Continuing . . .
It's my belief that the horizontal extension of the SmartStow baffle is meant to ensure that when the mower is in the vertical position, the fuel in the bowl can't leak out the vent because the opening will be above the level of the fuel. But, as noted earlier, I put my mower with the modified vent in the SmartStow position and left it for several hours and there was no sign of leakage, despite not having the extended baffle.
I did not attempt to test the mower in the vertical position, but guessing it may very well leak.
I had thought that as well. But your substitution of the 594098, which is for the 163 cc but not SmartStow, seems to be a viable solution, with precautions as described below.
Did some bench testing. I clamped the spare
non-SmartStow carb I have (799584) on a camera tripod so that it can be set horizontal (normal run position) and vertical (stow position).
With a short clear vinyl tube attached to the fuel inlet, I flowed distilled water into the bowl until it would not take any more. This is simulating the fuel tank supply, and the inlet valve closing when the float gets high enough.
I then closed off the end of the vinyl tube, rotated the tripod mount 90 degrees, as in the vertical SmartStow position, and waited. There was no sign of leaking, even after several hours.
When the carb is vertical, the fuel inlet is the lowest passage going outside; next lowest is the atmospheric vent (with the flat, circular "baffle"); and, above that is the main nozzle jet with it's tiny opening.
I don't know at what level the water would be in the bowl when the bowl is vertical, but imagine it's at least as high as the atmospheric vent, and would not have been surprised to see water coming out from that vent. I even rotated the carb another 45 degrees (almost up-side-down) and again expected water to come out of the atmospheric vent, but it didn't. The only rationale I can offer for it not leaking is that the water level is above the atmospheric vent opening, and with no other large path for air to get into the bowl, the water doesn't go out the vent.
With the carb in the vertical position, I then pulled the plug off the end of the short vinyl tube; water flowed out, but in this case it's coming back out through the tube that is normally attached to the fuel tank, so that would not be a problem. (In this case, the weight of the water trying to go out what is then the bottom of the bowl area is sufficient to allow atmospheric pressure at the vent to push water back from the vent baffle and supply the air needed to compensate for the water going out the fuel tank fitting.
From this it suggests that both the SmartStow and regular current black plastic carbs of the same design might normally (when all is working properly) drain back into the fuel tank when lifted into the vertical position, reducing if not eliminating the possibility of leaks.
However, if for some reason the bowl doesn't drain down into the fuel tank e.g., inlet valve remains closed, in the longer term liquid could seep out through the atmospheric vent if the liquid is higher than the vent. This could happen because the main jet, albeit a very small opening, terminates in carburetor's venturi air passage. Consequently, air could be drawn into the bowl area very slowly through the main jet and allow liquid to leak out the vent.
In the SmartStow carbs, the extended vent baffle opening is above the liquid level, and there's no risk of liquid/fuel leaking out that path when air finds its way into the bowl area.
The problem for B&S is how to correct the issue of the atmospheric vent being blocked by fuel when operating (my understanding of the cause of the sputtering), while ensuring that leakage in storage can't happen. Looking at the innards of the carbs and the current passages for fuel and venting, I think it could require a significant change in the design of the upper half of the carburetor.
As an interim measure **, if the mower will be stored in the vertical position, to protect against the possibility of leakage if the bowl does not drain back to the fuel tank, a fuel shut-off valve could be added to cut the fuel supply and then run the engine until it stalls. At that time, there will be little fuel left in the bowl, and the risk of fuel exiting through the atmospheric vent because the bowl didn't drain back to the fuel tank when the mower was lifted would be low.
That's the way I see it at this point, but do I have it right? Comments, further ideas etc., encouraged and welcome.
**We know from user reports that B&S has successfully addressed the sputtering problem in at least some 190 cc SmartStow engines used by Toro, so there is a fix that, presumably, retains the SmartStow function. Goal now is to find out what that fix is, and see if it can be applied to the 163 cc SmartStow engine.
Update July 18, 2021: FWIW I ran multiple "fill - drain" tests, and every time found the bowl would self-drain when the carb is lifted from horizontal to the vertical/stow position. It took about 28 ml to fill a completely empty carburetor. In the vertical position, 20 ml drained back out the inlet tube. With the carb vertical, the remaining 8 ml is at the level of the lower edge of the large hex drain opening in the bottom part of the bowl, and at the level of the inlet valve, so well below any other paths for fuel to leak out when in the vertical position.
