That is not a good test for a pump. You must be pretty powerful to operate that pump. The test for pumps is to run output into a container during crank to see volume while pump is installed on motor. (dizzy primary lead removed) Some aftermarket inline glass and non glass filters don't stay full but fuel in carb will start motor and fuel will flow if all is well. Take the filter out of the system with a piece if tubing clamped in it's place and check improvement. If flow from pump is good and taking out filter does no good= carb needle valve/dirt, air leak on suction side or clog in line/tank or not a fuel issue at all.. Get an all metal, sealed can filter.
Is your filter before or after pump?

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