Timing problem

I think I see the problem

With the piston at TDC #1 it's valves are not closed, as evidenced by the backfiring. But the next cylinder in the firing order; it's valves are closed. So when the spark goes to #8, voila it runs. However # 8 is near the bottom of it's stroke, so firing it this early means there is not a lot of energy that gets to the flywheel, and hence the throttle has to be opened way further than normal. And of course the vacuum is in the basement low.
So like said earlier; by nm9 in post #72 check the cam timing!
Or at least do a leakdown test; but I'd just go straight to the cam-timing.