If those are not factory belts that is probably your problem.

2/3 x 40 9/10 inch belt

MTD Lawn Tractor Transaxle Belt, 47-3/8 x 3/8-in 954-04208A parts | Sears PartsDirect

MTD Lawn Tractor Transaxle Belt, 47-3/8 x 3/8-in 954-04208A (replaced by 490-501-Y007) parts - manufacturer-approved parts for a proper fit every time! We also have installation guides, diagrams and manuals to help you along the way!

www.searspartsdirect.com

2/3 x 90 4/5 inch belt

Lawn Tractor Ground Drive Belt, 2/3 x 90-4/5-in (replaces 754-0467A, 954-0467) 954-0467A parts | Sears PartsDirect

Lawn Tractor Ground Drive Belt, 2/3 x 90-4/5-in (replaces 754-0467A, 954-0467) 954-0467A parts - manufacturer-approved parts for a proper fit every time! We also have installation guides, diagrams and manuals to help you along the way!

www.searspartsdirect.com

That is not something you will find at Advance Auto.

 

Thanks Dave55. The drive belt is original and in good shape. I did replace the Vari-Drive belt with a none OEM belt. I used an expensive one another YouTuber had used. So I bit the bullet and ordered the correct one from the Sears web site. Dave55, I don't show that the "Transaxle" belt you listed above is the one for my tractor. I ordered part number 954-0241A which is replaced by 490-501-Y087.

 

I was reading somewhere, the way the variator pulley works on both belts at the same time, they should always be replaced in pairs.
The center sheave in the variator pulley floats between both belts.

 

I did take apart the vari-drive pulley system and there is no issue there. The two bearings that hold it are in great shape. Everything else looks good. I am hoping replacing the smaller belt... again with and mfg part, will resolve it.

 

So I replaced the vari-drive belt with OEM with no change. I think I hear "belt flopping" when it behaves like I describe above. I wonder if the main drive belt has stretched too much? I am going to mount my cell phone underneath and film what is going on under there.

 

The system is such that both belts should stay tensioned when you adjust the gearing. One gets higher as the other gets lower in the VEE more or less. The mechanism should keep them both equally tensioned or there would be slippage. They HAVE to be tensioned or it will not work. Yours apparently has tension in the lower ranges that does not maintain as it is shifted to the higher. Something is not moving as well as it should. I have not taken one apart to know what to expect, but you should be able to see where things are positioned in the lower ranges and compare that to where things are in the higher. Something(s) must move, and one or more are not moving as synchronously as they should. IOW as one tensions and moves up/down in the VEE, the other should also maintain tension as it moves up/down in its VEE.
I do know that an older MTD I owned had a shift lever on the controls that allowed up and down shifting while on the move(supposedly) when I would try to upshift, I felt as if the lever would break given the force needed to effect the shift. I kind of stepped on the clutch/brake to de-load the belts so it could shift more readily. I did not like it, as I really felt I could break the lever with little more force. Slipping the clutch allowed much easier shifting. FWIW.
All I have read indicates belts replaced in pairs, and only by the OEM belts. Doing otherwise will allow the belts to ride differently in their respective VEE, making it work weird. Tech term, I am sure.
tom

 

if it's like the mtd 600 series there is an adjustment procedure in this manual

https://web.ncf.ca/da229/smallengine/mtd-white-manual-OGRM-5002_L145g-l160g-winnie.pdf

 

Well, things are headed in a positive direction I think. See the attached photo. I got the OEM drive belt and replaced it. You can see the new one is wider and I can tell is a bit thicker. That may be all due to wear. Or it could be a cheap aftermarket. I have replaced it but it is too late for a test drive. That will happen in the morning. Fingers crossed.

 

The test drive went well. It was the belts. Thank you all for your suggestions.

 

based on your belt number that's a 700 series step through. working on one now with bad transaxle.