synch problems - no vac on #3

79 kz1000st
Bike wasn't running right last year, pulled carbs off today to move needle clip and clean them. Went to synchronize and i get no vacuum on #3. Only way to get the other three up to ~20 is to back #3 screw right off. As soon as i start to screw it down again it kills everything. If i synchronized all four to #3 none even read on the gauge. Thought it might be a dead cyl but it has 120 psi and burns fuel just fine (looking through colortune ). Any help is appreciated.

Bueller?

Valve clearances within specs?

Water spray test on header pipes to confirm combustion?

Good Fortune!