The CAA has identified an emerging trend of rough running Gypsy Major engines and even engine failures. These are believed to be caused by the use of 100LL avgas in Gypsy Major engines, and the CAA is contacting owners of aircraft powered by these engines with updated maintenance advice
Owners of all affected aircraft – such as Tiger Moths, Chipmunks and Austers – whether on C of A or Permit to Fly are advised to ensure routine engine checks are carried out by a maintenance organisation or person experienced on the engine type. Analysis suggests 100LL could cause reliability problems, particularly with cylinder heads, valves and valve seats.
Checks should also be carried out if low compression is noticed when turning the propeller by hand or if the engine runs rough, although these issues are not always attributable to 100LL fuel, of course.
UL91 is recommended as the most appropriate fuel for a Gypsy Major engine, while unleaded mogas should never by used, as the octane enhancers can damage the engine, particularly non-metallic elements.