I think it was a bit under powered, it just fell outta the sky. Tried to fly it with a 400DF and a 1350 TP 3S lipo.

FIrst flight of the MX400

It sure looked good there for a minute.

I didn't have very good luck when using my 400DH in my MX-400, it's a nice light weight motor but isn't happy unless it's running in that 90% range. I really like the 450TH paired with a 10-12T paired with a 1500-2100mah is the way to go. Of all the motors I have tried for this heli it's the best all around motor in terms of being easy on my Lipo's and having plenty of punch.

It should have had plenty of power. I have a 400DF and run 12t (summer) or 13t winter. It never dropped like that. I can get well over 2600 rpm at less than 100% throttle. I do run 85 to 90% throttle when not in gov mode, and 85% in gov mode.
But it does run sort of hot at about 130*F on return on a summer day. I changed to a Scorpion HK2208-28 which does not seem to have as much instantaneous torque as the 400DF, but runs barely above air temp and uses 22% less current.
What ESC did you use with the 400DF ? I had to use a CC35 because other esc's could not soft start it.
The 400DF was easier on the batteries, returning with the batteries cooler than some other motors I have even though the 400DF motor can ran hot. Now I use the 400DF for a backup motor. I got it 2 1/2 years ago, and replaced it and a 430L 3550 with Scorpion motors which "beat" them off the heli !

CH

I tried a couple different speedos and it didnt seem to help. I went to a 450 type and geared it down with a 10T and it pulls hardly any amps and flys alot better.

It fell out of the sky because the oneway was bad.