Couple be a couple of issues. Lets start with the plugs. What number are they? Stock for a cb750 are D8EA. They should not be DR plugs as those have built in resistance and the plug caps already take care of that.
D9 is a colder plug and d7 is a hotter plug. You would not use a d7 for anything really unless you are running in cold temps. D9 is good for long distances or hotter temps. By temps, I mean ambient air temps, not motor temp.
Also, it is more common for the inside cylinder to be hotter than the outside. The outside cylinders are exposed to more cooling medium, i.e. the fins.
Now, what do you mean by "needle Seat"? the needle is mounted in the slide and so there is no depth change. They then insert into the main jet which screws in. Unless somehow the thread got tapped further into the jet holder, these should not sit at any appreciable difference in height. Now, if you got needles from a 77 or older, those had slots on them to change needle height.
When assembling the carbs, you bench synced them right? You basically take the smooth end of a 1/8 inch drill bit and set it in #2 and set the idle knob so that the carb, when closed, just barely holds the drill bit. You then set the other 3 carbs to do the same. Then you should also set the idle mix screw on each carb to manual spec. This depends on if you have the K or F model. I think 1.5 turns out from a lightly seated position for the K and 1.75 for the F. I think.
I should also ask if the carbs were thoroughly cleaned. Did you manually pull out the idle jets and clean them? If not, they are most likely still dirty. Carb sprays and air rarely get them fully clean. When you clean a carb, it should be clean enough you would drink through it. This includes the accel pump and housing.
Float height should be set to 14.5 mm +-.5mm. This is done by holding the carb at an angle so that the float closes BUT so it does NOT push in the pin on the float needle. Final height check can be done with 4 clear fuel tubes hooked to the drain nipple and turned up. Then crack open the drain screw and the heights should be virtually equal.
From here, I would check into your ignition. I assume you still have the points, so I will go by that.
Point gap should be from .012 to .016 or so. Again, check manual but I think I have the right numbers. When I ran points, I set it to the lowest number, .012. Reason for this is that these systems are weak. Yes, reliable but weak. Keeping the point gap lower helps with dwell time, also knows as coil saturation time. A more saturated coil gives better spark energy, at least till full coil capacity, then it just bleeds off as heat. That is not a concern with points though.
From there, you must make sure your timing is accurate on static timing. I use a light strip for this that goes out as soon as the point opens. This is good for static timing. You will want a timing gun for high speed timing, mainly to check to see if the advancer is working right. I would also suggest getting a dwell meter with tachometer. Not required but very handy. This will make sure your dwell is at maximum.
Again, these old points systems are weak in terms of spark energy, so maximizing dwell is critical.
The secondary use of the dwell meter comes with the tach for tuning the idle mix of the carbs. The bike tach is terribly inaccurate for this. You can do without this if you want, but best performance will be had with it.
There is more but this is enough to start with.
Oh 1 more thing, if you did not get new spark plug caps, check the resistance of the old caps. Should be about 5000 ohms. Much higher and replacement is in order.
The ride IS the adventure. The destination is just to get gas!
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