Run them any way you want – one weight, two weights, or no weights. However you must run each primary weight with the same amount of weight (inserts) in it as all the others, or yes, you will be out of balance and would get uneven pressure on the friction discs. And obviously you cannot remove the primary weights, otherwise you will be going nowhere fast.
You don’t need anything else in the hole with the tungsten. The gap is not as great as you describe, but in either case it doesn’t matter, because there’s isn’t enough room between the weights and the speed collar for it to come out, and the gap narrows as the weight goes up the ramp. In other words … don’t worry about it. ;-)
Yes you could loosen the rear and that might take care of it. In a roadrace kart two races can be the limit on front tires, depending on the track; as always, it could be a combination of factors.
Been a while since I ran there, but for a ratio, 14-79/80 will get you close.
With this ratio I think you’ll want the yellow (not black) springs. Or vice versa, not a Horstman guy. Whichever springs are for the lower than usual ratio.
