44IDF unresponsive idle mixture screws Options

[3liter914-6]

Hi folks,

My brother's 914 has been sitting for a decade or so ever since I installed a 2270 for him to replace the stock 2L he blew'd up. Being the good brother I am I recently decided to actually install most of the parts I've been secretly buying for him over the years and after a bit of work and a lot of money on Webers and digital distributors and a fancy Tangerine racing cable linkage it's shaken off its cobwebs and is now up and running.

It fires up pretty nicely, but it's a bit rough running with some popping. I disconnected the linkage and went through the adjustment procedure per Weber's lean best idle procedure. Everything goes well until I get to the idle mixture screws where nothing really happens. Google searches tell me this is caused by 3 things:

1. Vacuum leaks
I've sprayed ether at the manifold base and at the carb/manifold interface without noticeable change, so this leads me to believe it's not the issue

2. Dirty idle jets/circuit.
I just rebuilt the carbs, even though after opening them up it seemed unnecessary. Gave them a 24 hour soak followed by blasting out every orifice and jet I could find with carb cleaner and compressed air. So I'm not inclined to think this is the issue.

Arguing against the above is that this is on all 4 carb barrels, if it were a leak or clog I would think there would be some affected/unaffected.

3. Incorrectly sized idle jet. This could make sense given that they all have the same jet and all cylinders are affected. Per the stamping they're 55s which more or less seems to be the accepted default 44IDF jet size, so it's surprising to me that would be far enough off in size to be totally unresponsive.

Any other troubleshooting I can attempt that might help answer the question of do I need different (larger?) idle jets? Anything I'm missing?

Thanks!