Unfortunately, SportCruisers are known to encounter vapor lock issues due to the compact cowling design and limited airflow. Some models of SportCruisers possess varying fuel system configurations that omit a fuel return line setup. Our suggestion is to transition to Av-Gas 100LL to assess whether the problem abates.
If you have a Garmin or Dynon system and a fuel flow transducer installed, downloading engine data might reveal spikes in fuel flow, possibly reaching up to 9 gallons per hour (GPH) or more. These erratic fuel flow patterns and spikes could indicate a vapor lock, as vapor passing over the transducer impeller accelerates it, thus causing the fuel flow spike.
Rotax emphasizes in their operators and installation manual as well as their Service Instructions the following guidance: The fuel system must be designed with consideration for vapor lock contingent on ambient conditions such as pressure and temperature, along with the types of fuel used (vapor pressure class). Vapor lock could lead to engine stalling. Depending on aircraft regulations, like the "Hot Fuel Test," compliance might be necessary. In case issues arise during testing, affected components like the fuel pump's supply line should be cooled.
For vapor lock prevention, insulate all fuel lines on the suction side of the fuel pump against engine compartment heat and route them away from hot engine components, ensuring there are no kinks and they are suitably shielded. Under extremely critical conditions (such as vapor formation problems), considering routing fuel lines within an air-cooled hose could be wise.
Mandatory installation of a fuel return line is advised. If the ROTAX® fuel distributor piece with a regulator isn't accessible, fuel pressure must be regulated through a restriction in the fuel return line to maintain fuel pressure within ROTAX®'s designated operational limits.
There is a risk of vapor lock formation if winter fuel is used for summer operation. So use summer blend fuels only in summer and winter blend fuels must only be used in winter conditions.
The engine manufacturer suggests using an electrical auxiliary fuel pump if airworthiness regulations don't already demand it. This pump is valuable not just in case of mechanical fuel pump malfunction but also for maintaining fuel supply during vapor formation at high altitudes and temperatures.
AVGAS is rated by MON values. AVGAS 100LL possesses a high lead content which can lead to deposits in combustion chambers, piston rings, and oil systems. Nonetheless, AVGAS is approved for use and proves beneficial in scenarios involving prolonged storage periods, vapor lock, or when alternate fuel options are scarce. Altitude and ambient temperature must be considered when selecting AVGAS fuel to mitigate vapor lock risks.
AVGAS fuel additives such as Decalin® and Alcor®, known for scavenging lead deposits, haven't been vetted by BRP-Rotax. Field experience indicates that while these products significantly reduce lead deposits, they don't harm the engine when used correctly. Always adhere to additive manufacturer instructions, especially regarding safety precautions. BRP-Rotax has only field-tested Decalin Runup® and Alcor TCP® brands. Similar additives are discouraged, as BRP-Rotax cannot verify their compatibility with the 916 iSc B, 915 i Series, 912 i Series, 912, and 914 Series engine types.
References:
https://legacy.rotaxowner.com/si_tb_info/serviceinfo/si-912-016-r13.pdf
https://www.rotax-owner.com/pdf/IM_912_Series_Ed3_R0.pdf
https://www.rotax-owner.com/pdf/OM_912-Series_ED4_R1.pdf
https://www.rotax-owner.com/en/flightsafety/476-ai476
