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Hello all,

Firstly, I really appreciate all the help.

As I was finishing up my lawn yesterday (thankfully), the tractor started doing something very odd. The 'throttle' peddle that engages the drive belt just sank to the floor. I let it up, and pushed it back down again, and the thing lurched forward a few inches, but then immediately stopped again and the peddle sank again. I could do this over and over again to get it to engage for a few inches and then nothing. I looked under there, and from what I could see, the belt (brand new MTD original - this is only the 3rd or 4th mow with it) seemed like it was still engaged on all the pullys and routed between the two in the middle that pull it tight to engage the drive in the back. If I held the peddle down long enough, I could see smoke from the front that tells me that it was slipping.. maybe? I shut it all down and pushed it into the shed.

Any ideas before I go around looking for a missing spring or something stupid?

Nope ,
Sounds like you know the score
One of the springs has broken / fallen off
one of the pulleys has fallen off.
One of the belts has fallen off.
AS the pedal drops to the floor, most likely suspect is the spring.
Just had to weld a patch on the common anchor point for the front & rear springs as it was just about cut through.

thanks

I was hoping that it was a simple 'Ya, it fell off the back... ' or something as easily stupid to fix. I don't have the inclination or time to deal with it this weekend, but I guess it's better than trying to finish up with my push mower.

FWIW I like to put some plastic wrap over the fuel tank filler then screw the filler cap back on followed by removing the battery & battery support.
Pull the deck out, you will find it easier to do if you slip some 2x4's under the deck first to slide it out on.
Now stand the mower on its bum.
Soooo much easier when you can see everything square on in good light
The main tension spring , part # 25 in this diagram https://www.partstree.com/parts/troy-bilt/mowers-lawn-garden-tractor/13ax60kh011-troy-bilt-super-bronco-lawn-tractor-2008/drive-system/ has a habit of cutting right through the tang on the frame it anchors to.
This one was welded up but in pinch if the spring as not cut right through you can slip a bolt ( grade 8 ) through the slot and anchor the spring to it between a pair of washers..
This one got welded up then a washer welded to the tab doubling the contact area for the spring.

thank you greatly.... !!

I got the thing up on its butt, and looked at the engagement. My springs do not connect to the deck at all. They connect to the peddle bracket, and I don't see any place where another spring would go. That said, it seems like the idler pulley for the upper transmission belt is hosed. I'm going to replace it. I don't think that's the main problem, though.

The main pulley under the seat that connects the bottom belt with the tensioners to the upper belt has this weird middle section that slides up and down. It seems like it does this to make one groove deeper than the other, and in my case, almost waaaaaay too deep for either of the two belts to actually engage. Also, when I have the peddle down and the lower belt fully engaged, the lower belt hits the tension bracket for the upper belt and seems like it pushes it in a way to reduce it's tension, and that makes the groove for the upper belt too big and makes it not engage at all.

These are both brand new OEM belts. I've double and triple checked the part numbers, and they seem correct for the model number under my seat.

Anybody have any help?

The throttle pedal is connected to the pair of pulleys together on a pivoting arm, sort of in the middle of the belt run from the front to the back.
As you press the pedal the plate pivots and puts tension on the belt.

The top belt should have a really heavy spring connected to the arm with a small pulley on it.
Depending upon which model you have that spring will go forward to a tab that also has the 2 thin return springs for the brake & throttle pedals or go directly across the mower & mount into a hole in the chassis.

Tricky bit.
That spring puts a fixed amount of tension on the top belt which pushes the sliding bit in the "funny double pulley " down.
As you put your foot on the pedal, the tension in the front belt exceeds the pressure in the rear belt forcing the sliding bit up.
The harder you push the pedal the higher the sliding bit ( from here on called a sheave ) up, which makes the effective diameter of the front belt pulley section smaller thus turns the entire double pulley faster so the mower goes faster.
it is a different way of making a pair of stepped pulleys that you see on pedestal drills and home lathes to change the speeds.

if your pedal goes to the floor & stays there then there is not enough tension on the front belt.
Just the same it should return under the influence of the light return spring which may also be missing as they fall off & people do not notice it because the tension from the back spring will force the pedal most of the way back up.

Interesting. so that's why this is called a 'variable speed' transmission

The upper belt does have a large spring, but there isn't a ton of pressure on it. I can move the tension lever with relative ease. I wonder if it's connected to the correct spot, or if the spring has enough tension on it. I'll flip the mower again to look, but I'm pretty sure that the tension spring for the upper belts tension arm goes across the body and not to the same mount point on the pedal mounts.

Do you think that a worn bearing in the idle pulley on the tension arm for the upper belt would be enough play to cause this type of behavior? It's really the only part I can visually see as being an issue with the entire setup. When things aren't moving, the pedal has a lot of tension. When it starts to move, I assume the sheave moves up (or down) and causes the excessive play. When I was under there last time, I adjusted the throttle pedal with the little adjuster to make it tighter. Again, it worked for about 3-4 feet and then the upper trans belt popped off. That didn't happen before. It would just not move and the front pulley under the engine would slip and smoke.

Should I get another new lower belt (along with the replacement idler pulley)? Maybe the one I got (on Amazon) was gray market or defective.

Once you get to grips with how the thing works mentally it all becomes apparent.
If you push the sliding side up too far then in theory that could cause the top belt to run off the pulley like popping a pimple.
Also double check that all of the bolts are in the tranny housing.
It should have 4 on the axels holding it up then 2 more stopping it rolling forward when the belt goes tight.
That is not so much of a problem with vari drives as hydros but not an out of the world chance.

The one I am working on at the moment was running very slow & stopping going up hills and that was basically caused by the 1/5" slot the spring had cut in the mount as shown in the photo.
The replacement spring was also stronger than the old one.

The actual vari drive pulley itself can go bad it is a cantilever shaft on a pair of bearings and they do wear allowing the entire set up to tilt forward when pressure is applied which reduces the tension in both belts.

Once set up they are great and will run forever, much better than Hydros that will eventually die & cost a bomb to replace.

The big pulley on the tranny can strip out the splines on the input shaft and the selector ( F-N-R ) dogs do wear out, particularly is the owners has a habbit of changing when not fully stationary.

Just to follow-up on this post. I replaced the upper transmission idler pulley with the totally destroyed bearing. I also looked closer at the mount point for the spring that connects the idler arm to the chasis, and I do see that it has eaten a notch into that mount point, just like in your photo. I flipped the spring around on the arm so that it goes around the back side of the arm rather than the front side. As also stated before, I adjusted the peddle adjustment so that it was all the way tight. Those things seemed to take care of it, and the tractor is a bit faster now. At least it seems like it is.

The pulley with the buggered bearing would have been the prime cause and of course the spring will just make things worse.
When you adjusted the arm that works the two tensioning pulleys, did the front belt go loose enough not to run with the pedal up ?
If not you will need to back it off a little.
The front belt must have enough slack so as to slip when the brake is applied or the motion pedal is fully up.
If not you will damage the belts whenever you put the brake on, you will also damage the dogs on the F N R gear change.

That front rear pulley ( good english eh ) takes a beating and I usually replace it whenever I replace both belts.
I have been sold the wrong pulley a few times as MTD use exactly the same system on the lawn tractor series but with 1/2" ( 4L ) belts and somhow the smaller 4L's have gotten mixed up the the larger 5L's the right pulley is 3.5" diameter and of course the belt fits fully within the pulley.
The wrong one is 3" diameter ( 3.125 really) and of course 1/2 wide so only 1/2 the belt fits inside the pulley.
The wrong pulley works but wears the belt to a sort of step shape which will cause it to start slipping when going slow when the tension is low.
Generally you will change 2 rears for every front, that is just a mechanical thing, the rear is 1/2 the length so it wears twice as fast ( sort of ).

Because the rear runs a triangle the belt has an easier life than the front which gets bent backwards severely by the tensioning pulley and that action splits standard 5/8" belts.
Because of this rears wear thin and fronts tend to split and break up.
You have done all that you can other than change over the variable pulley

Awesome, thank you for all your help!

The peddle has more than enough slack, so it doesn't engage anything for a at least half-way peddle travel. It must have been pretty loose before. It still feels faster than it used to, so that's a nice bonus.

I checked the new pulley, and the belt fits perfectly, and fully, inside the grooves. It matches the old one, so it looks like they sent me the right one

I did my full lawn this morning, and everything went perfectly. No slipping, and nothing else fell off. This thing has been a beast. I got it used, and I think the previous owner just kept up on oil changes. At least they did that, though

again, thank you!