912ULS rough at throttle back

I had a similar issue to this that turned out to be worn gearbox splines. The vibration would start when throttling back and 'unloading' the engine. You could try different rpm and change the aircraft pitch to increase the load on the engine. If the engine is loaded then any wear in the gearbox splines will be taken out and the vibration will be reduced.

If it is the gearbox splines you will need a new propshaft and drive dog. Not cheap I'm afraid. Let's hope it's something similar!

I had a similar issue. It turned out to be the "slack" created in the throttle cables due to the radius enroute to the carburetors. The carburetor full throttle springs are not strong enough to remove the slack so under certain conditions the cables are not the "exact" same length to the carbutretors.. My solution was to add a few pounds more spring tension, a stronger spring. You will probably need to rebalance after doing this but the carbs (vacuum) should follow each other through the complete rpm range when you are done. I suspect if you hook up your balance gauges now you will find out when the engine is running rough the carbs are not in balance.

Hi RX01, if the vibration continues, your gearbox will suffer. Is the mags check rough above idle and below 4000 ish?

Determine which ignition system A or B causing the rough running and begin locating the cause from there.

hope that helps for now and hopefully others will offer their remedy from experience.

The mag check is fine in both inginiton lines. No rough running and rpm drop with no difference between mags