Our triple tandem has a 1 1/4" Chris King threadless headset, and a looooooong stem that I can no longer captain. I find plenty of 90 mm adjustable stems that would do the trick, but they are all 1 1/8". I figured the solution would be simple, a 1 1/4" to 1 1/8" adapter/riser, and between that and the adjustable stem I could ride as verticle as I deemed neccesary.
Searched at least 10 pages of Google hits, and found nothing.
Am I going to have to fabricate a custom stem, or is there another simple answer I am neglecting to think of?

I have an old C'dale M700 and had the same stem problem as you. I finally gave up looking, then one day accidentally discovered that Santana Tandems makes 1.25" stems. I bought mine here, (click on "Stems" in the left margin to jump down to them):
https://www.precisiontandems.com/catframepart.htm

Thanks, looks like there's plenty of options!

Looks like for the increase in height and decrease in reach I am shooting for it will have to be a shorter steeper angled 1 1/4" stem plus a 2" extender, rather than a converter/riser and an adjustable stem.
Both methods seem expensive and overly reliant on excessive clamping, but what can you do? Beats putting titanium in my cervical spine though!

"Beats putting titanium in my cervical spine though!"
But think of all of the weight you'll save over stainless steel!

That's why I have'nt had the surgery yet...I'm holding out for carbon fibre.
The catch is that the less hardware they put in, the more dramatic and long lasting the external fixation....like bone grafting/fusion only w/ no hardware...but a screwed into place halo! And a solid YEAR before fusion takes place if it indeed DOES!
Ape hangers rule.