This is a Walbro WTEA-1 carburetor which is an air head (stratified) version. Tuning is different than non air heads so yes they tune richer on the H side. This particular uses spring loaded limiters which has to be fully depress to release the mixture screws normally. It sound like busted the limiter. They are replaceable but are expensive. Also note if you attempt to rebuild this carburetor you will the new W tool as the old W tool will set the metering lever wrong. PN 500-528-1
The basic H setting is 2-1/2 turns from seated and L is 1-1/2 turns.
As far getting the carburetor off it isn't that hard but you get a copy of the Husqvarna 455 Rancher 460 Parts and Service Manual. It will instruct you on how the remove and install the carburetor.