i assume when you say rod with the spring you mean the accelerator pump and not the needle valve. the bic pen spring trick supposedly makes the spring stiffer and makes the accelerator pump give its squirt a little sooner, never made much difference on mine. the pump rod not sliding smoothly should not cause what you are describing. if the accelerator pump in not working correctly you will get a flat spot when you jab the throttle but if you accelerate smoothly it should rev right up. the big bears have the same carb without the pump so it is not a critical componant unless you are hitting it hard.the exhaust port should not cause an issue short term but you need to get it fixed or you can ruin the exhaust valve. if you still have the parking brake find the 2 wires that run out of the brake lever and follow them down under the hood and unplug that connection, the switch sometimes fails and the cdi thinks the brake is on and enables the rev limiter. i doubt that is the problem because you say top end is not effected but it is a good mod as it serves no real purpose and can cause lots of issues. i am a little confused on the time line, did you put the zoom on and it ran fine until you stored it or did you put the new carb on after the long term storage. either way everything you describe makes me think plugged pilot circuit. my suspicion is you are going to need to rebuild your brand new carb. i am not sure what else it could be