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seagull

Well Known Member
This is what I have done and my results.

I extended the bowl vent tubes and inserted them 1” into the rear of their respective K & N air cleaners. The improvement was a smoother running engine and a GPH reduction in excess of .5GPH.
After doing the mod I flew locally and immediately could see the lower GPH on the Dynon. The power feels better but it could be subjective. I flew two flights of equal time and altitude. With fuel refill at the end to confirm the GPH was indeed lower.

The testing;
I installed 2 tubes connected to pressure sensors, the tubes came back inside the cockpit to my gauges. One tube was parallel to the stock vent tube under the carb bail alongside the existing tube. The second tube was put inside the K & N air cleaner. Both tubes were installed on the left side carburetor. I flew from 1500’ to 6000’ and at all air speeds and attitudes. During the flight the pressure reading was mostly equal on both sensors, but when it did vary the pressure at the vent tube (under the bail) was always higher in comparison to the one inside the air cleaner. My suspicion is the air swirling inside the cowl varies in pressure depending on the angle and speed that it enters the cowl.

The simple explanation;
When the air pressure above the fuel in the bowl (the vent tube) is higher than the air pressure below at the carburetor intake, it pushes down causing more fuel to go through the jets causing a richer mixture.
When the pressures are equal the carburetor meters the fuel as designed.

The carburetor vent tube is supposed to be in a location that has the same air pressure as the air entering the carburetor. A Rotax with the stock airbox has connections for the vent tubes. When the vent tubes are connected to the airbox it is in the same air that is entering the carburetor throat.

I now have 30 hours on this mod and am convinced it has made a positive improvement. With this mod I am averaging 3.8 GPH on flights averaging 110 -115 kts TAS. I fly very close to GW, typically take off from 1500’ climb to 6-7000’. My ground time averages 5 minutes, (engine is pre-warmed). I pulled the plugs after 15 hours and they were light tan, looking better than they have in the past. My EGT’s are in the low 1300’s.
 
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Do you have pictures on the 'install'?

I assume you mean the clear plastic tube in this pic? How'd you get it into the air cleaner?
 

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Do you have pictures on the 'install'?
I assume you mean the clear plastic tube in this pic? How'd you get it into the air cleaner?

I drilled a hole slightly undersized in the center of the air cleaner, the hose is a snug fit. Originally I used a small tie to keep it from moving inward, (top pic). I later was concerned about the tie reducing the hose ID so I took a spark plug tip, (the threaded part we remove), drilled out its ID and inserted it 1' into the hose to serve the same purpose, (bottom pic).

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Now that right there is darn clever. I have read that the clear tube should be in the same pressure area as the carb intake. This is the first one I've seen easily done on our ULS motor.

What is that little yellow box on your engine mount standoff? Crack detector?
 
See first photo above... Notice the safety wire on enrichment cable where it enters the adjustment ferrule. I mentioned this once in a thread and Van's said not necessary. It keeps the cable housing engaged in the socket if the return spring should fail. Just makes sense...
 
See first photo above... Notice the safety wire on enrichment cable where it enters the adjustment ferrule. I mentioned this once in a thread and Van's said not necessary. It keeps the cable housing engaged in the socket if the return spring should fail. Just makes sense...

Agreed. I have safety wire in place there as well.

And Seagulls approach here is very interesting. Curious what the potential downsides/risks are?
 
"Curious what the potential downsides/risks are?"

Likely it will not happen but if the spark plug ferrule works loose it could be ingested into the engine -- $$$$. Also, I wonder if any residual vented fuel in the tube might pose a carb bowl vent air flow issue? I like the overall concept.
 
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"Curious what the potential downsides/risks are?"

Likely it will not happen but if the spark plug ferrule works loose it could be ingested into the engine -- $$$$. Also, I wonder if any residual vented fuel in the tube might pose a carb bowl vent air flow issue? I like the overall concept.

The "Ferrule" is tight in the tube, even if it wasn't I could not get through the hole in the air filter and could not go the other direction through the fitting at the carburetor.

As for the "residual" concern, this is exactly the way Rotax designed the carburetors to be vented through their air box.
 
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Interesting idea. I wonder if a hose bulkhead fitting would be a good tie-in to the filter.
My thinking on that was;

I wanted the hose to terminate in the center of the air filter to be in the middle of the pressure area, maybe this doesn't matter......

The hose through the hole is simple, KISS, and makes the corner better than a fitting.
 
One possible drawback is what will happen if (when) floats sink and carb float bowl overflows. I have witnessed this firsthand and it is horrifying to see. Facet electric fuel pump flows something like 1 GPM, and when not shut off by float needle valve, the fuel has got to go somewhere. The drip pan is a joke but slightly better than nothing at all.

My concern is… with the engine off, if the floats sink with the fuel pump operating, raw fuel will pour into the air filter at a high rate. Fuel will follow the throat of the carb and end up flooding the intake manifold and whichever cylinder has its intake valve open when the engine is stopped. The potential for hydraulic lock and bent connecting rod should not be overlooked.

I’m not familiar with the airbox setup on some Rotax engines but my guess is there is design consideration to eliminate possibility of fuel entering the carb throat.

I should be sleeping and not up at 3:30 AM worrying about this stuff...
 
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One possible drawback is what will happen if (when) floats sink and carb float bowl overflows. I have witnessed this firsthand and it is horrifying to see. Facet electric fuel pump flows something like 1 GPM, and when not shut off by float needle valve, the fuel has got to go somewhere. The drip pan is a joke but slightly better than nothing at all.

My concern is… with the engine off, if the floats sink with the fuel pump operating, raw fuel will pour into the air filter at a high rate. Fuel will follow the throat of the carb and end up flooding the intake manifold and whichever cylinder has its intake valve open when the engine is stopped. The potential for hydraulic lock and bent connecting rod should not be overlooked.

I’m not familiar with the airbox setup on some Rotax engines but my guess is there is design consideration to eliminate possibility of fuel entering the carb throat.

I should be sleeping and not up at 3:30 AM worrying about this stuff...

That is one scenario, BUT I doubt the small tube would be able to relieve the massive flow in your situation. Lets assume you keep the stock vent into the drip pan with your scenario and the pan overflows, the fuel will hit the hot manifold, catch fire and then what?

I think if choosing I would rather have a flooded engine with a bent rod that quits in flight than the engine compartment on fire, maybe that's just me though.

Actually I believe if the bowl flooded and the overflow went into the air cleaner it would leak through the filter element dripping out the bottom missing all the hot parts. An idea would be to seal the end of the tube inside the air cleaner and put holes perpendicular to the tube so overflow fuel would squirt to the sides rather toward the carb throat.

I just finished a late movie, earlier here, but hitting the sheets anyway.
 
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I expect that if overflow fuel flowed through the small tube into the air filter as pictured, it would simply drain down through the side of the air filter and not make it into the carb air intake.
Nice idea and nice work seagull to come up with this solution!
 
I’m still thinking about this. The current overflow hose is pointed vertically down. If the float needle valve does not shut off, fuel will flow very easily thru the overflow tube and gravity will be your friend and assist with a syphon. Flow is fast and volume is plentiful – I’ve witnessed it firsthand.

If this longer vent hose runs horizontal, or worse yet is inclined, the fuel overflow will meet resistance and likely take an easier path. Here is where you might have a problem. If overflow is restricted the fuel will purge through the main needle jet directly below the vacuum-operated piston. The needle is tapered and does not close off the main jet. If fuel takes this path, it will certainly fill the intake manifold and find its way to the open cylinder. I have experienced this with motorcycle carbs...
 
If overflow is restricted the fuel will purge through the main needle jet directly below the vacuum-operated piston. The needle is tapered and does not close off the main jet. If fuel takes this path, it will certainly fill the intake manifold and find its way to the open cylinder.

If this condition happens the engine will run extremely rough. There will not be enough fuel entering through the needle to cause hydraulic lock. As pilot you will know something is wrong and reduce throttle or shut off the engine.

Keep in mind Bing does not call that hose an overflow, it is a vent hose, for air venting. It is there to allow the bowl to achieve the same air pressure as the intake of the carb. The vent is not designed to flow volumes of fuel out of the bowl so the situation you are envisioning with a stuck float and fuel flooding the engine will happen regardless where you put the vent tube.

The stock RV-12 configuration with the vent hose dumping into the drip tray is only safe if you have "drips", anything greater than a drip is a setup for disaster sitting over a hot exhaust.

Considering the large number of planes setup this way and lack of reports of floods, fires, and crashes, I don't think it is a big concern either way.
 
Bing and Rotax can call the clear hose anything they want. “Overflow” might raise some eyebrows and detract from sales. It certainly functions as a vent to normalize atmospheric pressure in the float bowl, so call it a vent. I have personally seen fuel pouring out of the tube on the right carb of a friend’s RV-12 (100TT) shortly after landing.

Folks with motorcycles learn to shut off fuel valve. Search BMW (Bing) carb leak onto the floor of your garage. Lots of unhappy people. Motorcycle is different setup due to fuel tank above engine for constant head pressure. RV-12 has carbs mounted higher than fuel tank – so no fuel pressure unless electric pump is running.
 
Motorcycles park with a kickstand, this puts the carb & floats at a slant the needle seat doesn’t work properly that way, that is why they leak.

I didn’t get the kick stand option with my -12.
 
If those float bowl hoses are an overflow, why would Rotax route it to their air filter box in the first place?

While it can function as an overflow, I doubt that's the intent of the Bing /Rotax design with those tubes.
 
True, but you did option for the Bing sinking floats like all the rest of us did…

I have had sinking floats. It is a problem that is noticeable by a rough running engine then confirmed with a weight test and corrected with new floats. Very annoying but nothing that would cause the castrophy you are promoting.
 
Walter-

I think you may be onto something by routing the carburetor air vent back to the air filter. Based on Rotax documentation, it appears that may be a good option to consider.

I've never liked the way the vents are plumbed to just spill fuel onto the drip tray over a hot exhaust. As Jim has mentioned, when there is an issue with one of the floats, fuel just pours out of that vent tube. I had one experience where prior to engine start I caught a whiff of fuel and fortunately didn't hit the starter. Upon investigation, the left carb was overflowing a lot of fuel. (Interestingly, I did NOT have a heavy float, nor was there any debris found in the bowl. I did flush the fuel into a clear tray and verified the needle valve would shut the fuel flow off. Never found the root cause of that event and have not had any issues since ... either one float was stuck or more likely the needle valve was stuck).

I really don't see any down side to routing the vent tube back to the air filter as you did as long as the vent tube is kept at a downward slope. Should there be a fuel overflow into the vent tube, I think as previously mentioned by others, most of the excess fuel would just drip out of the filter element.

My only pause for concern is to make absolutely certain it is impossible to ingest any hardware into the carbs. I think some sort of fitting that could be inserted into the air filter would be a better choice for the vent hose ... something like either of the following two fittings:
https://www.mcmaster.com/5058K798/
The 90° fitting could be inserted into an undersized hole in the filter then sealed from the outside with a dollop of engine RTV. The 90° fitting is available in both 4mm or 1/8" ID tubing.
https://www.mcmaster.com/2653K11/
The straight fitting (only available for 1/8" ID tubing) could be inserted into an undersized hole in the filter until the aft wall of the filter is located in the center of the fitting then sealed in place from the outside with a dollop of engine RTV.

I think it would be near impossible for either fitting to be ingested if installed correctly and something I'm looking forward to trying this in the future ... will likely begin with the 90° fitting.
 
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John G,

The method I used has nothing that could get sucked into the intake. The vent tube fits snugly through the rubber at the end of the air cleaner. The ferrule is inside the tube causing the OD to be increased so the tube is set at 1" inside the air cleaner.

Adding fittings as you suggest is unnecessary, I like the KISS principal.

I am going to modify the end of the tube inside the air cleaner like this photo. This will assure any overflow is directed to the sides of the air cleaner and having 4 holes will insure even pressure.

The end of the vinyl tube is sealed with heat from a small torch, heat it all around then twist the tube. The holes (4) are made using the wrong end of a heated 1/16 drill bit.

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