filter, adjusted the carb, nothing would help. Had an old tractor mechanic come look at it. He took one listen and said had to be the gas.
We had a small 80 acre farm so were getting 150 gallons at a time from the local co-op in a 300 gallon barrel. Wasn't a recent fill, probably had used 2 tanks out of it and had ran fine on those. We drained the tractor, put some new fuel from a station in town in, and it was back running like it used to.
Had the co-op come test the fuel in the barrel. They said there was nothing wrong with it. So just to see what would happen I put some in a '71 Ford truck we kept around the farm for fencing and plowing snow. It immediately did the same thing, ran rough and no power. Told the co-op to come get it.
They claimed that they used up what they pumped out of our tank in their forklifts and never had a problem. Who knows, but what we drained out of the 3020 and the pickup didn't even do a good job starting brush piles on fire. We changed suppliers after that.
