Does it just miss a beat, slow down, quit entirely?
Are there ari bubbles in the fule lines, foaming in the tank, holes in the lines, tank pressurized?
Does it do it on the gound and/or in the air?
Does it matter if the nose of the plane is high or low?
Is there a kink in the klunk line in the tank due to a sudden stop landing which caused the clunk to flip forward.
What kind of engine is it? What prop? Tank pressurized?
What kind of plane is it in?
Did it ever run properly in the present installation? What (if anything) was changed?
Have you changed the glow plug?
Are you sure the fuel is fresh/OK/ suitable?
Is there an in-line filter? Is it dirty/obstructing fuel flow?

Lots of questions but you haven't told us much yet to start with.

Oh yeah, is it too rich?