912 uls carb vent lines

[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).

[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

[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.