I bought a used John Deer stx38 riding mower. Worked fine until recently, when it began running weakly, especially uphill. Now it stalls as soon as the blades are engaged. Blades are sharp, I cleaned the carb, put in new fuel, checked the oil, cleaned the deck, top & bottom, new fuel filter & jumped the seat switch. No change. What could be the cause?

Hi,
The STX38 is a Rare design for John Deere and has a couple of unusual quirks because of it. It is one of the few model that have a gravity-fed rather than pump-fed carb. Many of them have owner-installed fuel shutoffs in the fuel line because the carbs tend to "drool." Over a period, the fuel line can begin to deteriorate internally and shed bits of rubber that can get stuck between the fuel inlet of the carb and the top side on the inlet needle seat and restrict flow. The other big idiosycracy is the throttle shaft/carb body can wear allowing the shaft to flop to and fro and allowing in air. This results in poor performance because that air never travels through the venturi of the carb and thus does not have any fuel mixed with it. I include a carb drawing with arrows to parts and areas. The only solution is a carb replacement for this.
Peace,
Clarke