914 Low Power on Takeoff Attempt

Hi All

The engine TCU monitors airbox temp that you can see on the BUDS software.    if that temp value exceeds 40C it suggests that you may need an intercooler in the system.  The TCU will ease off full boost to try and prevent detonation and keep it running.  This almost always happens on hot days with full power where it is trying to maintain 40 inch HG on WOT.  At max continuous, 33 in HG 5500 RPM, it is almost never going to happen.  Anything that is over 108 % power will open the shuttle valve to get air pressure for the floats from the forward pressure point in the airbox.  This is to increase fuel burn on high power (prevent detonation).  

One question or perhaps i missed this, is this a fixed pitch or inflight adjustable?  The engine has to have the setpoints adjusted before first use whenever you remove and replace the linkage from the system.  This resets the throttle position sensor on the 2/4 side carburetor.  

Cheers

The prop is an Airmaster electric constant speed.

I'll get the TCU data this weekend, so it should be interesting.

I feel like there is a very, very good chance that you have a leaking/broken/missing tube from your pressure regulator. Your experience sounds almost identical to mine. Everything was fine one day and the next day it wasn't. Differential fuel pressure dropped to 0, and the engine stuttered and died.

First, STOP. Don't run your engine at peak power with 0 fuel pressure. That is a lean condition and you can't be doing your engine any good.

Next, look at the vacuum hoses in your engine. I have attached a highlighted image (the base image was taken from 73-00-00, pg 8, from MMH_912-914 Series_ED1_R6.pdf) showing how all the hoses should be run. They are color-coordinated, so blue does not connect to yellow (even if in the drawing it might look that way). If the yellow line has a leak, then you will see exactly what your data shows. The regulator keeps pressure ab out 2.5bar above the airbox pressure, but once the airbox pressure exceeds atmospheric the regulator no longer has a correct regulation signal. The upshot is that it stops getting the fuel pressure you need, and your carbs start blowing air into the carb float bowls, instead of sucking fuel from the bowls into the venturi. Whoops!

Once I was armed with this understanding, I found the leak within a few seconds. It took a mirror to see the broken line, but it was that easy. The frustrating part was that the engine had been examined by countless mechanics, and none of them had been able to diagnose the simple problem. 

The challenge with the 914 is that there are not a lot out there, and so there's not much tribal knowledge about them. Even when I spoke with Rotax directly, they were telling me incorrect information about the 914. Fortunately I had the service manuals and was able to figure it out on my own.

I took a look today and didn't see any obvious leaks in the lines, but I will look again, more closely.

Thanks!

Craig

I did download the TCU log files (link below).  Honestly doesn't seem to be much there, but maybe someone else can tease something useful out. The zip file is attached.

Are the logs only for the last flight? In that case, my last flight was fine.

https://www.dropbox.com/scl/fi/ob5bzw5g2841wb7irrw5k/Logs_9576763_2023-12-03_09-30-10.zip?rlkey=c14wbwz80d0yjble9f4hzivdb&dl=0