[SamUK]

I tried to attach a video, but it doesn't seem to want to attach.

Try again, I've had this happen on a few occasions.
You can add a video from your mobile phone (cell) pretty easily, as far as I know if on a laptop the video needs to be on YouTube or a file sharing site of some form, I don't know for sure but every video I've seen on here has been file shared prior to uploading.
I always use my iPhone, I find it quicker and easier.

 

[SamUK]

I would personally set the air/fuel mixture slightly rich whilst running in your new build, you can back it off a touch afterwards.
If you haven't altered the air/fuel since changing the air filter etc, it may indeed be getting more air than fuel and running lean. Refer to your manual for the correct setting and go with that, remember to count the turns so you know if it was lean or not.

 

[SamUK]

Ahh ok, I have no experience with Efi ATV's unfortunately, I'm useless to you now lol.

 

[SamUK]

No problem. I'd be tempted to look at the timing again before firing it up for another test run. If it doesn't sound right, it's odds on that it's not right.