Well first the engine quitting in vertical is the high end. you have it too lean. But don't mess with that since your low end is not adjusted right. First what type of engine is it? I will give you a brief low down to setting the mixture:

First you have to adjust the low end. Unlike glow engine the low end on gas effects the top end. You can actually have the low end very rich and the high end very lean and the engine will run but not very good.

Reset low and high to factory setting if you know them
If not open the low and high end 1 1/2 turns after you close them completely.

Start the engine and let it warm up and then idle at 1000-1500 rpm. Quickly move the throttle to 2500-3000 rpm. JUst like glow if it quits the engine is lean. If it saggs or popps then comes up you are rich. I would try to get the low end as lean as possible.

Once the low end is set work on the high. Lean the engine until you get peak rpm and then adjust 200 rpm rich of peak.

That should fix your problem

Chris