OK, so I think it's wear. I think it's the cam rubbing against the body/resting point of the carby and over time wearing it down, hence the washer to lift the cam higher, which is also why the washers are so thin.
Unfortunately, I only had one and figured that with the problems I'm having I wouldn't have been able to resolve the issue with just the one. That said, I wanted to test it quickly and didn't have any other brass washers. So, I grabbed a steel washer which fit but was probably as thick as at least 4 of the brass washers, and the issue appears to have been resolved; There are now issues with the throttle moving and cut-off, I'm assuming because of the super thick washer, which I'm not worried about because I'll replace it with the correct brass washers when I get them.
The poppet valve was originally set to C but I moved it to A to try to reduce the amount of fuel coming in. I did that before as one of my troubleshooting steps too but obviously, it didn't make a difference, though worthwhile noting.
Thanks for your help everyone and if I swap out the washer for the brass washers and find I've got more issues or it doesn't resolve the issue after swapping out, I'll be back... from outer space!!.. haha.
