The carb on my Magnum 40 won't close down. It'll go from fully open to about half closed but that's it. This isn't a servo problem. It' seems to be mechanical with the throttle lever. The engine runs fine but can't be idled. BTW, this problem presented itself for the first time about i5 minutes my last flight when I tried to slow down for a landing. No dice! So I had to fly it around for 10 more nervous minutes until it finally ran out of gas and I was able to bring it in for a deadstick landing. I have crashed recently so maybe something got knocked out of whack.

What's up with this?

OK. So I'm an idiot. I just discovered the idle set screw and it was WAY loose!!! A few turns and things were OK again. I assume you tighten down the little nut underneath the screw head to lock things down. I imagine a little locktite would be in order to.

I really do have to establish a better routine for pre-flight (and post repair) checks.

Glad you found the problem. I was going to suggest that you check the idle set screw.
Dennis-

I don't know for sure about the larger Magnum carbs, but on the smaller ones, there are two screws, the idle stop and another screw that actually holds the carb barrel in to the carb body. If you have the two screws, you can actually just remove the idle stop screw and put a dab of jbweld or something on there to seal the hole up. One less thing to vibrate loose. Loctite is always a good thing anyway.