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  #1  
Old 04-27-2018, 09:18 AM
Sdemeyer Sdemeyer is offline
 
Join Date: Apr 2015
Location: Olympia,wa
Posts: 46
Default Vapor lock?

My 6A with an o320 -150hp , carb does not like mogas and I am having a hard time understanding whats really going on. Seems that when the OAT gets anywhere close to 80 and after a long taxi, when I apply takeoff power, the carb floods after a few seconds, creating a huge cloud of black smoke out the exhaust and then the engine completely quits. This happens almost all the time if I have the boost pump on, but not as often with the boost pump off. My fuel pressure never exceeds 4.5 PSI with the boost pump on and with it off, its .5 psi, when this happens. If I run straight avgas, I never have a problem. I installed the 150HP for the sole purpose to run mogas so this is a little frustrating and I would like to find a way to make this work, if possible. The mogas I use works perfect in my other planes but they are not RV. This is a new engine with new fuel pump, carb and everything else related to the fuel system. I always thought vapor lock was lack of fuel, not too much fuel.What am I missing here?
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Olympia,WA
RV6A quickbuild, Flying as of 10-14-2017
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  #2  
Old 04-27-2018, 09:31 AM
Mike S's Avatar
Mike S Mike S is offline
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Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 14,080
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When you push the throttle in for full power, are you just shoving it in quickly, or easing it in over a 2or 3 second period?

The higher OAT and long taxi sure do point to an overheating of the fuel issue----have you looked for places where the exhaust pipe and fuel line are close together without having a heat shield in place?

How long has this been going on------how long has the plane been flying?

What is the field altitude?

Any work done on either the carb or fuel system prior to the start of the problem?
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Mike Starkey
VAF 909

Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

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  #3  
Old 04-27-2018, 12:58 PM
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airguy airguy is offline
 
Join Date: Oct 2006
Location: Garden City, Tx
Posts: 4,101
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You're simply heating the fuel to the boiling point, and then it's flashing to vapor and pushing the mix into the engine, flooding it out.

It's not what you want to hear, but the solution is injection with high pressure to keep the fuel from boiling - or much better cooling FWF which is hard to do on an RV. The problem will usually be worse in winter than summer (my experience) due to the winter-blend fuel that has an even higher vapor pressure.

I run premium autofuel (E10) in my 9A, standard Bendix injection but no engine driven fuel pump (dual electrics) with an AFP purge valve and it works great - once you figure out the technique - but it's definitely different than 100LL. Of course, it's also sub-$3 per gallon...
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Greg Niehues - VAF 2018 dues paid
Garden City, TX
N16GN flying! http://websites.expercraft.com/airguy/
Built an off-plan 9A with too much fuel and too much HP. Should drop dead any minute now.
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  #4  
Old 04-27-2018, 01:08 PM
tsneidin tsneidin is offline
 
Join Date: Jul 2005
Posts: 105
Default

Have you got pictures of your dual electric pump install?

Quote:
Originally Posted by airguy View Post
You're simply heating the fuel to the boiling point, and then it's flashing to vapor and pushing the mix into the engine, flooding it out.

It's not what you want to hear, but the solution is injection with high pressure to keep the fuel from boiling - or much better cooling FWF which is hard to do on an RV. The problem will usually be worse in winter than summer (my experience) due to the winter-blend fuel that has an even higher vapor pressure.

I run premium autofuel (E10) in my 9A, standard Bendix injection but no engine driven fuel pump (dual electrics) with an AFP purge valve and it works great - once you figure out the technique - but it's definitely different than 100LL. Of course, it's also sub-$3 per gallon...
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  #5  
Old 04-27-2018, 01:25 PM
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airguy airguy is offline
 
Join Date: Oct 2006
Location: Garden City, Tx
Posts: 4,101
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Quote:
Originally Posted by tsneidin View Post
Have you got pictures of your dual electric pump install?
Last time I opened it up for putting in padding for the pumps and lines, lots of prior pictures as it took me a while to reach my final configuration that I was happy with. I had to build a special mount plate for a pair of AFP pumps, one from each tank, and stretch the standard doghouse that covers the boost pump area to house it. There were about 8 or so iterations of this design before I called it good.

http://websites.expercraft.com/airgu...y&log_id=71325
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Greg Niehues - VAF 2018 dues paid
Garden City, TX
N16GN flying! http://websites.expercraft.com/airguy/
Built an off-plan 9A with too much fuel and too much HP. Should drop dead any minute now.
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  #6  
Old 04-27-2018, 03:54 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 2,689
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When fuel reaches the boiling point inside the carb bowl, it allows the float to drop below normal (the boiling fuel doesn't offer the same buoyancy as normal fuel) and this allows the bowl to overfill, causing a rich condition (fuel metering is combination of jet size and fuel height in bowl, in addition to negative pressure from the venturi). The boost pump adds extra volume exacerbating the problem. Mogas has a lower vapor point and boils cooler than avgas. Winter blend is even lower than summer blend mogas. If this problem is a regular event, I suggest a fuel return circuit to feed cooler fuel to the carb. I have one and use it religiously when taxing in hot weather or after a heat soak condition.

Larry
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Last edited by lr172 : 04-27-2018 at 03:58 PM.
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  #7  
Old 04-27-2018, 04:38 PM
rv7charlie rv7charlie is offline
 
Join Date: May 2006
Location: Pocahontas MS
Posts: 2,819
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Hi Scott,

I've run E-free premium mogas in both the 160 hp carb'd -4s I've owned, for around 20 years. I've had *one* issue with vapor lock, and that one was totally my fault. Old, *winter blend* gas (forgot it was still in the tanks; wasn't flying much), 100+ degree day, short flight followed by a long session sitting on a very hot ramp. I'm confident that the low-center mounted gascolator contributed to the problem. The engine started running very rough when I got to around 1k feet on takeoff.

After the incident, I removed the gascolator, made sure I don't run winter gas in the summer, and I've never had any issues since.

As others mentioned, look for areas where the exhaust pipe can radiate directly at the fuel line, gascolator, carb, etc. Note that it doesn't have to be that close; just a clear 'look' at the fuel system.

I should mention that there can be some variation in the vapor pressure of mogas. If your state has different rules about vapor pressure, your gas might have higher vapor pressure than mine. I live in MS, so it's rarely not hot here. :-)

Charlie
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  #8  
Old 04-27-2018, 06:38 PM
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snopercod snopercod is offline
 
Join Date: Aug 2016
Location: Asheville, NC
Posts: 1,145
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I had the exact same symptoms with my O-290-D2. After extensive testing and head scratching, the problem turned out to be fuel boiling in the bowl due to heat soaking from the oil sump. I fixed the problem by installing a 1/16" phenolic gasket (and fiber gaskets on either side) between the carb and the sump.
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  #9  
Old 04-27-2018, 07:10 PM
Sdemeyer Sdemeyer is offline
 
Join Date: Apr 2015
Location: Olympia,wa
Posts: 46
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Thanks everyone for the diagnosis and advice. What everyone is saying makes sense. I do have Winter blend fuel at the moment and we had a record breaking 80 deg day yesterday so I'm sure that contributes to the problem. I have all lines forward of the firewall, firesleeved and I also installed blast tubes toward the mechanical fuel pump and gascolator but it didn't do much for helping. I will run avgas for a few months just to be sure fuel is the issue and its not something else. The last time we had a warm day was back in September and I had the same problem. Not sure if my fuel was Winter Blend back then.
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RV6A quickbuild, Flying as of 10-14-2017
Mustang II Built and Flying
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