Engine not getting fuel

Hi. I’m running an engine on my test stand, but it’s got a problem with not getting fuel.

When I try to prime it by covering the venturi, I see fuel coming up the line to the inlet at the carb on the compression stroke, then it goes back down the line after compression.

So it takes more turns to prime it. The engine will then start and run, but only till the fuel in it is burned, then it dies.

I’m using a starter motor to start it.

What could be causing this?

It’s a Super Tigre G75, that’s had new bearings and a new piston ring.

Could the carb be needing cleaned, or something else?

I will put in my two pennyworth
In a normal set up there is nothing physically stopping the fuel flowing back from the carb so it will tend to flow back.
As the engine won't keep the fuel flowing my first guess is an air leak where the fuel tube joins the carb.
My second guess, and less likely, is the tank fuel level is so low such that the carb vacuum from the venturi is not strong enough to keep the fuel flowing.

Sounds like a air leak or a blockage, any bubbles in the fuel line?
Try taking the needle out and blowing through the spray bar.

Thanks for the suggestions. I’ll try some of those and see how it goes.