lincsman has brought this to us (Back in 2013):
I have recently fitted an adjustable Duo Flush Dudley Turbo 88 in my
toilet cistern. I can not get it to perform a half flush on a regular
basis. Has anyone out there fitted one of these and had the same
problem? It is driving me mad as I have tried to adjust this so many
times. All help would be appreciated. Many thanks.

I am having the very same issues with our two toilets, in both of which
I installed the Turbo 88. They do the short flush, using a rubber
washer, which is a lose fit around the piston rod, just below a blue
hard plastic washer then the C link. The rubber washer prevents air
getting into the syphon, if the flush is quickly released. If however
the flush handle is kept pressed, the rubber washer is intended to
allow air in to break the syphon early, to provide a short flush -
except the rubber washer seems to have no positive means to get it to
rise and clear the port.

It can manage to do a short flush, but only if the flush handle is
turned very quickly, or if it is released then repressed rapidly - but
neither method can be relied upon.

As said, the rubber washer is a lose fit on the piston rod, I would
have expected it to have had some slight contact with the rod, enough
to at least let it be drawn up by the movement of the rod.

Yes, I have written to Dudley and they say it should work, assuming the
static water level is as it should be - my two are, yet neither works
as they should.

Harry Bloomfield wrote :
Yes, I have written to Dudley and they say it should work, assuming the
static water level is as it should be - my two are, yet neither works as they
should.

Now solved....

Rather counter intuitive, but simply lowering the water lever by around
12mm fixed the issue on both of my toilets. My thinking all along was
that the higher the cistern level, the better the chance of it working
as it should on the short flush and anyway, I was struggling to get the
level just to the 9L mark on the cistern.

Harry Bloomfield wrote:

Now solved....

You mend bogs at 5am?

On 05/06/17 07:44, Andy Burns wrote:
Harry Bloomfield wrote:

Now solved....

You mend bogs at 5am?

When you gotta go, you gotta go...


--
No Apple devices were knowingly used in the preparation of this post.

Andy Burns presented the following explanation :
Harry Bloomfield wrote:

Now solved....

You mend bogs at 5am?

I occasionally have restless nights, so rather than thrash about in
bed, I get up and do something useful. These have not worked properly
since I installed them 12 months ago, despite my checking I had
properly configured them several times.

What the (very limited) instructions don't say, is that they will not
function reliably on the short flush, if the water level is set too
high.