Gord Trail and I finally got his surging problem figured out. Couple of problems and they are all the usual ones
Go Figure eh
Anyways, start first with the fuel system. Check that your fuel strainer assembly is clean and there is no dirt in the screen. As was mentioned, check the main metering jet and ensure both the large orifice on the side is not plugged nor the small orifice on the end. (this one was plugged slightly on Gord's). It does not take much dirt to clog it. Then, check the inlet screen and look for teflon tape particles. (another problem on Gord's carb). Most of the white teflon tape IS NOT meant for gasoline applications.
If this is clean, then make sure that the side of your fuel chamber is not bowed in. If it is, then most likely the float is rubbing against the side and possibly causing fuel starvation. If this is the case, then crack the carb and while you are in there check the float for holes and all the passageways for dirt and stuff.
Also, check your rpm's and ensure that at slow idle it is around 400 to 600 rpm depending on where your Cub likes it. Ellie likes it at around 475 to 500, Gord's Cub likes it more in the range of 600 - 650.
If that doesn't solve it, then probably the main culprit is in the governor. There are two places to adjust the governor. One is the bumper spring tension and the other is the spring that controls the weights inside the governor. They may need to be adjusted.
NOTE: To be safe, make all adjustments to the governor with the engine stopped. YOU DO NOT WANT YOUR HANDS TO GET SMACKED WITH THE FAN