There are a lot of specifics missing to go into too much detail, but generally speaking, "surging" or "hunting" is a result running out of fuel. (or air to emulsify and deliver fuel) What generally happens is this: the carburetor has a bowl on the bottom that holds the fuel that will be delivered to the combustion chamber, The carburetor has a series of passages and orifices the control the amount of air and fuel that get mixed together and delivered to the engine. Generally, there is a "high speed circuit" and a "low speed circuit". I would be willing to bet that the "low speed circuit" on your carburetor has an obstruction in one of those passages or orifices. If you can create a consistent failure pattern it will help you to diagnosis the problem. If you are able to make the engine surge, you should be able to watch the carburetor. You should see the throttle plate opening and closing as it hunts. There will be a rod/link attached to that throttle plate that should attach the other end to the governor arm. while it is surging, gently hold the gov. arm still (not at high rpm but low) If the engine falters and stalls or almost stalls, then the fuel delivery system is probably the culprit. If while holding that arm still, the problem seems resolved, then the failure may be in the governor system. There are a lot of other things that play into these situations too: fuel pumps, air filters, spark plugs, fuel filters,etc.. and the list goes on. But hopefully this will give you enough to start.
