How do I know if I need a carburetor sync?

I got an 83 KZ750 this past week that hadn't run in a few months. Cleaned out the 4 carbs and after some starter fluid and playing with the choke, I got the bike to run and rev consistently, however if the RPMs dropped to below 3000, it would just die. When the bike was above 3000 RPMs, the left two cylinders would run great, however the right two appeared to only be getting gas sometimes and were spotty when running (you could hear it and see it). Is this a carb issue? I was told the carbs needed to be synced.

It's not a sync issue. Your carbs are dirty.

I cleaned those carbs thoroughly. It can't be that. Bowls, needle, jets, all the brass parts, diaphragm. etc. were all gone through and cleaned out.

Did you remove the pilot jets and main nozzles to make sure the orifices are open? Did you remove the pilot screws to make sure all the passages are open? How far open are the pilot screws?

Also realize that the airbox and air filter needs to be installed or the bike won't run well if at all.

The pilots are very small. You have to make sure they're clean with a guitar string or wire brush strand. A sync is the last step after all else has been done like valve adjustments. If you take out the carbs they must be synced again.
Steve

Maybe do a rough 'bench synch' first then balance them using vac gauges, to avoid any possible gross maladjustment masking any other problems..

You know when it is right as you get that 'whoom whoom' sound back when blipping the throttle.

The idle circuit(s) always block first, as the orifices are so small.

Make sure you didn't miss some hidden jets. Sometimes there are jets on top of other jets, and sometimes there is a jet hidden under a plug.