Save the OEM carb if it is OEM. Clean it a few more times. Keep it for a backup.I have an 18TwinII on my Case 446. I cleaned the carburetor 2wc and it was still surging. Then the fuel pump started leaking so I bought a new carburetor and now it runs great. The original had 3 bolts on the fuel pump and the new one has 4 so either will work. I'll look and see if I can spot that spring on my engine.
slomo