An obvious thing to do is to check for stiction.
And on that note, don't lubricate your slides. The oil will just pick up very fine dust and get gummy with the heat of the engine. Might be possible that the PO did and it's causing you some grief.
Also, if the fuel flow ( jetting really) was different between the two carbs, I'd expect a lot of vibration, as the cylinders would be out of sync. Your problem, as stated in your other post, is that the engine gives up holding revs in fifth gear. Seems like a fuel delivery issue to me (as opposed to metering). Have you taken off the float bowls and measure how much fuel you get out within a minute of opening up the fuel tap? I don't have the number off hand, but you can find out if enough gas is getting to your carbs.
Could be a collapsed line, clogged petcock screen, pinched fuel line (including under the airbox), poor tank venting, etc. It just doesnt seem right that a bad diaphragm or difference between carb jetting would let you get to 70MPH and hold it for a minute or two before it gave you some real problems.
