My Magnum XLS .46 runs great and strong great top end great idle however when coming back from full throttle the engine takes about 20 seconds to "settle" into an idle. Why is this?

Idle is lean

So unscrew the lowspeed a tad?

yes. If that doesn't do it, it's lean because of an air leak.

Denis

yeah it sounds like it is running a touch too lean on the low speed mixture screw adjustment.
You also want to make sure the carburetor is pushed down as tight as you can push it before you tighten up the screws holding it in. You want to compress the O-ring on the carb to crankcase fit. It is easy to not push hard on the carb to get a good seal there.

I ran into a problem in the past. I had a engine that would run good and idle good on the ground. But after flying it for a while in the air, it didn't want to idle anymore. So after studying the problem, I determined that the engine had heated up good from flying around and started to leak or suck in air past the front bearing. I wound up installing a new front bearing where I left the front dust cover ot seal in place to help stop it from sucking air.

I had run the engine WOT on the ground for several minutes to simulate flying (not perfect I know), but then the engine would exhibit the problem. So after checking all the other candidates for air leaks, that left only the front of the crankcase where the crankshaft went through the bearings.

On another engine that had a oil groove going forward almost all the way to the front bearing that didn't work from day one. I had to fill in the groove with JB-weld epoxy and let it cure real good. Then reassemble the engine and that helped it a lot and it ran Ok afterwards.

Thanks for the detailed response! I will have to wait until I have a plane for the engine as I crashed the plane recently (not because of the engine) haha so I'll have to wait till I build this other one.