Just rebuilt a briggs engine and it runs at low speed but loopes at high. I have rebuilt the carb. Adjusted the valves etc. It spits some through the carb. I set the valves at .006 in. and.008 ex.

Sounds like you could still have a fuel problem on high jet.
When one runs smooth at an idle and surges on high speed.main jet problem.
Re check your valves check your manual I thought both were 004-006