Topic: Odd problem synchronizing carburetors

[70CB750]

I was having hard time trying to synchro carbs with my antique Hg device.  I would get them all lined up at 22" of Hg, but the #4 would hang at 5.  Finally, I had enough - and run out of mersury:) - and  rode her to Charlie Gnarly, since he has vacuum gauges and done it before.
 
The fist attempt confirmed what the mercury showed - 1 through 3 was pretty decent but # 4 was way low.  We swapped sliders between #3 and #4 and it helped a bit and during the process the #4 eventually caught up and Charlie was able to line them all up and she runs like I never ever had her running before. 
 
What was wrong with #4 is something I don't know, The swap of slides (3 and 4) helped, but did not fix it, and I think it could be the engine getting really hot (we didn't use box fan till almost the end of tunning)  to bring the vacuum up???  Could it be a vacuum leak through the carb slide?  My carburetors have the rubber sleeve over the throtle cable adjusting nut (the one on the carb lid) missing - they were rotten bad and there could be some vacuum leaking???
 
The boots carburetors to cylinders are new and the engine has a good compression.
 
Open to ideas and suggestions, thanks!
 

[crazypj]

Couple of things, high vacuum means slide is low, ergo, low vacuum means slide is high allowing more air to pass under it.
Carb sync is the last thing you do after full service, check/re-set valve adjustment, check ignition timing, sync carbs.
 If reading is real low it's a good idea to do compression and leak down tests, you may find #4 has low compression due to wear on valves/rings, etc (one cylinder always has at least one valve partially open so you get rust on faces, bore, etc)
 If it isn't within 10% of the other cylinders it will never run right

[70CB750]

The compression test was within 5% of 150psi.  Valves are adjusted and timing spot on. I did all of that several times and rode it for couple hundred miles before trying any serious carb synchonizing.

[crazypj]

In that case, slide was set way too high
Swapping slides didn't really do anything except put slide closer to where it should have been in the first place.
 Looks like you got lucky, bike must have been stored somewhere dry 
BTW, don't worry over actual numbers, just get everything level (it's usually in the 18~25cm/Hg range if motor is in good shape)

[70CB750]

Thank you, thats what Charlie said too about the numbers. Do you think I should try to find or improvize the rubber boots over the carburetor cables? 

[ekpent]

Those rubber boots-caps are probably on there more to keep dust and water out of there which is always a good thing.

[crazypj]

The rubber boots shouldn't effect carb sync, unless carbs are worn out enough for air to pass around slides (or you have a hold down screw missing)
 They do keep the slider rods 'cleaner' though