I have no leaking in my carb, but found that the carb I got does not possibly like using a foam airfilter, it has a few miss at high speed but OK at idle. I can set the point gap and almost get it perfect, but will be trying a paper airfilter that is less restrictive and see if it does the trick. This is a Briggs 3hp and it works fine with another vacuject carb that sucks gas up from tank below, the newer one is with a float and tank above.
Relating to what your situation is with leaking fuel, I read that venting gas cap is not at fault and was checked. True a venting cap malfunctioning that causes pressure build up in the tank does have more than gravity flow pressure and would have been a problem as the float needle valve will not seal in the seat. Also with too high a gas tank height, but I don’t think that that is an issue. If you can place another tank or the one you have at a level where the outflow is even with the input of the carb inlet, then that would be the minimum height and could be tried to test to see if tank height is too high.
I thought for a moment that the height of gas tank if there is no leaking would mean then there is no effect on the richness of the mixture since the float is working. That is because the float height sets the level of gas in the bowl which is the determining pressure through the jets. If there are adjustment to the jets then that can alter the richness further. My carb has no adjustment valves and the float I would possible look to adjust the height, but I’m guessing it is fine and it also may not be adjustable as most cheap ones are not. It has been a year and a half I've had this new carn that I just recently had tried on the engine and I know it is spotless clean. I had the bowl off and seen the float but had not actually looked to see if there was a metal tab that was adjustable to tweak.
Now to relate this to your situation when you shut off the gas and it stops leaking, then there is still gas left in the bowl and you get to running the engine any speed from idle to full throttle till the gas runs out in the bowl. That I understand was mentioned by Bairdco and agree with that point. Atleast then you know the carb can run the engine fine and fixing the leak is what your zoneing in on. You have two of these carbs with the same problem is somewhat disturbing to me, but you if can check on date codes if there are codes and if there near or match each other, maybe the carbs made with defect.
I have some links from Briggs site and the PDF file mentions things about carb issues and floats that I found interesting. Only did I find one good video on adjusting a float for a Briggs engine and it was a racing engine carb that has two floats on each side of an axle of sort that controls the valve.
There are tips on carb problems and the PDF link which I also added below in the the next link:
http://www.briggsandstratton.com/support/frequently-asked-questions/Carburetor%20problem%20troubleshooting/?body_0$tbKeyword=float+adjustment
The PDF below with some good details of carbs with floats (Carburetion Troubleshooting
Quick Reference Guide):
http://www.briggsandstratton.com/su...~/media/77782F652D3E4328B82988841A4DCD32.ashx
Setting the Float Height on your Walbro PZ Carburetor video on Yutube:
This video has clear information to see
http://www.youtube.com/watch?v=VnyrRtaA2Wo
******************* STRIKE OUT BELOW..... YOU ALREADY SOLVED ISSUE ON CARB*****************************
though it is nice to know you have a quite advanced carb that has notches to adjust float!
I read about soldering a redrilling jet... I suppose if you cannot get a jet or they don't make anymore that is using the old noodle... good for you!
*** delete --> Like to know what turns up and how it gets fixed from leaking and running right.
MT