Toilet won't flush without holding down handle

I recently replaced the flush valve and ballcock for a customer with a Fluidmaster kit - have done this many times before.
Now the toilet won't flush completely unless you hold down on the handle to let all the water out of the tank. The Fluidmaster had an adjustable cup - I replaced it with a regular one, and also tried one with the foam insert with no change. The toilet is an Eljer (I think), with the foam liner in the tank. Water level is set to the correct height - can't go higher with the fluidmaster design.
Anyone have any ideas? Customer is getting unhappy. Replace toilet?
Thanks for comments
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Well, the foam liner is going to reduce total available volume. Was it used because of condensation due to a leaking flapper valve?

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It will reduce the volume, but the flush should work once the stopper is lifted and the stopper should stay up. Perhaps it is not lifted high enough or some other adjustment is needed. Ed
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Yeah, it sounds like it's sinking, and therefore closing too quickly. I've got a Korky that will close as soon as you release pressure on the flush valve, but that's the way I wanted it to be.

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wrote:

Shorten the chain so that flapper is pulled high enough such that flotation forces exceed suction.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
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