Black Motor Oil/Rotax 912ULS

My nearly new RV-12 had 17 hrs when I purchased it, now about 80 hrs. The oil turns black very soon after each annual change. My A & P does not find any mechanical reason for this, but finds the spark plugs and exhaust tailpipe very black with soot indicating an over rich mixture. The engine performance appears normal, no apparent oil consumption, fuel burn 4-5 gpm. Is this problem normally associated with heavy carb floats without any other symptoms? Another RV-12 owner believes it is a problem caused by excessive vibration in the gearbox, but I don't see the correlation. Anyone have black oil problems without other symptoms? Is it really just heavy floats? Thanks for any advice.
Norm

Is your choke fully pressed in and against the stops on the carburetors?

What are you seeing for Coolant temps on the heads, and exhaust temps at 5200 RPM and level flight, say, at 5000 ft AGL ?

Running the AeroShell SportPlus 4, usually by 25 to 30 hours, there's some color in the oil, been changing it every 40 to 45 hours on the Hobbs.

Summer time will see added make up oil of 1/4 to 1/3 of a bottle of oil in that time, the motor seems to use a little bit more oil in hot weather, climb outs at full Gross weight will see the oil temp needle nudge barely into the yellow at around 230F, then I back it off on the climb rate.

Are there different notches to clip the needle valve on the throttle body and how far or how deep it sits near the main jet? Aren't those supposed to be changed out? If you are seeing black plugs at 5000 to 5500 Rpm, the motor is surely getting most of it's fueling off of the main jet. Where that circlip sits on the needle valve if there is 3, 4 or 5 notches is very important.

Black Motor Oil

NinerBikes, thanks for your information. The 12 is in the shop for the two Nose Gear SB's and annual Condition Inspection, so I can't verify the temps, but I think they run normal, and as you do, I level off a bit when temps approach the yellow during climb. Using AeroShell SportPlus 4, only flying about 20 hrs per year so don't see any yearly consumption. I will pass on your suggestions regarding the choke adjustment and the needle valve clips to my mechanic. Because the dirty oil problem has been there from the very beginning, I am thinking it could well be related to the choke adjustment and/or the carb fuel/float level. I think there is something in the builders notes about sending the carbs to someone to have the floats replaced, maybe that induced some error. BTW I never see fuel on the hangar floor or in the engine compartment, use only non-ethanol 92 octane Mogas. I will report when (hopefully) the problem is resolved. Thanks for the help.
Norm

Norm: Check your PMs. -- David

Just a suggestion? if you are only flying 20 hours/year that may present a problem using auto fuel. Even though you are using non-alcohol fuel there are two possible problems with not flying regularly. First, auto fuel doesn?t retain octane rating when stored for weeks/month and second, and this might be more important, fuel blends change for winter and summer. Vapor Pressure rating is less in winter Mogas and this presents a problem if winter Mogas is used in summer when ambient temperatures are higher. Very distinct possibility of having vapor lock. It?s been known to happen especially on tightly-cowled RV-12. Best to use fresh auto fuel and use extra care when suppliers switch over for VP rating especially in the spring.

I think if I were to fly so few hours per year, I would just use 100LL. Avgas is specially formulated to not degrade very much over time. Avgas has excellent storage characteristics and good for preventing vapor lock.

For what it?s worth?

Black Motor Oil

Thanks Jim, it is good for me to reconsider why I chose to use Mogas. I think it was because Rotax recommended it, allows a longer oil change period and maybe the lead is not good for the gearbox, but many other aircraft engines use 100LL fuel and have gears and other things that use the same oil as the engine, and I don't need the longer oil change nor the inconvenience of hauling fuel to the hangar and more. Looks like I should be using 100LL. Also, I suspect the annual inspection will find I have bad floats causing the black oil. Yuck!
Thanks Jim.

Did you weigh your floats, and if so, what did you find?

In my experience, black soot on the exhaust is a sign of an excessively rich mixture. However, it will not typically make the oil black. Black oil is typically a sign of exxessive blow-by getting past the rings and will occur even with proper mixtures.

Larry

If you start using 100LL regularly, I suggest you also start using Decalin RunUp, a lead scavenging additive. It doesn't eliminate lead issues, but drastically reduces them.
I use it in my Subaru powered RV whenever I use 100LL instead of MoGas,

Black Motor Oil

NinerBikes said:

Did you weigh your floats, and if so, what did you find?

Click to expand...

Randy, found max wt. for a pair is 7 grams, for a single 3 1/2 grams. This may be within limits, but maybe should just replace anyway? Re-use the lighter ones? Fuel consumption seems normal to me, flight yesterday 4.5 gph @ 5000 RPM. A&P doing the work for me.
Thanks guys.

At the very least, you will want to save the old floats. Let them "dry out" and they will once again be < 7 grams/pair. Not OK to use again for extended period, but good enough to throw in baggage compartment with spare tools in case carb acts up on the road. Ask me how I know this can happen...

Good advice on any serviceable part that is removed. I save carb sockets, fuel pumps and spark plugs just in case I need to bridge some time waiting for new replacements.

Norm,

Did you get this resolved, or find any indication of why your oil is black? The owner of N836BL seems to run his at 5200 to 5350, maybe 5400, and the color of his plugs is a normal dark tan in color.

Too much idle time will darken the plugs up.