Bandit

I know you want answers but lets slow down a bit here. Speculation is less than worthless.

1.) "I don't think the filters were over oiled but before they were not oiled at all."

The filters were purchased, received and installed several years ago. Bought off eBay or Amazon, I can't remember anymore but you may have the receipt. As I did not have proper filter oil on hand and the sale did not mention a supply of oil, I contacted the vendor who indicated they were sold lightly pre-oiled without additional reserve oil. To be honest, my initial impression upon receipt was uncertainty whether they had been pre-oiled or not. They were very clearly not soaked (been there, done that, like Mark had to fix that). But then, they did not look or feel totally dry to me either. I have used and oiled KnN filters in the past so not totally ignorant but I ended up convincing myself they may have had a very light oiling at best. There were other more pressing items on the To Do List and I never had the opportunity to either clean or oil them after install.

2.) "I just really question whether a combination of that and the vent line route could have really brought forth a mixture problem due to the atmospheric pressure differentials inside and outside the carb.  In my head I wouldn't think so but when the throttle is opened up all the way those filters are really sucking in a lot of air."

Terminating the vent lines into either an airbox or air filter (doesn't matter to my thinking) results in a more equal atmospheric pressure differential, IMHO. If the Rotax requirement is to be as equal as possible to the air entering the intake throat (not entering the airbox or filter), how could it not be preferable to hanging in the breeze under cowl somewhere? Even if proper length vent lines are terminated at the carb and secured by the float bowl bail (which is the Rotax configuration when not using the optional airbox and most likely meets Rotax's ram-air and vacuum free zone requirement for the majority of installations), stands to reason when referring to a KF installation they are subject to significantly more buffeting and other forces that impact and increase atmospheric pressure differential under cowl. I don't think throttle position matters at all if the lines terminate within the airbox or filter, carb pressure is still always going to be closer to the air pressure at the intake than if outside the airbox or filter. I also think if there were any suction risk under normal operating conditions at all, Rotax would not route carb vent lines directly into the airbox.

3.) "There certainly does seem to be quite a bit of evidence of prior instances from other aircraft where vent line routing caused power loss.  Just Google Rotax 912 vent line power loss and there is all kinds of issues with it."

I used your search parameters exactly and concluded the following: every single loss of engine power that may have had carb vent lines in the equation appeared due to improper length of vent line, improperly terminating in a ram-air or vacuum zone and a possible blocked vent line. All of which have -zero- to do with routing the vent lines into an airbox or filter. I went through several pages of search results but if I've missed something, please do help me out. When I researched the topic years ago I never found one single instance where venting the Bing 64 carbs into a 912ul airbox or filter of any kind caused so much as rough running, let alone was blamed or linked to a fire or a loss of RPM due to fuel dump caused by faulty or failed carb float(s). Doesn't mean it couldn't or didn't happen here, there is always a first time, but I sure would be interested in any FACTS or empirical data that confirms it actually happened before.

I agree the issue of carb venting must be taken very seriously, it has been demonstrated many times just how sensitive and important the Rotax venting requirements are. The first flight of your plane is perfect example of why proper vent line length when terminated to atmosphere under cowl is critical. That entry sure got my attention! I don't think removal of the carb heat airbox is a factor, guessing the majority of 912 installs in KF airframes omit it due to cowl and carb location. However, that is completely different scenario and question than routing vent lines to an airbox or airfilter. These engines are installed in so many different airframe/cowl/pusher/tractor configurations Rotax necessarily leaves it up to the plane manufacturer to meet their requirements (ram-air and vacuum free zone) in whatever way they so choose. Rotax specifically recommends and provides provision to vent carbs directly into their airbox. My Aeropro has carb vent lines into the airbox, as do many other SLSA's. 

If Rotax finds routing vent lines to an airbox a preferred method, is routing to a filter appreciably different or problematic? Nowhere in any Rotax BRP-Powertrain install, operation or maintenance manual does it prohibit or direct against venting carb lines into a round or conical filter which, of course, are Rotax approved air filter configurations that Rotax also provides. The way Rotax ALWAYS errs on the side of caution, it they believed that somehow it was ok to vent lines into their airbox but for whatever reason venting into a non-oem airbox or particular or different style filter was a liability waiting to happen, you would think there would be an big fat Notice or Warning box in the manual(s) somewhere.

Well, after some further thought and with benefit of now owning a plane with an airbox, I think maybe there is a caution to note. Here's why: not sure precisely how the Rotax airbox works but on my Aeropro airbox, the vent line nipples enter thru the side of airbox to the outside the filter media. Therefore, anything coming though the vent line has to make it's way past the filter to enter the intake throat. If you route a vent line into the backside of a round or conical KnN filter, you enter to the inside of the filter media. Therefore, an unrestricted path to enter the intake throat. Obviously two different installs that -may- react differently to a possible fuel dump caused by a bad float. Not trying to argue anything, but is a vent line even large enough to deliver a sudden fuel dump into either airbox or filter in an amount sufficient to cause an immediate and large loss of RPM? Let's say fuel makes it's way into the KnN filter via the vent line which enters in the lower portion of the filter below the intake throat. At the time it enter the filter I presume it is a solid stream, rather than atomized. So does gravity cause it to fall and pool or be absorbed by the filter media, or does it get sucked upwards into the intake throat? Is that even possible? I have no clue how much suction force that would take, other than I would guess a lot more for a free flowing filter and less for an obstructed filter. Does anyone know for sure?

I also found some interesting info on the float failures. A soggy float, at least as far as I've read, doesn't generally manifest itself in the form of a catastrophic failure, suddenly and completely all at once without some foreshadowing 'tells'. Most posts I found discussing failed floats included comments suggesting they were alerted by odor or rough running, as opposed to total or significant loss of RPM. Heck, some with intermittent symptoms even continued to fly before completing their diagnosis, yikes. So, no prior fuel odor on the ground or in flight? No prior evidence of fuel saturation on the filter element? No prior unusual or erratic engine RPMs? No prior engine miss at all, at any RPM? No prior evidence of fluid, fuel or condensation, noted in the clear vent lines during pre-flights? That would seem unusual to me, but I'm no expert even with 20/20 hindsight and just thinking out loud. You may never know with certainty exactly what happened, can't let it eat you up.

An un-oiled K&N filter has almost no filtering properties to it.  

I prefer using the bottle oil as opposed to the spray.  One drop per pleat, it will soak through the dry media quickly and you are set.

May be true for a 912, but in a 2-stroke with mixed gas the cleaned filters will be the same color as the oil that is mixed with the gas after a few minutes of running.

After You've got me thinking now.  I had just oiled the K&N filter so it makes sense that it would put even more suction on that vent line drawing fuel right into the carb throat.  At least I would think that saturating the element would add more air restriction which would have to change the pressure differential between the float bowl and the inside of the filter.  I mean I had just oiled the filter the day before.

The proper amount of oil will add very little restriction to the flow of a K&N filter.  Unless you way over oiled them this wouldn't cause a problem.

You've got me thinking now.  I had just oiled the K&N filter so it makes sense that it would put even more suction on that vent line drawing fuel right into the carb throat.  At least I would think that saturating the element would add more air restriction which would have to change the pressure differential between the float bowl and the inside of the filter.  I mean I had just oiled the filter the day before.

The proper amount of oil will add very little restriction to the flow of a K&N filter.  Unless you way over oiled them this wouldn't cause a problem.

All of this new-fangled unproven stuff makes me want to go find a C-75.  I flew one for hundreds of hours with no problems. 

Old EDMO

Nice pictures!  Maybe I'll get home in time to do some ski flying myself.  Drove 725 miles today from Greenville SC  on the way to Ct, From there home in a week or so.  Somehow  I bet winter will be still happening in Mn by then.   Jim Chuk

I have heard that Poly-Tone doesn't take gas spills well, Is that true? I am not against Poly-tone by any means and the price is very appealing compared to aerothane, For those of you that have used Poly-tone, are you happy with how it all turned out and would you do it again,

I shot mine with epoxy primer then shot polytone on top.  No issues with it and it's lived outside in the Alaskan weather since 09.

If you have pinked edges sticking up take a hobby iron and iron them down after you brush in the polybrush.

Very true, but you will have way more sticking up if you are shy on the polybrush below the tapes.  Jim Chuk