the 8 inch prop is on the edge of the abilities of the .15 to swing. That's one reason the 7 inch works better. The heavier load makes problems show more.

Often surging is a sign of an air leak in the fuel system. Watch for air bubbles in the fuel line.

If you don't see air bubbles, look for places air could be leaking at the carb, such as the fuel nipple threads, the needle valve shaft (assuming on-the carb needle) or even under the carb base where it mounts to the engine. (loosen the mounting screws, press down firmly on the carb with your thumb to compress the O-ring, and re-tighten the screws. Maybe look to see if the O-ring is even there....)

The idle mix on a dual needle type carb can take over the engine if set too lean. With the carb barrel closed, turn in the idle needle to see how far in it is. It should be at least 1 turn out from the seat. (assumes its not an air bleed carb... most of O.S.'s LA series are air bleed.)

Another thing that happens with some engines is there's a "secondary peak" when tuning the fuel mix. Occasionally an engine will redevelop rpm when its running too lean. When this happens, the lean run is VERY hot, and susceptible to surging. You can open the needle to make the engine run slppy rich and then re-tune slowly, finding the first peak in rpm, then richen the mix appx 1/8 turn to cure the problem if that is the cause.