Start the engine the put your finger on the governor and manually increase the throttle slowly till the engine is doing full revs
Listen for the engine missing lean.
If it does then make up an altrnative fuel tank.
IF you are going to succeed in this business a secondary fuel supply is an essential tool and there are plenty of old tanks around.
Most workshops will have one on a stand or hanging from a rafter .
I use an old Victa pro walk behind tank as it holds a gallon and has a combined tap & filter in it.
Before that is was a Tecumseh tank for the same reason.
It does not need to be big, one out of cheap Chinese monkey bike will do just so long as it has a 1/4" fuel tap and holds about 1 pint.
Hook up alternative tank and the problem goes away = fuel supply problem.
Drop the tank to floor level and problem comes back = fuel pump problem.
Takes all of two minutes saves many hours