Yes it does have the anti tilt locking device but it still seems to wiggle a little.

 

doesn't harm the seat at all...

 

Hmmm Are the bolts on the anti tilt thing bottoming out before they pull the device tight? The two bolts on the outsides of mine did just that , bottomed out before really tightening the anti tilt thing. I changed out all the bolts to longer Allen head bolts. My socket wrench was a pain to get in there, the allen wrench is pretty easy. Mine really doesn't move.
Dan

 

I have the AMC-600-2 double motorcycle carrier. I was a fairly aprehensive about the whole concept but I gave it a shot anyway. I use the carrier on the back of my 2003 Yota 4Runner LTD V8 2x4 with factory hitch rated at 500lb.tongue & 5K lb. gross trailer (+/-).
I also admit that it was very easy to run 2 additional 'outriggers' made of 1.5" aluminum angle iron that I can bolt (2 bolts each) directly to the frame of the 4Runner and are subsequently bolted to the carrier. I then use holes drilled in the outriggers to secure the bike handlebars to the carrier via ratchet straps.
You can see what I am talking about (hopefully) in this pic:

http://www.engagesite.com/yucatan201.../DSCF3656.html

As you can see, I have a Yamaha XTZ 250 Lander and my wife's Yamaha 125 XTZ on the rack. That's right ... 540 lbs.! We toted this rig 2700 miles from central Mexico to the Yucatan and back!! (friends let me tell you...that's some rough miles and probably 1,000 'topes'!)
Absolutely no problems. Granted, my a$$ is a little droopy and I scrape the tail of it fairly often but no problems. Just got back from another 500 mile round trip (on Mexican roads) Again, no drama.

So, though I cannot speak directly about the single AMC 400 carrier, the AMC-600 does a great job BUT I DID MAKE SOME MAJOR MODS.
I'd feel safe with single carrier mounted in a solid receiver.

 

Good to know it was more of operator error, and inadequate tie-downs,and not a design flaw. Even though I have gotten used to the fact that the bike is secure on the carrier, I still have visions of it falling off, sliding down the interstate at 70mph, on it's side, while spinning in lazy circles until the semi behind me bounces over it, splattering green plastic out both sides.
Good to know about the ramp coming loose too. I'll have to pay attention to that. I sometimes set a 5 gal gas can on the bolted on ramp, and run a strap around it and the rack. Keeps the gas smell out of the topper. If I'm not hauling gas, I just toss the ramp in the back of the truck. The back glass opens that far even with the bike on.
Dan