Hey Dan, The things I've read as suggestions in this thread are to retard your timing a bit, perhaps down to 23 degrees, and to check for intake leaks.
Another thing to look it is do your mixture and throttle cables move fully to the stops on the carburetor in each direction?
I think technically it is possible to check flow in a carb on the bench, but you probably need something that will move a measured amount of air through it as well, which is probably going to be quite a job to construct.
You might have mentioned this, but are you running 100LL? Is the fuel "fresh"?
And as usual, Kitplanes has some good advice in this article:
https://www.kitplanes.com/carburetor-maintenance/
I’ve tried both 25 and 20 deg timing on both, and encountered thermal runaway in both cases (up to 1.5 dF/sec CHT rise). I had fresh 100LL in the tanks, nothing mixed in that I know of. As for intake leaks, the engine idles smooth at approximately 12” MAP with the throttle closed, and I have SDS intake flanges installed. Both throttle and mixture controls were verified to go to both stops.