You use a new plug because an old one has all the burn characteristics of what it was doing before the mods and after. A new plug will only have the characteristics of what it's doing since the new mods. If it is running lean, a new plug will show a white looking tip. If you are using the old plug, the tip already looks like what it has been since it has been in there and you can't really tell if it is running lean (or rich for that matter) because of the mods.
None the less, if you are stepping on the gas and it is falling on its face, you are going to have to pull that carb off and pull that main jet to make sure it is wide open. I suspect there is a piece of trash in it.
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