The newly purchased deck belt turning the blades is extremely loose

take the belt off and be sure the pulleys turn freely. sounds like you mowed through water and they may have some rust on them or the bearings in the spindles are bad. just take the belt off, turn the pulleys and find the one that is bad and repair it.

loose is a subjective term. As long as it is not flopping around or slipping on the pulleys it should be ok. You don't want it so loose it will fly off.

My idea is not about the belts, but is about the deck. If the tensioning mechanism on the deck is getting stuck in an intermittant position than the belt may be just tight enough to turn blades while slipping through the pulleys, this may cause enough friction to start melting your belts. If this is the case you will have to remove your mower deck and see what is preventing the mechanism from functioning properly.

when the deck( blade) is engaged it puts a load on the engine. that tells me theire is some type of bolckage to the belt/force of the movement and is keeping the engine of moving freely. check to see if there is any blockage of the blades, or the belt is loose or off track.

I also need a deck belt diagram because a manual didn't come with my 746 SRL yard machine

Hello,
My name is Dane and I am going to help you solve your problem. You said that the "Pulley" is loose when the blades are disengaged. I hope you meant that the belt is loose, because the pulley should never be loose. If it is loose, you should remove the pulley and make sure the KEY inside the center hole is still ok. Sometimes when the pulley gets loose it chews up the KEY and allows the pulley to turn on the crank. If the KEY is ok, just reinstall the pulley and tighten it.
I believe that your belt is probably just stretched and worn. The mower deck belt gets it's tension from lowering the mower deck. the lower the deck the tighter the belt. Just replace the belt. I hope I've helped! Thank you! Dane

I dont know the belt size but if you go to mtd belts in Jackssmallengines.com you should find it. Stephen.

check for a broken belt tension er or maybe grass clippings under belt area, check for a broken or loose pulley bearing, even a bent shaft on the deck.

you have to raise the mower then drop the rear of the deck by removing the 2 pins on the blade adjustment lever.once you do that it should go pretty easy

My mtd 20 hp mower, blade will not engage, found a large spring and a small metal piece that came off and I have no Idea where it goes in order to get this blade going again, can some one give me some info Please..