That could be it (tank height) or check the clunk! for clogging. Chances are that test stand has been sitting around for a while.
Does it have a filter?
Is it vented?
Back flush the entire feed line and vent.
Hope this helps.

I am posting this for afellow club member who is not on the forum.  Keep in mind I have not seen his set up.  Just looking for some help for him because I am not an engine guru.

He has put 3 engines on his test stand and the only way he can keep them running is placing his thumb over the Venturi .  Once he removes his thumb the engine dies.  Since it is more than one engine that does this, I am thinking maybe the tank is not in the right place.

Any ideas?

Thanks
Mike

Most basically, restricting the air coming in with no other changes and the engines runs means he has a very, very lean mixture.

With the tank full and the carb/venture end of the fuel line well below tank level, will fuel freely gravity drain? If it doesn't, as already pointed out, clean/flush fuel tank and lines.
When the engine is running, are any air bubbles visible in the fuel line? I just has a brand new Sullivan tank/line with a micro pin hole in it, drawing air.
Can you blow air or force fuel through the needle/spraybar with whatever method used to fill the tank?

And my last idea: Having just run 3 different engines on a test stand, each with it's own run issues, I found 2 with mis-installed spray bars. Most have a single hole which MUST be 90 degrees to the airflow (or reasonably close). Any other position reduces the pressure differential reducing fuel flow.

Duff Fuel .