If solder wasn't sticking to a brass jet, either you didn't get it hot enough or you didn't get it clean enough. You need a good flux to clean it while it's hot - cleaning it when it's cool doesn't help because it will oxidise while you are heating it.
It sounds as if you found where your plastic sleeve was obstructing one of the passages of the carburetor, and cut it away to fix that problem. The remaining problem now is that the soft plastic sleeve probably won't last any time at all, submerged in petrol.
At least you are making progress, even if it is almost certainly temporary progress.
