My 48'' Husqvarna riding mower deck cuts too low when set in the in the highest position.

Hi, I'm not sure of your issue exactly, but here's a link to Husqvarna mower manuals. You can look your model up and look under the section for the cutting deck.
http://lawnandgarden.manualsonline.com/manuals/mfg/husqvarna/husqvarna_lawn_mower_product_list.html

It sounds like when the unit is under more load it stalls out. It could be that bearings on mower need to be greased or replaced. Or it could be something internal. I would personally check compression with a compression tester. They are fairly cheap and easy to use. They simply thread in spark plug hole, you turn unit over and observe gauge. Probably a good tune up with valve setting checked/adjusted would be a good idea too.

I’m happy to assist further over the phone at https://www.6ya.com/expert/layne_0fccff498c319c32

use a volt meter test the voltage when the engine is off then start it up and see if the voltage goes up it works

Place your mower on a hard flat surface ... your garage floor if possible. lift the deck to the transport position. Get a tape measure - two if you can. With the engine OFF and the KEY out of the ignition, and the SPARK PLUG WIREs disconnected, move the blades 90 degrees from the fore aft axis of your tractor. Measure the distance from the floor to the outside tip of the blade. Make the deck level by adjusting the hanger.

Next, move the blades to the fore and aft position and measure again. Adjust fore and aft until the blades are level.

Double check aide to side ...

Double check front to back.

Lower the deck and check again both directions ...

You shuld be able to get your blades to with in one sixteenth of an inch of being level.

Of course, your tires should all be properly inflated BEFORE you start any of this adjustment process.

I hope this helps you ... it is laborous ... but you can do it. I did!

Thanks for your interest in FixYa.com

http://www.askmehelpdesk.com/tools-power-equipment/husqvarna-lawn-mower-deck-belt-doesnt-fit-381569.html

That link has a diagram that should help.

my husqvarna lth 120 will not move backwards or forwards

Well I guess laying down next to it and taking a look under the mower deck is beyond the question. One could ask about how many teeth the horse has or just look in its mouth. However, I am going to guess 3 blades as 2 blade units usually are 42" cut maximum. Mine is 48" and has 3 blades.

When you put it in the highest setting can you see the deck actually moving in an upward direction? If not the linkage might be broken or damaged. When you raise the deck up or down with the lever it should move, a deck setting in the full up position is a pretty high setting you have to have some long grass to be cutting in the full up position and still think its to short. You might be able to reposition the deck, if when you look under the unit you see a bracket that controls the deck height setting that has a number of holes in it(maybe even 2 brackets), there should be a pin(s) in one of the holes this could be set in a low position and therefore you can change position to change height of deck.

Make sure the mower rake angle is not too much,On level floor point one of the blades in a front rear orientation,measure height of blade from floor the front measurement should be no more than 1/4" lower than the rear measurement.If the front of the mower is too low the angle the belt has to run is too much and the belt will twist and wear rap[idly.To raise the front of the deck there are either one or two adjustment rods that attach the front of mower deck to frame tighten this adjustment up to raise the front of mower deck.

Is this the first time cutting grass with a lawn tractor?
How tall of grass?
Use the grass cutting heigth adjustments on the tractor to a higher setting when cutting tall grass. Cut again to a lower height setting on the second pass. Grass higher than your tractor is not good for your tractor. Follow the manual instructions for best results.