Tricopter pitching down with throttle

Hi,
I am developing a tricopter vehicle and it is pitching down a lot (~5 deg) when the throttle is reduced . But it does not pitch up when the throttle in increased. Does anyone have any ideas what can cause a pitch asymmetry like this? The 2 tilting motors in front (10kW) are smaller than the motor in back (17kW), and the motors in front use 2 blade 32x 10.7in props, whereas the back uses 2 of the same props forming a 4 blade prop. I considered the possibility that the back motor has more rotational inertia, which could make the back motor lag the front motors. But unlike my vehicle, this would generate a symmetric pitching effect, such that throttling down would pitch the vehicle down, and throttling up would pitch the vehicle up. This symmetric effect is seen in many drones. Does anyone have an idea what can cause a vehicle to pitch only down with a decrease in throttle, and not pitch up with an increase in throttle?

Here is a link to see the vehicle as well as construction details:

I have tested the vehicle high up out of ground effect and this asymetric pitching behavior is observed:

Video:
https://drive.google.com/file/d/1XF2af9BRVl0FTPep8M5Kbumt_HAGXxAh/view?usp=drivesdk

Log:
https://drive.google.com/file/d/1sIP8zdrSVs5vN-fAD5S_UVAoOIKZN91y/view?usp=drivesdk

Thanks, Wiliam

Note that additional information and discussion on this problem can be found at the following link:

Here is summary of the latest information. i am building a very large unmanned tricopter, which weighs about 180 lbs and is the size a of a large station wagon. We have have gotten it to lift and hover. But when reduce throttle, the vehicle pitches down and has had some hard landings. We developed a bench test that enables us to see this effect while the vehicle is on the ground. We bassically balánce the vehicle at its CG in a beam of wood, and study how the flight controller stabalizes the vehicle. Increasing throttle does not affect the balance, but pitching forward pitches it forward a lot. Orrigionally we had more P than I terms, then we tied more I than P with the same result, but we saw reaction of the Pitch to the stick control was more delayed. Now we are trying equal P and I term and are seeing much better results on the bench. But when tried it the air out of ground effect, the developed a possible pitch oscillation and a rolled left for no apparent reason.

I am trying to read the log and it looks like the the Pitch oscillation is me over reacting because of a 1/2 sec vehicle reaction delay, and a little pitching of the vehicle due to a reduction in throttle, as well as a mysterious momentary cut out of the front left motor. I have no idea why. If you could look at the log and see what you think, I would be most appreciative. The vehicle is underpowered and we have noted that the motors are hitting their max values sometimes. With fullly charged batteries the vehicle hovers at ~70% throttle. Unfortunately during the last test it took us a few tries to get in the air, so the batteries well a little depleted. Next time we will make sure the batteries are fully charged. He is a link to see the video and log file of the last test, as well as a more detailed analysis of the flight.

https://drive.google.com/drive/folders/1kNFwSXsqi_Dqwi5JRvsccrIy0YeqgIZe

Well, work on that…