Hello Roger
I am over 80 hours now with the vent tubes routed into the rear black flat ends of the air filters.They are just a snug fit protruding 1/2" or so inside the filter. They seem to work well. Question: If these vents are so sensitive to air pressure, will not the pressure inside the filter be lower than outside? Or am I just trying to find a problem that does not matter? Why doesn`t Rotax just recommend this configuration?
Cheers Don
There's no air pressure inside the filter just volume of flowing air which is at ambient temps and barometric pressure, I'm surprised that you haven't had gas leakage but then how would you know, it would be sucked into the carbs, the Rotax routing is thru the carb clip that holds the bowl on facing to the rear, I lengthened mine out so that they are closer to the rear of the engine just to be on the safe side in case one started overflowing, the thought of fire while flying scares the Hell out of me, short of having a fire suppression system there's not much one can do.
I know a lot of people that have the vent tubes situated in the air filters and have never had an issue. Rotax has their vent tubes routed to their airbox. The Flight Design CTSW with the original air filter setup has the vent tubes routed into the airbox. It's okay.
Roger Lee LSRM-A & Rotax Instructor & Rotax IRC Tucson, AZ Ryan Airfield (KRYN) 520-349-7056 Cell
The stock setup on my Ikarus C42B with an 80hp 912 is having the vent tubes going behind the air filters through a hole in the black outer rubber ring of the air filter. I recently replaced both air filters and followed the directions in the Rotax manual, ie. tube under bowl clip, but the engine ran rough at cruise. The pressure inside the air intake tubes on the C42 must be so high at cruise speed that the engine is not getting enough fuel when the tube senses ambient pressure from the engine bay.
I've got the same problem with my right carb on my 912ULS. This started happening immediately after I installed the carbs after being rebuilt by Rotech (Canada). This does not appear to happen when the engine is not running. If it is running and I turn on the electric fuel pump, it spits fuel out of the air intake side (I guess). Electric fuel pump delivers around 3.8 psi when the engine is not started. I have a manufacturer installed air box between the carbs with a single air filter. The air box is slanted down on each side with two drain hoses (one on each side for each carb). One of these hoses spits out fuel. I mentioned this to Rotech and they said that they pressure test the float valve by turning the carb upside down and apply 7 PSI of air pressure. They said it passed their test. I've synced both carbs (mechanical and pneumatic) using a digital Carbmate and have 0" differential pressure at idle. I can't figure out why it is doing this. Today I decided to fly for an hour without using the electric fuel pump during take off/climbs and when I parked the plane for about 30 minutes it had a puddle of fuel underneath the plane. It did not spit fuel while exiting the aircraft after the flight, but I suspect that the fuel is heating up under the hot cowling (without any air flow to keep it cool) while it was standing there for 30 minutes and the fuel in the float bowl is expanding and finds it way out the drain. I never recalled it ever doing this before, so there must be a common problem. I'm at a loss as to what the issue is. Any comments or suggestions?
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