i have a similar problem to this which i posted about yet no one replied. i cant post the URL , but i can post the whole embed link on vimeo. the sound comes from the same position as yours, to the left of the engine by the oil cap, kind of top left part of the engine. i dont think its the fuel injectors at all, yet it the chuff chuff.
<iframe src="https://player.vimeo.com/video/227113935" width="640" height="360" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe> <p><a href="https://vimeo.com/227113935">dooper engine</a> from <a href="https://vimeo.com/user69247881">john cooper</a> on <a href="https://vimeo.com">Vimeo</a>.</p>
just copy and paste the part in " hyphens.