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  #1  
Old 03-25-2021, 02:07 PM
seagull seagull is online now
 
Join Date: Dec 2019
Location: San Bernardino
Posts: 280
Default YMMV “Your Mileage May Vary”

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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Last edited by seagull : 03-25-2021 at 02:10 PM.
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  #2  
Old 03-25-2021, 02:12 PM
JFCRV12 JFCRV12 is offline
 
Join Date: May 2015
Location: Liberty Township OH
Posts: 192
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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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Last edited by JFCRV12 : 03-25-2021 at 02:15 PM. Reason: added a picture
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  #3  
Old 03-25-2021, 02:59 PM
seagull seagull is online now
 
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Location: San Bernardino
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Quote:
Originally Posted by JFCRV12 View Post
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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  #4  
Old 03-25-2021, 03:15 PM
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Tony_T Tony_T is online now
 
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Location: Lacey, WA
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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?
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  #5  
Old 03-25-2021, 03:19 PM
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Piper J3 Piper J3 is offline
 
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Location: Hinckley, Ohio
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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...
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  #6  
Old 03-25-2021, 03:20 PM
seagull seagull is online now
 
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Location: San Bernardino
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Quote:
Originally Posted by Tony_T View Post
What is that little yellow box on your engine mount standoff? Crack detector?
EGT thermocouple connector.
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  #7  
Old 03-25-2021, 03:28 PM
DHeal DHeal is offline
 
Join Date: Sep 2006
Location: Windsor, California
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Those yellow EGT connectors are at:

https://www.omega.com/en-us/search/?text=SMPW-CC-K-M
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Last edited by DHeal : 03-25-2021 at 03:32 PM.
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  #8  
Old 03-25-2021, 04:55 PM
JFCRV12 JFCRV12 is offline
 
Join Date: May 2015
Location: Liberty Township OH
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Quote:
Originally Posted by Piper J3 View Post
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?
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Old 03-25-2021, 05:23 PM
DHeal DHeal is offline
 
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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.
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David Heal - Windsor, CA (near Santa Rosa)
EAA #23982 (circa 1965) - EAA Technical Counselor and Flight Advisor; CFI - A&I
RV-12 E-LSA #120496 (SV w/ AP and ADS-B 2020) - N124DH flying since March 2014 - 1,030+ hours (as of May 2021)!
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Last edited by DHeal : 03-25-2021 at 05:29 PM.
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  #10  
Old 03-25-2021, 05:48 PM
seagull seagull is online now
 
Join Date: Dec 2019
Location: San Bernardino
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Quote:
Originally Posted by DHeal View Post
"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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Last edited by seagull : 03-25-2021 at 06:02 PM.
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