Rotax 912 carburator vent lines

[P Morel]

I've been trying to get rid of the fuel smell I get after landing from the vent lines of the bing carburetors.

I originally had the tygon vent lines plumbed back into the larger KLN air filters but once I replace the larger filters with smaller, that was not an option. Besides, plumbing back into the filters really didn't resolve the venting problem.

I'm curious if anyone has taken the vent lines and either extended them down the side of the firewall or even "T"d them together and then routing the line down and away from any heat source. My biggest concern with altering the length of the tygon vent line is if there would be any adverse effect on the line pressures that would cause the carbs to expel additional fuel or not.

I know this has been an ongoing topic with the Rotax 912/Kitfox since the intake manifold has to be reversed to accommodate the oil tank.

I'd like to know how some of you resolved this problem and I'm willing to give it a try on my plane.

[sturdee]

Vent lines are short probably around 4" and are secured under the bowl clip, any lengthening will effect the altitude mixture during climb. Any leaking through the vent lines indicates your float levels are not set right! Or carbs badly out of balance causing severe shaking during low rpm. Get them checked out if they are flooding fire risk if you don't have drip trays fitted under the carbs as left side right over exhaust. Carb balancing takes ten minutes if you have the simple electronic balancer, safe flying mike mk4

[Dave S]

Pmorel.

1) Mike's comment about the length of the line is right on. The tube/line is used for balancing the pressure in the carb and anything that changes it, like extending the line will adversly affect the intended purpose. Ideally the tubes would be connected to an Airbox, but that is not an option on Kitfoxes with teh center/rear mounted oil tank.

2) The fuel odor indicates the the carbs are likely overflowing after shutdown,(assuming all other possible sources of fuel release/spill/leak have been ruled out - might want to be sure the odor is showing up via the vent tubes).

3) Question....Are you saying the odor shows up as soon as you land; or, is this a problem that shows up after the engine is shut down?

Sincerely,

Dave S

[kitfox2009]

Hi Paul
I extended mine and ran them into the back of the K&N air filters. Been that way now for over 100 hours and everything seems right.
Cheers
Don

[P Morel]

Vent lines are short probably around 4" and are secured under the bowl clip, any lengthening will effect the altitude mixture during climb.

Thanks for all the help. The fuel smell occurs after landing and throttled back to arouk 2k rpm. As I increase the rpm, the odor dissipates that indicates to me the engine is smoothing out with less shaking. I'll be sure to make my vent lines around 4".

[MotReklaw]

Hi Paul,

I vented those tubes into the K&N filter on my second 912ULS. I used the original tubes and they worked fine, plus no more accidental dripping onto the exhaust on startup or shut down.

In my Zenith days, lots of discussion was made about keeping those tubes short and in the vacinity of the carbs for altitude compensation. One of the top Rotax mechs in AZ said that's what he did on 912's that didn't have the airbox (put the tubes in the filters) We never had a problem. It was almost a straight shot from tube carb tube outlet to the K&N filter. Don't know what kind of filters you are using.

[foxkit3]

You can cheet with your vent lines by taking a plastic vaccum line tee, find one that looks like a Y if possible take and cut your vent line to the proper length install your T so the straight threw portion is inline than route your now drain line nice and low

[HighWing]

You can cheet with your vent lines by taking a plastic vaccum line tee, find one that looks like a Y if possible take and cut your vent line to the proper length install your T so the straight threw portion is inline than route your now drain line nice and low

I have wondered if your idea would work. It sounds very clean and simple.
Lowell

[jrevens]

I'm guessing that the issue is trying to make sure that the pressure where the common line terminates is the same as that at the inlet of both carburetors. Are the air pressures the same at both carburetors, or do they vary due to factors beyond our control? It seems like Rotax would recommend connecting these to a common line (directed away from exhaust pipes, etc.) if this wasn't an issue. If you have a common air-box for both carbs, part of that potential issue is solved, but the pressure at the end of the tube still has to be the same as the common pressure in the box. Am I thinking right about this?