I agree with all of the above suggestions, but here is an odd one I ran across once. I used to use thin walled very flexible fuel line in the tank from the clunk to the fuel pick up tube. Worked fine on some 40 sized engines and even a lower powered 60. But with a high powered 60 the same set up caused the engine to surge. Tried everything, none of the lines leaked the engine ran fine on the bench so I swapped out the tank with the one on the bench that it ran on. The surging stopped. I figured the flexible thin walled silicone line would partially collapse, then open when the engine slowed down, then collapse as the engine picked up, over and over again. So you might try that.