Carb sync issue

I would check to see if the cables are moving equally at both carbs. Measure the exposed cable with calipers at the carb and compare. Do the measure going from idle to 1/2 then measure both. Also from full to 1/2. The measurement must be exactly the same. The return springs will help to remove cable slack keeping them equal.

First I'd do what Walt suggested on cable movement check. Then most of us never use 2500 rpm to sync the higher rpm anymore. It's just too much in the idle circuit. Most of us use 3300 - 3500 rpm to sync. When I used to sync at 2500 rpm the gauges did split as the throttle advanced. That all stopped when I started using 3300 - 3500 rpm. Then the needles on my gauges stayed in sync as the throttle advanced.  I also don't set such a high idle rpm, but that's up to you.

Do you set the idle stop screw the same on both carburetors when checking idle, or do you do separate adjustments?

Going to check all settings one more time today. Thanks for the tips!

You set the idle stop screws equally when doing the mechanical sync, but you may need to adjust one or the other during the pneumatic sync to make the gauge readings equal or to adjust the idle rpm.

Thanks Roger! 
I don't know how many times I have read the Line Maintenance Manual and checked the videos her on the site... and yes, all I could see as a logical step was to adjust both the same amount when mechanical sync (did not hook up the hoses for idle check while still on the mechanical sync step). Engine runs good, easy start, no stumbling at all.

After a longer test, I was ready to hook up the CarbMate. Did the "option 2" for connecting the hoses from the LMM. Engine runs a bit rougher as noted. I only needed some smaller adjustment on the idle stop (within 1/2 turn) to get balance at idle, this time around 1800rpm (I am not the pilot.. just responsible for the maintenance. Asked him, and 1800 is good at idle).

At full throttle, I got 5200 rpms, the propeller is a ground adjustable... and I think the previous owner set it coarse to fly the airplane to our base, still as expected. Runs smooth, all pressures balanced.

While slowly retarding the throttle, around 4000 rpm, the right carburetor is drifting, and it is always too high. After some adjustments, I can see some differences but not enough. I really can't get it down enough, even tried to lift the other carb to make the meet in the middle, but no cigar!
It is almost like the carb is changing the setting by itself... haha (I soon need a beer).

Gave up.. took it back in the hangar... have reset both back to mechanical sync again... and everything seems normal.

Previously I found the right carb to be a bit loose in the rubber mount, suspected vacuum leak, witch may explain the higher pressure.. but after wiggling and checking everything, it is now snug tight (ordered new ones with o rings still)..

Don't know what to do... I really fear the next pneumatic balance operation.

To sum it up: i am able to mechanical balance the carbs, runs good. I am able to adjust idle rpm as expected. I am able to pneumatically balance the carbs at idle. I see balanced pressures at high rpm, but between 3000-4000 it is all gambling......