It sounds as if there may be some restriction or air leakage on the suction side of the primer, and it would be worthwhile making a couple of simple checks.
First, try loosening the fuel tank cap and see if either the primer behaviour or the throttle response improves. If it does, you have a tank venting problem, which is not unusual and can be fixed.
Second, check the condition of the filter built into the fuel pickup on the end of the suction pipe inside the fuel tank, making sure it is clean, and check whether there is a split in the suction pipe. I have read that the BG55 blowers are prone to having trouble with aging of that internal fuel pipe - it may crack and suck air, especially if it is an early production blower with black fuel hose (later production seems to have green fuel hose). You may need to block one end of the hose and blow in the other, to see if it leaks.
