Question Burble in the engine (CubCrafters FX-3)

[Neal]

This hasn't happened in a year and today it happened twice. I flew down a half hour from me to get gas and topped off the tanks. Flew back and felt a "thump" saying to myself "what was that?" Was it a bird strike? I looked around and continued on then it happened again and it was the engine. I pushed full rich and climbed to a higher altitude "just in case" and all was well from then on. I've had this discussion in the past on the CubCrafters forum and the conclusion as it was experienced by others was possibly air in the lines. But oddly I have not had this issue since the last time which was about a year ago. I was running lean of peak around 21 squared. Maybe it was too lean, I don't know.

Curious if anyone else has experienced this and what the cause is presumed to be?

 

[Waldom]

The mixture set a bit too lean could cause an occasional "bump", but I doubt you leaned any differently than you usually do.

I'm sure you've already done this, but did you check the fuel tank and gascolator drains for contamination after you landed? I have experienced contaminated fuel from both fixed pumps and fuel trucks that I had fueled from many times before without a problem.

 

[Neal]

Thanks for the reply. I did not check after I landed, fortunately I have yet to see any issues in sump testing. It may have been too lean. It's rare when this happens and it's been a while. Will continue to monitor. Sure gets your attention.

 

[Neal]

2.0 today and no issues, and preflight sump checks clean as always.