need advice on what could cause this slight engine problem

The engine's way lean. Without worrying about the timing curve (which I agree is important, but not posted) I can tell you the 750 is out of the box way lean and you should be able to run any spring int he secondary and a 4" arm engine won't stumble. So without knowing the timing, I'd say...
- make sure the timing is at least 12° initial, preferably more like 18°, but you don't want more than 32° total.
- Up the primary jets to 78s to start, and I think you'll end up at 80s in the front too.
- Make sure the power valve is where it needs to be. The 6.5 supplied in the carb is probably too low depending on the camshaft and compression. So get a vacuum gage at the parts store and see what you are pulling for vaccum in drive at idle. Then get a power valve rated 1.5" lower.
- Up the secondary jets to 82 and see what it does witht he rest of the changes.
It should pull cleanly to around 5500 if you have the cam and heads for the stroke.