Update: Used the mower yesterday evening for just the 3rd time, and the problem seems to have mostly gone away as it just sputtered for about half a second one time only. This time I filled the gas tank to about 3/4" below the vent opening. Maybe that might have been the original cause since I was overfilling the tank????
Has the fuel level proved to be the solution?
I've had my Toro 20340 for a little over a year now. It did the same thing the first time it was used. It never dies, just seems to slow down, then recovers.
Over time I've noticed that it doesn't seem to "hesitate" (for want of a better description) when first started (temperature typically in the 70s), but it will when it's fully warmed up. Also, when warm, if I raise the front wheels just clear of the ground, and rock the handle side to side, it will hesitate. This had me thinking it's either fuel related, perhaps the level in the carburetor bowl, or governor-related, as the governor mechanism is in the sump, and the oil there will be sloshing around as well and perhaps could affect the position of the weights.
I've removed and fully cleaned the carb (even though it didn't need it) as this is a common issue with the plastic B & S carburetor, including removing and flushing the jet assembly, to no avail. In doing this, I noticed that the needle valve is white plastic, the same as the float, with a rubber tip, whereas the replacement part (59458) is shown on-line as being metal. Not sure if this too could be factor.
I should add that it has hesitated once or twice where there weren't significant bumps that I noticed, but perhaps there was some shifting as I moved along.
I've been searching forums and reviews, and found other reports of this same symptom. However, it does seem to be limited to this Toro model, which has the B & S 104M02-0131-F1 engine. As far as I could find so far, this engine was used only on this particular Toro mower. (It's the Stow-and-Mow version, which allows the unit to be stored vertically. I understand that there might be some modifications in the engine to accommodate this, and I'm thinking these changes could be the reason for the hesitation symptom.)
Also, in regard to the governor, I'm going to revisit the oil level. As the hesitation seems to occur after it's warmed up, I'm wondering if the oil level could be a factor. It's set to be at Full when cold, but will check it to see how much higher it is when fully warmed up. If it's much higher, I might reduce the level (but still within the permissible range), to see if that might have an effect.
Otherwise, it starts well first pull, and runs smoothly the rest of the time.
Hope to get the wisdom and experience of the group here.