I have a pre-1994 wall-mount toilet, and it has low consumption (1.6) diaphram with a Sloan side mount flush control. The problem is you have to flush twice to get everything removed from the bowl. Sometimes--even a third time is necessary. Do I have to replace the (1.6) diaphram with a (3.5)? The toilet itself is an American Standard with nothing but that written on it.

Yes. The diaphragm must match the consumption rating of the bowl.

will holding the flush control (as opposed to press and release) make the toilet flush longer?

The standard Sloan Flush Valve is 110, but you mentioned a "side mount" application. If I remember correctly.. those closets were popular in the 40's and were equip'ed with 3.5-4.5 gpf kit (A-36-A). Using the A-19-AC Relief Valve.
Using any other kit would produce less flush waters and require a second or third flush.
Best Wishes

If the toilet is pre 1994 is is unlikely a low consumption toilet. The valve can be low supply which will connect to a high consumption but the effect would be low performance as the high consumption unit can consume all the valve can put out. For high performance use a high supply with a high consumption as they were designed to work together that way.



Edited 2 times.

Also the diaphram Sloan A-56-A or the A-156-A has no measuring effect on the flush or the amount of water. What regulated the flow was the A-19-AC and the refill ring's position on the relief valve, back then.
Best Wishes