You do not need a spring in the carb. The crankcase of a two stroke engine provides both the positive and negative pulses needed by the fuel pump in both the Walbro and Zama carbs. Very few of those carbs have the spring inside, and the ones that do are for four stroke engines. Don't add a spring on your two stroke app; it might cause the pump to not function, or function erratically. Just a pulse line from the crankcase to the carb is all that is needed. There are two bosses on the top of the carb fuel pump cover. Either one can conveniently be drilled and tapped for an external pulse line if your intended carb doesn't already have an external pulse provision. Do not confuse the carb primer bulb connection for being the pulse line. Let me know if you need a pic of this.

AV8TOR