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  #1  
Old 05-04-2018, 06:45 AM
JRo JRo is offline
 
Join Date: Dec 2011
Location: Poestenkill, NY (near Albany)
Posts: 126
Default RV-12 fuel problem

I'm having a fuel problem in the following configuration: engine warm, full throttle climb: (My electric fuel pump is switched) Fuel pressure indication suddenly goes low (around 2 psi) but engine continues to run smoothly. I switch on the electric pump & pressure returns to normal, but fuel FLOW indicates high (around 10 gph) and I smell fuel. I switch pump off, flow returns to normal, fuel pressure goes low again, fuel smell dissipates.

Pre- and post-flight inspection shows full tank, clean gascolator, clean float bowls, all lines secure & tight. The fuel pump had already been replaced in accordance with Service Bulletin, and I am on the latest & greatest in a long, tiresome & incredibly expensive evolution of carburetor floats (The last 2 sets of floats were $300!).

What's my problem?

Thanks,
Jim
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  #2  
Old 05-04-2018, 06:59 AM
Mich48041 Mich48041 is offline
 
Join Date: Apr 2008
Location: Riley TWP MI
Posts: 2,693
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Since you can smell fuel, it could be coming from a carb vent or from the engine driven fuel pump vent holes. I suspect the fuel pump because the engine runs smoothly. Even new pumps can fail.
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  #3  
Old 05-04-2018, 08:29 AM
JRo JRo is offline
 
Join Date: Dec 2011
Location: Poestenkill, NY (near Albany)
Posts: 126
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Thanks Joe. Any way to test the pump? Could heavy floats explain this problem?

Thanks, Jim
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  #4  
Old 05-04-2018, 08:32 AM
JBPILOT JBPILOT is offline
 
Join Date: Nov 2007
Location: Jesup, Iowa
Posts: 1,598
Default Questions - -

When sitting on the ground, what is the flow reading with elec pump on and off ?
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  #5  
Old 05-04-2018, 08:48 AM
DHeal DHeal is offline
 
Join Date: Sep 2006
Location: Windsor, California
Posts: 721
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When I get an unusual whiff of fuel smell in my RV-12 (occasionally occurs during a high-power climb), I usually suspect a sinking (heavy) float -- I too have replaced many defective (and costly) floats. If the float weights are OK, then it could be a misadjusted float height or a leaking/improperly placed float bowl gasket. Also, a bit of debris in the fuel return orifice could cause the fuel pressure in the carb bowl to be excessive. Excessive engine vibration can also cause fuel to vent.

The occasional brief "low fuel pressure on climb-out but engine continues to run normally" condition is noted by many RV-12 operators. A definitive reason and cure for this appears elusive.

ps -- RCAP was my old aeronautical hang-out during my 1969-1973 RPI days -- good times.
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  #6  
Old 05-04-2018, 11:15 AM
RFSchaller RFSchaller is offline
 
Join Date: Oct 2010
Location: Phoenix, AZ
Posts: 2,322
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I have a switched pump too. I see fuel flow go up about 1 GPH when I turn it on.

I had the same symptoms last summer without the gas smell. It turned out to be vapor lock from old auto gas. I attributed the jump in flow indication to be due to vapor bubbles going through the red cube.

When I smelled fuel a couple of years ago it was caused by leaking bowl gaskets, and I good see fuel in the carb drip trays.
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  #7  
Old 05-04-2018, 05:14 PM
Piper J3's Avatar
Piper J3 Piper J3 is offline
 
Join Date: Nov 2014
Location: Hinckley, Ohio
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Quote:
Originally Posted by RFSchaller View Post
I have a switched pump too. I see fuel flow go up about 1 GPH when I turn it on.

I had the same symptoms last summer without the gas smell. It turned out to be vapor lock from old auto gas. I attributed the jump in flow indication to be due to vapor bubbles going through the red cube.
The "jump" in fuel flow with electric pump running is the added fuel returning to the tank. Many threads here in RV-12 VAF talk about the necessity of running both fuel pumps continuously on the ground and in the air. There are safety-of-flight reasons for doing this...
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Hinckley, Ohio
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PPL/ASEL 1970 - Sport Pilot since 2004
80 hrs Flying Aeronca Chief 11AC
1130 hrs Flying 46 Piper J-3 Cub
RV-12 E-LSA #120058 AWC Jul 2012 - Bought Flying Oct 2015 with 48TT - Hobbs now 400

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  #8  
Old 05-04-2018, 09:07 PM
RFSchaller RFSchaller is offline
 
Join Date: Oct 2010
Location: Phoenix, AZ
Posts: 2,322
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Jim,
The jump I was referring to was during the vapor lock, not the increase due to increased return flow.

Rich
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  #9  
Old 05-05-2018, 08:18 AM
John-G John-G is offline
 
Join Date: Jan 2012
Location: Northeast Ohio
Posts: 525
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Looking back, this seems to be a reoccurring spring/early summer theme. There are many posts here sharing similar symptoms. Yes, the carb floats and fuel pumps have been a big issue in the past and shouldn’t be overlooked … but I “feel” there is another contributing factor that shouldn’t be overlooked or taken lightly … use of winter blend auto fuel on warm days.

The winter blends of auto fuel used around the country are formulated to vaporize at lower temperatures for better winter emissions. Unfortunately, this means during warmer spring and early summer days the use of winter blended auto fuel can be sketchy in our tightly cowled time machines because any significant heat buildup under the cowl will have the fuel on the verge of vaporization.

I feel it prudent to consider running 100LL or a blend of 100LL and auto fuel during the few weeks when the air temperature is getting hot, but the summer auto fuel blend is not yet available at service station pumps.

Use of winter blend auto fuel may not be the OP’s main issue … but considering it is likely that the majority of RV-12 owners do use auto fuel, vapor lock is a real possibility this time of the year. Until summer auto fuel blends become available in your area, consider using some 100LL in the interim to help mitigate the issue.

Happy flying,
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www.dogaviation.com
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  #10  
Old 05-05-2018, 03:48 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 7,754
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Quote:
Originally Posted by Piper J3 View Post
The "jump" in fuel flow with electric pump running is the added fuel returning to the tank. Many threads here in RV-12 VAF talk about the necessity of running both fuel pumps continuously on the ground and in the air. There are safety-of-flight reasons for doing this...
The slight increase in pressure wont change the flow rate in the 1/4" diam return line very much.

I have seen failing engine driven pumps dump fuel out the vent outlet and cause a jump in fuel flow when there is input pressure from the aux pump, but then have the leakage stop (and the fuel flow drop), when the aux pump is shut off by pulling the fuse.

Don't overlook the possibility that the engine driven fuel pump might be failing.
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