[Alvy]

A couple things/questions come to mind. Number one there are different CCA rated batteries for the box that you have but 340 is more than enough.

With a voltmeter can you tell us what the battery is dropping to under load while starting on its own? Or do you possibly have access to a battery load tester?

Can you describe how it cranks without the jump pack? Does it get to a certain point in the revolution and then slow dramatically or stop for a moment? This is a compression release engine and those symptoms could indicate an issue there.

Also static timing the engine, adjusting valves, etc. helps a lot with cranking speed. Maybe try to post a video for us to see how slow it is to crank without the jump pack, this will help narrow it down more because the description of the problem is a little vague.

List can go on and on from excess volt drop across the starter solenoid to faulty starter generator internals, pto out of adjustment still, etc.