Belt twisting after pulley swap

Hey guys, I have a kind of general question but it's kind of long-winded but I don't know how to explain and ask my questions any more briefly.

I have a Murray Classic with no deck that I did a rear pulley swap on to speed it up a little so I could putt around the neighborhood on it a little. There was about a 9" pulley that I removed and replaced with a 3" pulley. My stock drive belt was 95" long. I replaced it with an 86" long belt. That belt is just a little bit too long, but I thought that was kind of a good thing because the 3" pulley makes the belt take a slightly different path than it did before, and interferes with part of the body. I thought no big deal, I'll add a second idler pulley to both tighten up the belt and to re-route it a little for the clearance I needed.

I just finished doing that a few minutes ago. Got the second idler pulley mounted using a little fabbed-up bracket and it looks good EXCEPT that right now the flat pulley is pushing against the v-side of the belt instead of the flat side of the belt to give it tension.

When I first started up I shifted to reverse and the mower QUICKLY (good thing) backed out of the garage. I pushed in the clutch, shifted to first and the mower QUICKLY moved forward. Cool! I shifted into 2nd, that was fine, 3rd, 4th, then 5th. When I hit 5th gear the tractor suddenly slowed down and I smelled burning rubber. I dropped back to 1st gear and headed back towards the garage up a hill (my driveway). I made it 90% of the way back as the mower slowed down to a stop with some smoke coming from underneath. I looked and the belt was still routed correctly but it had twisted and I'm pretty sure that was causing it to slip on the drive pulleys which made it get hot and smoke.

So I'm wondering if they make idler pulleys that are designed to push up against the v-side of the belt and if so where do I get one. Or if I'm going to have to get back up under there and try to figure out a way to mount the idler belt so it pushes against the flat side of the belt. It "seems like" because of using the smaller rear pulley that I need to push the belt "outwards" with an idler pulley so it more closely follows the original path. If I push it inwards I'm worried I'd have to push it so far that it would contact the other side of itself. Hard to put into words. Any advice is appreciated. Thank you.

You looking for a v-idler?

Yes, that's what I needed, thank you.  I got the v-idler pulley installed and now my mower will do 15 mph. WOOOT!!!