brush cutter carb flooding

Fuel flowing from carburetor while not being used as fuel tank pressurizes checks.
 
First make sure to use the correct fuel pump diaphragm as kits usually have couple different ones. This diaphragm is not to be confused the metering diaphragm term. Also check the gasket and diaphragm order on both sides of the carburetor. First carburetor body it is on the fuel pump side diaphragm then gasket and on the metering side it is gasket then the diaphragm.
 
Now with the fuel pump side and metering needle, meter lever, spring in place pressure test the fuel pump side. It should hold 5 psi (air). If this passes then pump side is sealed. I had only a couple carburetors over the years that had a bad needle seat to carburetor that leaked. If it don't hold the pressure then place the carburetor in a liquid to see where leak is which in your case would probably be at the needle and seat area.
 
Also verify that all the welch plugs are in place and sealed.
 
Since most of these carburetors are open on the metering side there is no real value test that you can do except on those with the built-in primer. If the carburetor has a built-in primer then you can pressure test it to see if it is leaking in reverse flow. Same reverse test on external primers. These primer check valves are what keeps the fuel tank pressure from flooding the carburetor through the discharge port. 
 
One other thought is to make sure the external primer if used is not connected backwards.
 

Then you are probably working the WT-45A (WT-45-1) which use kit K10-WAT. Walbro don't include the spring as part of these kits and is normally not needed. As for if the spring has weaken it is possible but doesn't normally happens. Walbro does not supply this spring (98-304-7, NLA per Walbro site) but it can be possibly sourced from Stihl but it will be fairly expensive to obtain. Stihl part number is 1120 122 3001. My dealer here usually charges shipping on special orders.
 
As alternate pressure test you can use a small syringe connected via the inlet side and use the submersion method looking for leaks. It wouldn't give a proper pop off test but will show if you have a seepage problem. I never needed to polish needle seat on these cubes and the use of the tooth paste may have actually damaged the seat. IF seating proves to be the problem then the only recourse is a new carburetor. Although I don't recommend a clone carburetor as I had problems them they can gotten for around $15 + shipping.
 
If you do these cubes as part of your regular business then I recommend getting a Mityvac 8500 and an ultrasonic cleaner along with the W tool. Both the Mityvac and ultrasonic are usable for other equipment too, especially the Mityvac as you can pressure/vac test the 2 cycle crankcases.