no compression cyl 1&2 912 ULS

[jiott]

John, I sure am curious about the source of the 5400 rpm recommendation. I find nothing in the Rotax Operator's Manual except an operating range.
Yes I did the recommended gearbox removal/inspection/service at about950 hours-sent it to LEAF. They reported it was in very good condition, clean and only replaced the usual springs and washers (I use nearly 100% mogas clear). It never showed any unusual backlash, however I didn't measure it, or any unusual starting kickback or noises. My prop is ground adjustable, set to slightly favor cruise speed, so it is loaded fairly well at my 5200 rpm cruise.
I find that the gain in speed from 5200 to 5400 rpm is a minimal 2-3 mph and an increase in fuel consumption of about 0.5 gph-not worth it IMHO.

[jiott]

Nothing I've ever read by Rotax that says 5400 rpm is best. 5200 is fine; been running there for 1300 hours. Its a sweet spot for me for fuel economy and speed. No carbon buildup noticed at all.

[jrevens]

Nothing I've ever read by Rotax that says 5400 rpm is best. 5200 is fine; been running there for 1300 hours. Its a sweet spot for me for fuel economy and speed. No carbon buildup noticed at all.

Jim,

Have you done any service/maintenance work on your gear box (PSRU) in those 1300 hours? Also, what kind of backlash can you measure at your prop tips? I’m just curious, and interested in the experience of those with more hours than I have on my engine. I was listening to a conversation at OSH regarding a recommendation to keep the rpm at something like 5450 during normal cruise to help minimize wear on the gears that can occur when the prop is more lightly loaded & more backlash is possible.