John - Just a couple of observations/thoughts on your linkage and return spring stuff. First, I'd suggest finding a return spring that looks more like the top of the three pictured, but at least a 1/2"-1" shorter in overall length. The installed spring you have pictured doesn't appear to be under much if any kind of tensions at the rest position. It should have at least some tension applied even in the rest position, otherwise it may not pull your linkage to the idle stop like it should.
I'd also look at the vertical rod from bell crank to carb as it appears it might be making contact with the exhaust manifold. It could be if it's coming in contact with the manifold through its range of motion that the contact/drag is preventing the linkage from fully returning to the idle position.
Last suggestion has to do with checking your bell crank and linkage for full range of motion. I'd suggest disconnecting the pedal rod at the bell crank, checking that the bell crank and vertical rod movement allow full range of the carb throttle arm without the pedal rod. If the bell crank and vertical rod action the throttle arm through idle stop to full open and full self return (with the return spring attached), look to see if the pedal rod easily still lines up and connects with the bell crank at the idle stop position. If the pedal rod is just a tiny bit too long, that could force the bell crank and vertical rod up off the idle stop position. Especially true if the return springs isn't still under tension when things are in the idle stop position.
Glad to hear the Carter 2100 is performing well.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
