Engine issue on takeoff.

I am not saying it's the issue, but Rotax recommends that the carbs be overhauled every 200 hours. I have seen 912s that were hard to balance; after rebuilding, there were no issues. The cause could be a weak engine fuel pump. Has yours ever been replaced?

Hi Joseph

What you are experiencing is vapor lock!

You are still using winter blend fuel, which has a high vapor pressure, and it is now too warm to operate safely with that fuel. If you want to continue flying, you MUST switch to 100LL Avgas!

Trust me, we have seen this happen countless times, sometimes with very serious consequences.

A clear sign that you are experiencing vapor lock is your fuel flow spiking above 9 GPH. This happens because the vapor is passing through your fuel flow transducer, causing the impeller to spin faster as the vapor moves through.

You can do a blend of 100LL and auto fuel, but our recommendation is to switch entirely to 100LL and simply follow the additional oil and spark plug change intervals. Once your state transitions back to summer blend fuel, you can safely switch back to auto fuel.

Here is a link to an article we wrote on the topic https://www.rotax-owner.com/en/flightsafety/476-ai476  Keep in mind that while we discuss the mandatory fuel return line in this article, it is NOT the ultimate solution to the issue you are experiencing.

The ONLY way to resolve your problem is to switch fuel.

The best combination for your setup is to run 100LL Avgas, use Decalin or TCP fuel additive to help scavenge lead deposits, and use fully synthetic XPS engine oil.

See the attachments for more information.

Also, replace the floats in your carburetors, as they are beyond the acceptable limits. You need to use the new-style floats with the "R" stamped on the side.

I can almost guarantee you still have the older-style floats, which were affected by a service bulletin (SB) and should have been replaced a few years ago. While your floats are not the cause of the issues you're experiencing, they still need to be changed.

Thanks for the response Ronald. Carb inspection and 5 year rubber replacement was done at 350 hours 12-3-2020 (current time is 503 hours). The electric fuel pump has around 20 hours on it. Engine driven pump was installed in 2017 at 173 hours. 

Thanks. 

Thanks, Rotax-owner. I appreciate the information. I checked the floats, and they have a "B" on them, but they are out of spec, so I will definitely replace them. I am a bit surprised to see vapor lock at a 61-degree OAT. Either way, I will definitely be switching fuels.

Thanks again!

Joseph,

Was the aircraft sitting outside on the ramp for a time before you flew it, and was the cabin warm inside when you got in?  The location of the fuel tank on the RV-12 can make it susceptible to solar heating of the fuel if the sun is beating down on the canopy.  The temperature of the fuel in the tank is a factor in vapor lock, and it may have been much warmer than OAT.