Side by side urinals not flushing correctly-Please Help

I am refurbishing the plumbing in an old bank building. In a Men's room I have two identical commercial wall mounted urinals right beside each other. The one on the left flushes completely, and the one on the right does not, and when flushing the left urinal water backs up a lot in the right urinal, but a small amount visa versa. I can flush the left urinal for five minutes continuously and it keeps flushing. At the same time water in the right urinal comes up but it never overflows. I removed the right urinal and snaked out the drain. There does not appear to be any blockage. For the life of me I can't figure out why this right urinal will not flush completely. (And yes I did verify that the diaphragms are the correct 3.5 GPF and working properly.)
Has anyone encountered anything like this or have any idea why this is happening?
Thanks for any suggestions.

pull the urinals and inspect the internal p-trap for cigarette butts or a dead mouse...whatever
snake the drain,,,and measure where the snake hits the vertical drain.
cut the wall, cut a clean out into the vent
patch the wall and install an access door, for future cleaning

Thanks for your reply. I believe the vertical drain is centered between the urinals. When I had the right urinal removed I ran my snake into the drain which was going to the left. I did not encounter any resistance till I could hear the snake hitting the drain of the urinal on the left. Could not get the snake to turn down the common drain. My guess it they used an equal cross centered between the urinals and that is where the drain and vent are located. The easiest thing I will try next is to run a snake down the vent from the roof just to make sure the common drain and vent are clear. This is on the third floor so the vent for these urinals is only about 8' from the cross to the roof. If that doesn't help I'll be pulling right urinal again and checking the drain more closely. Since these fixtures are probably 50 years old could mineral deposits inside the urinal be a possibility?

there is no telling what has been flushed in the urinal
if you can not clear from above
this access door can be glued to the sheetrock, homedepot,,,

I hate to mess up a tile wall (the backside is an outside wall 3 stories high, no way on that) but that thing is pretty cool.

lol....stop being afraid.

take a 4 1/2'' grinder with a masonry blade, and cut that tile
when done, glue the door over the hole,

wear a dust mask,,,
safety glasses
gloves
steel toe boots
full face shield
3 point harness
retractable laynard
contact the hospital to be on stand by
and locate a fire extinguisher

as you turn on the grinder, yell '''you gotta have D -struction before you have Con-struction''

LOL!!

Frodo is right you said your refurbishing anyway open the wall above/between the urinals you will find a common vent install a dandy cleanout.....snake it then frame in an access panel for further maintenance...while your at it change the 3/4 stops....and all flushometer parts so your starting with everything new....

Someone came in ahead of me and did all that. They installed some 20 flushometers changing over from Sloan to Zurn. Unfortunately for them they did not take into account that these old fixture use a ton of water to flush properly and they installed 1.5 GPF valves, so the situation I came into was that none of the fixtures were flushing completely and the place stunk to high heaven. I replaced all diaphragms using 4.5 GPF on the toilets and and 3 GPF on the urinals. That solved the issues with all the other fixtures except this one stinking urinal.

David40 said:

Someone came in ahead of me and did all that. They installed some 20 flushometers changing over from Sloan to Zurn. Unfortunately for them they did not take into account that these old fixture use a ton of water to flush properly and they installed 1.5 GPF valves, so the situation I came into was that none of the fixtures were flushing completely and the place stunk to high heaven. I replaced all diaphragms using 4.5 GPF on the toilets and and 3 GPF on the urinals. That solved the issues with all the other fixtures except this one stinking urinal.

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have you tried snaking from that common vent????

the only other way is pulling the urinal in question and snaking it from there put a small bend in the cable...and just bump the trigger or pedal...sometimes running the snake in reverse will do it just don't force the cable nothing good happen when you do that

Geofd said:

have you tried snaking from that common vent????

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No, not yet, I am going to do that on Monday.

David40 said:

No, not yet, I am going to do that on Monday.

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ok let us know