DT2 Losing power, very slow.

[juxtapos]

I aquired this DT2 as part of a package. I already have one that runs fine but this one has about a 3rd of the power.
The strange thing i noticed that if I turn the fuel line off, it somehow solve the issue until the carb reservoir inevitably runs dry.

But for a 30 seconds the motor come up to the same power levels as my other DT2.

So its seems as if starving the carb of petrol pressure alleviates the problem.
Any ideas as to what this could be?

[boats.net]

Remove and clean carb, check fuel pump for ruptured diaphrams.

[juxtapos]

Remove and clean carb, check fuel pump for ruptured diaphrams.

Have done. But there are no diagrams and no fuel pump as this has a built in tank.
Very small engine.