I have noticed that when my prints start and the correct filament is already loaded the printer still purges its filament, it even does it when the AMS is not plugged in and I only have the external spool active.
Is anyone aware of any custom start g-code that is available to fix the annoyance?
Yes this is the case, it will take a little while for you to get used to it 
I dont want to get used to it, I want to fix it as it is unnecessary and annoying.
Yes it it bothered a lot of people but nothing has changed. It should be easy enough to add a āNo purgeā checkbox to the print dialogs, but we wait. It used to bother me a lot, but Makerworld funding my filament has made it very easy to get used to the purge.
The g-code is well commented. With a bit of effort you could probably find the code that does the purge, then you could modify it as required.
Yes, it is an easy task to edit the G-Code for the purge, but not to determine if the filament being used is already loaded to the AMS slot or if the rear filament is loaded, for that function I think the FW and Bambu Studio would need work.
Dealing with unnecessary filament purging can be annoying, especially when everything seems set up correctly. Have you checked your slicer settings for any priming or purge lines in the start g-code? Sometimes adjusting those can help reduce or eliminate the extra purge.
This happens from the default settings.
@andr00ahayes1967 If you ever do manage to excise the purge, please do post and let us know whether there were any unintended consequences. I had always assumed it was done to establish a predictable and repeatable āknown established stateā prior to then retracting and doing its fastidious nozzle wiping. It would be funny indeed if this time consuming start-up purge ritual served no real purpose.
While annoying, those purges have a meritā¦
Same for having the option to not purge but we can only hope to a get a button for thisā¦
Letās say you did not use your printer for a few days - unthinkable I know, just humour meā¦
Then the filament in the melting chamber could be way off in terms of being of acceptable quality.
Even worse if something went wrongā¦
Ever noticed how easy you get get stuck with the nozzle at 250 degrees and unable to cancel this unless you turn the machine off ? 
The reason for the purging is that most filaments benefit from coming straight from the rollā¦
In the early days of 3D printing no one worried about purging if the same filament was used because we always had enough brim to use up whatever was left in the hotend.
I think there were lots of requests to add a āno purgeā option in Studio but so far Bambu did not do it.
Many years ago I relied on a little G-code Editor.
Was a bit like a slim version of Notepad++ with just whatās required for text editing and highlighting various codes in different colours.
Sadly it was ācalling home wareā of the shareware kindā¦
Meaning, every so often it would refuse to work unless you let it do an online check for a new version.
What can I sayā¦ The author eventually disappeared and with that the Editor stopped working.
These days Notepad++ is quite powerful, you can even record macros.
One should be able to just comment out the entire purge sequence - and while at it all those other nonsense moves done before the actual print eventually starts on the P1 series.
Too bad this requires to export and import the G-code all the time, which makes things harder than what they should beā¦
None of my other printers do this, and they are just fine, I have also noticed that when the printer scans the first layer it then does yet another filament purge, I have quite a long list of parts to be printed, however, once these are done I am going to try and negate the purge and the second purge line and see what the outcome is, I will of course record the camera output and make this available.
In the slicer preview you have a slider at the bottom.
If you move this you can see the g-code, include the ones for the purging.
Maybe it gives you a good starting point for your tuningā¦