The left side blade on my YTH21K46 is vibrating and is chopping up the lawn, it seems to be very loose when I inspected it compared to the left side blade. Is this a bad bearing or bushing problem?

Stephen Bent/damage blade or spindle, check your deck for support link pin missing, damaged or rusted out brackets, blade spindle loose/mount damages or rusted out, missing deck support spring,wheel/roller if equipped, damaged blades, Recheck tire pressure and front steering axles for bent/damage, etc.
When measuring deck for level, measure at blade tips not the deck on a flat level surface. Check the attached links,instruction and guides, Good luck
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Hi Randall, The link below is to a web page with a few different free husqvarna riding lawn mower manuals. I would've give you the correct download but you didn't mentioned the year, Anyway this should help you.... CHEERS..
https://www.google.com.au/search?q=Husqvarna+Riding+Lawn+Mower+46+Cut+Kohler+Yth21k46&oq=Husqvarna+Riding+Lawn+Mower+46+Cut+Kohler+Yth21k46&aqs=chrome..69i57&sourceid=chrome&ie=UTF-8#q=user+manual+for+Husqvarna+Riding+Lawn+Mower+46+Cut+Kohler+Yth21k46

Exsessive vibration is normally attributable to the mower deck. If the vibration is occurring while the blades are engaged and cutting this would indicate a need for further investigation. Check the blades for damage or uneven ware. If the blades are new or appear in good shape check the mandrals and shafts to ensure they are not Brocken or bent. You can get mandrel assembly's for around $60.00 online and replace all you need in one go.

vibration in a walk behind mower is most always a imbalance in the blade , there lots of things that cause this , from loose blade , to bent blades

remove blade and test with blade balance , there not expensive ive seen them with a sharping stone for less than ten dollars before

pull spark plug before you remove blade 4 safety

when you actuate the "engage blade" lever, two things may occur,
First, a mechanical action, a spring moves to put tension onto the belt.
Second; at the end of its run, actuate a switch (PTF switch) to engage the clutch that moves the pulleys that consequentially move the blades.
if the tensioner is not moving , check if spring has come lose, undone or broken, (more likely in your case) or check the PTF switch.
any other assistance, contact me, good luck.

If you need further help, reach me via phone at https://www.6ya.com/expert/gerardo_79638d1eac6a0a1f

bad safety switch on the seat or the reverse switch is stuck

Things you need to check, loose blades, blades out off balance, bad belt or loose belt, bearings in the blade mandrels worn.

Bad vibrations when engaging blades is usually caused by an out of balance blade. Out of balance blades occur due to damaged and/or bent blades. The fix is to find and replace the offending blade.

Sounds like a bent blade to me. Can you determine where or which side the noise is coming from? For safety, I would replace all the blades. I once ran over my son's baseball while mowing and bent the housing and that caused the blade to hit the side of the deck. I simply bent the housing back with a few hits of a hammer and back to mowing I went.

If it's like mine (MM 875), it has a plastic "clutch" that has little edges on it that broke off when I was chopping sticks with the mower ;-) Went to the local B&D place and got a new one for less than $3.

But now I'm worrying about the nut holding the blade on -- if I try to get it good and tight, the plastic "clutch" spins around off-kilter... Funny how I never even looked at the blade after I bought the mower used, but never worried about the blade being loose and flying off.