25hr Inspection of 912is - concerns

[tracstarr]

Got out to the hanger yesterday. Pulled all the plugs and checked gap -all looked good. Put all back together and made sure to really push on the plug wires. One plug appeared to have a far cleaner look than the rest - so maybe wasn't firing as well? That said my test run went much better. Although, when switching off lanes the rpm actually increased by about 120 rpm - not decrease. I've never seen that before. Switching both lanes back on it would revert to the exact starting rpm. A difference in rise? between each was 20rpm.

[3kdscf]

The differential leak test on the 912is took place after my oil change due to not having it available. Starting with cylinder 3 I found TDC using a rubber glove on the end of the connected hose. I slowly increased the pressure to 80psi and it was holding at 78psi. However, I noticed I was getting a gurgling/burping sound in the oil. This seemed very strange. I repeated on the remaining cylinders and noticed the oil gurgling starting anywhere from 10-40psi, although there was no pressure change detected on the differential. My oil reservoir was now nearly flowing over. What’s going on here? Is this expected? I wouldn’t think so. (see video)

Hey Keith,

Thanks for this observation, I thought I was going bonkers so glad I have company When I do the differential compression test on my 912ULS, I also hear this bubbling sound but I thought it was coming from the float bowl of the Bing carb but the oil system makes more sense.

I am always paranoid about inducing air into the oil lines so I always re-purge the oil system for anything beyond a simple oil change. For those who know a lot more about the 912 engine design than I do, is purging the oil system required after a differential compression test? Am I just making more work for myself?

[Dave S]

Dan,

On the 912, a differential compression test will not introduce air into the oil system.

Two things may result in air introduction to the lubrication system:

1) Turning the propeller backwords

2) Opening up the oil system anywhere.

[3kdscf]

Thanks Dave!

One further question, does removing and inspecting the magnetic plug count as opening up the oil system? That has always been unclear to me.

[Dave S]

Dan,

Removing the magnetic plug does not constitute opening up the oil system.

The plug is located to protrude inside the crankcase pocket right by the reduction gear. Sometimes a little oil will run out when removing the plug since oil pools in the bottom of the gearcase. The plug is pretty much in the splatter zone of the large gear.

Dave

[3kdscf]

Thanks Dave!

I will stop wearing myself out trying to spin the prop at 80rpm to purge every time I do an oil change

[Dave S]

I have always used to oil change procedure recommended in the engine manual by Rotax for the 912ULS - never been a problem.

[Rodney]

Just a suggestion - I have now added pulling the valve covers and checking the torque on the cylinder head bolts. Not sure why or how, but found one cylinder not torqued to spec.

Regards
Rodney