Still have not answered my question...
Are you applying throttle to get it up to full speed? or is it revving on its own? before the stall...
The causes for each issue are similar but treated a little differently.
If the thing stalls immediately after a major acceleration that you have applied, then the cause is most likely that your HSN is set too lean. Why is this? When you apply the throttle on an engine with the HSN set too lean, your throttle input draws fuel from the fuel tank into the engine. When you stop applying throttle, that fuel flow drops dramatically causing the engine to stall out. This is made even more dramatic if it is combined with a LSN that is too lean; however, I suspect your LSN might be just fine based on the fact that it starts right up on the first pull...
If the thing revs on its own and then dies immediately without you putting any throttle input what-so-ever, then your LSN is set too lean and your idle set screw might be a bit too close to closed. Why is this? The reason you get a huge revving and then a stall is that the engine starts up on the fuel you pumped into the engine during the priming of the engine. In many cases, this causes the engine to accelerate and then look for more fuel when that is burned up. If the LSN is too lean, the engine may not be able to draw enough fuel to sustain its operation, and it stalls. If the idle set screw is set poorly with the carb a bit too close to closed, the engine will not only starve for fuel but will choke itself into a stall due to the lack of proper air-to-fuel mixture.
To fix either condition, adjust the appropriate needle accordingly...my recommendation would be to:
1. Set the idle set screw so that you have the proper gap (approx the thickness of a credit card) in the carb.
2. Set the LSN and HSN to factory presets.
3. Return the engine starting with the HSN and then adjusting the LSN as needed.