First off, I am very much a novice when it comes to Rotax, even though I have had a 914 for four years. Off hand, I find the graphs rather interesting. I think of the Rotax 914 as 2 two cylinder engines bolted together. Generally odd behavior happens in pairs. For example one carburetor feeds two cylinders. In this case 1 and 3. If the carburetor was the problem, I would expect to see 1 & 3 track each other. Its hard to tell from the graph which is egt #1, but it doesn't seem like there is much correlation between the two cylinders.
Assuming the carburetor is good, since the other cylinder serviced by that carburetor is ok, it suggests something cylinder specific. Looking at the graph, I see a slight increase of EGT at times 35 and 40 minutes. To me that suggest the mixture is getting leaner, which causes the EGT to rise. And I'm guessing the mixture gets lean enough at a certain point (42.5 minutes), that the egt drops off rapidly. --I'm curious if the engine was running rough at this point? Any noticeable vibration that would indicate cylinder #3 wasn't producing the same power as other cylinders? If the engine was still smooth, I would suspect the probe wiring. If the engine was rough, I would look a little further. I would start with the simple things. I'd replace all the spark plugs with properly gaped plugs, since they are cheap. (I think less than $20 for all 8). While I had the plugs out, I would also check the compression and bore scope cylinder #3, looking at the valves. --But if you don't have a bore scope, I would skip that. If you still have the egt issue, I would check for an induction leak near cylinder #3. Here is a video of how it is done on non-Rotax engines https://swisscows.com/video/watch?region=en-US&query=how%20to%20check%20for%20an%20induction%20leak%20aircraft&id=38764B7E3FFFA0F2B31238764B7E3FFFA0F2B312 --That said, I'm not sure how well that method works with the Rotax dog clutch. I've heard a shop vac can be used to charge the intake along with soapy bubbles.
For a bit more on why I would be suspicious of an intake leak. When the MAP is higher than outside air (roughly 29.92 inches), the Rotax is forced induction. That is to say the intake manifold is pressurized by the turbo. You will push fuel/air mix out of the leak, but the positive pressure should keep the mixture about right. When MAP is below atmosphere, the engine will draw a vacuum and you will suck in air from outside, and this will lean the mixture. --And the lean mixture raises the EGT, until it gets too lean, then the EGT falls rapidly.
A word on Rotax 914 and carburetors. They don't have a great reputation. Specifically the floats have a horrible reputation for sinking (how many service bulletins?), which causes a super rich condition. So rich that the fuel doesn't combust in the cylinders. This is probably not your problem. Also, there have been cases of mechanics over tightening the bowl during servicing, which bends the bowl and causes other poor running conditions. This is also not likely to be your problem. --That said, when the carburetors have good floats and haven't been messed up by a mechanic, they work really well.
