How do you adjust the deck belt on a Toro LX 420 riding mower?

Try rolling when you engage instead of in the still position may be too much on the motor, either that the belt is loose or needs to be replace. Or u need a carb adjustment at the idle.

First put the front on a jack stand on high. then lower the deck all the way down. Look at the three places on each side to remove the pins that hold the deck on Then there should be two in the front of the deck, remove them. Remove the belt from the motor pully. Lower the mower to the ground and release the peices the pins were in. Remove the clip from the tension cable and remove the cable from the deck. Raise the mover to pull out the deck. Reverse to instal
I hope I helped you

Best way to check that out is to run it for six or seven minutes and see it the number changes by .1 and if it takes an hour of running to move the nkmber you will know that it reads hours nit tenths.

Make a loop with the three drive blade pulleys, the other two are tensioner pulleys, if they are flat pulleys as they should be the back side of the belt rides on these , if they have been changed then use them as tension pulleys anyway you can, hope this helps.

Assuming you have put the blade drive lever into 'disengage' check the blade drive belt tension. With blade disengaged, belt should have significant slack. You may need to adjust the control cable, adjustment is usually on the mower deck; or you may have some debris on the deck preventing the pully to release belt tension

check the connections from the battery to starter-check the fuses -and relays than start with the safety switches --seat-clutch-and cutter deck the safety switches are a kill switch

look at the pulleys for play also how is the spring tension ?

check for play in the bearings blade bearings and the idler pulleys

Not sure if it has exactly what you need, but here is the manual for the LX425 Toro

http://lawnandgarden.manualsonline.com/mdownloads/9ca9793f-c051-456f-867e-7bc6cc815831.pdf

Check the belt on top of the mower deck. Make sure the belt didn't slip off or break.