912 uls carb vent lines

[hallschuch]

hi guys, reading the rotax installation manual it states:
" The carburetor float chamber venting lines have to berouted into a ram-air and vacuum free zone ... These lines must not be routed into the slipstream orany other location that is subject to ram-air or vacuumduring flight or ground operations. "
So what solutions are available - besides directly into the cockpit? :-)
thanks in advance...Karl

[jiott]

I routed mine into the back side of the air filters, because thats where they go if you have the Rotax supplied airbox. Been that way for 6 1/2 years.

[jrevens]

I routed mine into the air filters also... it seems to work very well.

24166

[jrthomas]

Routing the vent lines to the air filters would definitely not be a "vacuum free zone." One of my buddy's ran his out the bottom into the slip stream (creating a vacuum) and it caused his engine to run rough at midrange. It's a catch 22 because a rough running engine will puke gas out of the vent lines. Mine are about 4" long and my engine is super smooth. My buddy did his like mine and now his engine runs as smooth as mine. A smooth running engine doesn't puke gas.

[DHeal]

As an example, on the RV-12 with Rotax 912 ULS engine the carb vent lines are about 4" long and the ends are tucked down and under the right-side bail wire that secures the carb float bowl. This is a pretty common configuration for engine installations that have simple K&N cone filters (instead of the more complex and costly Rotax-style aluminum air box).

[jiott]

When I first started my brand new 912uls the carb vent lines were tucked under the float bowl bale wires as suggested. It ran great for about a year that way. Then I decided to route them to the back of the air filters and the engine ran just the same as before, very smooth. They have been there ever since because it just makes sense to me; its just like the Rotax airbox, and if any fuel vents out the tubes it will go right back into the carbs to be burned instead of dripping down on the hot exhaust manifold. I understand that there is a slight vacuum inside the filters, but if you keep your filters clean it is very slight. It also should give good warning if fuel starts venting because the engine will start running too rich and will be rough. Anyway that's my thinking on it.

[Dusty]

My vent lines run to the large style K N filters, when one vent line came off I only noticed during an inspection. There was no noticeable difference on the egt guage.

[jrevens]

Routing the vent lines to the air filters would definitely not be a "vacuum free zone." One of my buddy's ran his out the bottom into the slip stream (creating a vacuum) and it caused his engine to run rough at midrange. It's a catch 22 because a rough running engine will puke gas out of the vent lines. Mine are about 4" long and my engine is super smooth. My buddy did his like mine and now his engine runs as smooth as mine. A smooth running engine doesn't puke gas.

Depends on how much restriction there is in the filter, and if the needed airflow is greater than the filter can accommodate. I think the situation is very similar in performance to when the Rotax airbox is used. If you're going to leave them as received & recommended by Rotax, you'd be well advised to have some kind of drip pan underneath like the neat ones Lowell (Highwing) made, or the ones used on the RV-12s. That can make the removal of the float bowls much more of a pain in the neck though. I can see why your buddy's ran rough, with long lines going out into the slipstream (exactly what Rotax tells you not to do). The end of the cone filters that I installed the little elbows in is pretty thick and rubbery. The fittings fit in the hole tightly and just barely poke through on the inside. Like Jim said, and many others have also discovered, it works perfectly there, and I can tell you that my engine runs very smoothly throughout its range. As they say, "your mileage may vary".

[HighWing]

A picture of my vent line. This set-up was to eliminate the possible risk of a fuel drip onto the muffler as a couple of guys had reported. It provided a drain without moving the vent to a non neutral location.

[hallschuch]

thanks everyone, all your inputs are truly appreciated - this is a very friendly community.

[DHeal]

It should be noted that the carb drip pans serve two functions: 1) to deflect venting carb fuel from falling directly on the exhaust pipes and 2) to somewhat insulate the carb bowls from heat radiated by the exhaust pipes.

[HighWing]

Likely true, but in my 2 Model IVs, I had the drip pans on the first one and the pictured (previous post) variation on the second because I found with the nine years I had the drip pans, they made routine inspections and adjustment of the carburetor much more difficult as the float bowl couldn't be removed without separating the carburetor from the manifold as the drip pans I used were bolted to the manifold (Picture below). In the normal running of the engine - ground and flight - there was no apparent difference with and without. This latter was emphasized when I started flying with the local group of six as we began exploring California and the back country of Idaho. We flew together for 7 or 8 years and none of the other guys had drip pans.

[kitfox2009]

I know this method will not meet the requirements of many of our "professional" builder/owners but for almost 900 hours now I have zip tied a piece of heat shield material purchased from a local auto supply store under the carbs. The auto performance folks use this to insulate starters etc from extreme engine heat. I attach it to the manifold and carb throat so it forms a "bag" under the carb. Keeps heat away and prevents fuel from running on the exhaust. Easily removed for servicing. I also run the vent tubes into the air filters.

I can post pics if anyone is interested!

Take care and be safe. I find the best way to self isolate is IN the HANGER or better yet IN the AIR!


Cheers
Don
Vixen C-FOXK 912UL IVO IFA

[Delta Whisky]

Don - I'm interested in the pictures - when you get time. Thanks, Darrel

[kitfox2009]

I know this method will not meet the requirements of many of our "professional" builder/owners but for almost 900 hours now I have zip tied a piece of heat shield material purchased from a local auto supply store under the carbs. The auto performance folks use this to insulate starters etc from extreme engine heat. I attach it to the manifold and carb throat so it forms a "bag" under the carb. Keeps heat away and prevents fuel from running on the exhaust. Easily removed for servicing. I also run the vent tubes into the air filters.

I can post pics if anyone is interested!

Take care and be safe. I find the best way to self isolate is IN the HANGER or better yet IN the AIR!


Cheers
Don
Vixen C-FOXK 912UL IVO IFA

Hello Delta Whiskey
This pic was in my "profile pics". I hope that I have moved it here correctly. If not just browse through them. Seems like it worked OK (yea)

[Woodymuler]

HighWing,
I really like your solution to this issue, to the point that I acquired the two little funnels after seeing your photo in March. It is now at the top of my short list and was pondering the use of clear drain lines from the funnels instead of the aluminum just for simplicity??
I see the zip tie holding the vent tube in the little funnel. Does the rigid aluminum tube keep the funnel in place or is it secured somehow next to the carb?

KF5 w/7 FWF
First flight, hopefully this year

[HighWing]

HighWing,

I see the zip tie holding the vent tube in the little funnel. Does the rigid aluminum tube keep the funnel in place or is it secured somehow next to the carb?

KF5 w/7 FWF
First flight, hopefully this year

I am away from the airplane, but this photo I found tends to suggest that the funnel is supported by the aluminum tubing and the Zip Ties are there solely to keep the vent tube inside of the funnel.
Lowell

[Delta Whisky]

Since the last post brought this subject back into the "New Posts" option, I thought I'd add a picture of what I finally decided to do. Basically I followed the lead of those that routed the line back into the air filter.

[kitfox2009]

Since the last post brought this subject back into the "New Posts" option, I thought I'd add a picture of what I finally decided to do. Basically I followed the lead of those that routed the line back into the air filter.

Exactly what I have done now for the past 5 or 600 hours on my 912 UL. Seems to work well.

Regards
Don

[jiott]

Mine are done just like Delta W except I put the hole into the air filter as near the bottom as possible to get as much downslope to the vent line as possible. Been that way for about 900 hrs. The only downside I see to this arrangement is if your carb bowls start to overflow due to sinking floats or excessive vibration you will immediately get a super rich fuel mixture which would cause the engine to run very rough or even kill it. If this starts happening you will get immediate warning, probably on the ground during warm-up. Those engines with Rotax supplied airbox do essentially the same thing with the vent lines-run them into the airbox which feeds the carb intakes, so the risk must be quite small.