your problem could be caused by a number of things

1. is it a new carb? if so may need to adjust the floats.

2. fuel pump going out is it electric or mechanical?

3. clogged/restrictive fuel filter

4. trash in carb floating around clogging jets

5. trash in tank causing loss of fuel

6. trash in fuel pump blocking fuel

7. does it have vacum or mechanical advance?

8. could have a old vacum hose or gasket leaking after engine warms up

9. and many other things could make a surge too