Let me suggest this: once you've got your tach, start by setting H & L to 1 turn out a piece. If it won't run on L = 1, go out 1/4 increments until it will idle without dying. That saw spec is probably to idle somewhere in the neighborhood of 3000.
Turn the idle screw until it's at 3500.
Next, turn your L in about 1/8 at a time, giving it a couple seconds between turns to let it catch up to the new settings, do this until the RPMs stop rising and instead begin to fall - but not so far that it stalls. This is what's known as lean drop off.
Next, turn the idle screw BACK to 3500, and follow this by turning the L a bit to make sure it's still at that drop off point. Might take a try or two to get it, don't worry if it bounces 50-100 RPM plus or minus.
Once it's stable at ~3500 and also at lean drop off, turn the L screw out until it drops down to about 3000 or maybe as low as 2800. Most Stihls run at 2800 but other brands seem to me to run better at 3000+. Give the throttle a squeeze at this point and it should have a nice crisp response, if it immediately bogs down and dies, it's too lean on the H screw. Chances are it will be, turn the H screw out 1/4 at a time until it has a nice crisp response then fine tune that until it's at spec. Probably around 12k.
Also don't hold it at 12k or anything near that high for more than a few seconds at a time.
You do that and you'll have a well tuned carb - if it STILL wants to die in the cut, you can try to richen it slightly on the H but if it's not easily overcome then I'd suspect other problems in the machine. Did you put the new carb on it yet?
