[Y-Blockhead]

I don't think you are missing anything. I have never dealt with a Tillotsen carburetor but Marvel Scheblers tend to warp also.

What I have found to work is to get the surfaces as straight as possible.

Then I use a softer material for the gasket, instead of the paper ones provided in the kits. I cut my own gaskets from 1/32" thick cork/buna-n gasket material.

Lastly, ensure you do not over tighten the screws. I tighten the screws until the lock washer just flattens, no more.

Hope this works for you.