That was, indeed, part of the problem, and it got me 150rpm. At the choke-side end of the secondary throttle shaft there's a 90-degree tab that bottoms on the carb body when the secondary is closed - I twisted it up a bit and dropped another 150rpm. Finally, comparison with a good shop tune-up tach suggests the cheapie 2" tach in the truck reads 200 high at idle - the indicated 1050 was actually 850.
The 300 now idles at a seemingly accurate 500. Spec calls for 600; I'll try to get the difference back from that tab on the secondary.
