Backcountry Pilot • Carburetor Question

Carburetor Question

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Wed Oct 09, 2019 6:32 am

Carburetor Question

Hey all,

Had a little issue with the plane the other day. A quick backstory: When I bought the plane (Aeronca Champ 7AC with a continental A-65) in April and was flying it from FL to Northern NY we had a carburetor issue in SC and had to make a forced landing. Long story short, we sent the carb out to get re-built and when it was reunited with the plane we flew it home with no issues. Since that time I have put around 45 hours on the plane and it has run flawlessly. Since we are 40 miles from the nearest airport with 100LL I have been using a mixture of 100LL when I can get it and ethanol free 91 octane when I can't. More car gas than 100LL. A week or so ago I noticed that when starting hot I had some fuel dripping out of the airbox. Chalked it up to a flooded engine, even though it had not happened before. Saturday I went out for a flight and the plane started a little hard, but it was the coldest I have tried to start it - around 40 degrees. Once it warmed up it ran fine and the run-up was normal. Took off and climbed out just fine. A couple minutes into the flight the tone of the engine changed and I immediately nosed over and headed back to the airport. It was not making full power and wouldn't pull redline even in a slight descent. Landed with no issue, and shut down. Took a look and there was fuel dripping pout of the airbox again. Ran the prop backwards as if I was trying to get a flooded engine cleared and it made a squishing sound and would shoot fuel out of the carb on every turn. Called the guy that re-built it ( who is a older guy with a ton of experience with these) and after describing what happened he asked me what kind of gas I was using. After some discussion he said it was from using car gas and to drain the tanks, and carb then run it with 100LL from now on and everything will be fine. I have no reason to not believe him, however it's pretty unforgiving around here and to be honest I am a bit nervous about firing the thing up and taking off, even if I run it on the ground for a bit. Do any of you have experience with this? If so any information would be appreciated.

Thanks,

Pete
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Wed Oct 09, 2019 8:08 am

Re: Carburetor Question

Pete,

What kind of carb? I assume a Stromberg? I’ve run LOTS of auto gas through several small Continentals, and a few Lycoming and never had a problem.

Latest iteration was a C-90 with Marvel carb, in which I ran nearly straight auto gas and never an issue.

That said, some rubber parts can be sensitive to some additives in auto gas, and there’s auto gas and there’s auto gas, if you know what I mean.

I’d run some 100 LL and in meantime find other carb “experts” and ask them.

And, you’re not likely to find those on these forums..... :D

MTV
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Wed Oct 09, 2019 8:18 am

Re: Carburetor Question

Sheesh, Mike, have you no faith in your BCP brethren? :lol:

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Wed Oct 09, 2019 9:11 am

Re: Carburetor Question

mtv wrote:I’d run some 100 LL and in meantime find other carb “experts” and ask them.

And, you’re not likely to find those on these forums..... :D

MTV


Luckily we have an expert in identifying experts.



Pburns, to get to the bottom of the issue you have to look beyond the boogeyman nature of car gas. It's true, some car gas has stuff in it that can react with certain fuel system components. It's a good policy never to use auto gas with ethanol added as in an unpressurized fuel system it can scavenge water and create corrosive agents that eat up aluminum and maybe natural rubbers. IIRC some auto gas has bacteria in it that can cause varnishing. But what does that actually do?

If you're getting fuel pissing out of the airbox I would suspect maybe a stuck or sticky float. What making them stick? Find the cause of the pissing, then determine why. It's definitely a sketchy situation that can result in worse than a dead engine. I'd rather be gliding than on fire #-o 8)

Only one way to become an expert on carbs, and that's to open them up. Maybe pull it off and take to your mechanic and watch.

-Pro bono expert on dealing with not knowing shit and surviving
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Wed Oct 09, 2019 9:57 am

Re: Carburetor Question

Assuming you have a Stromberg carburetor there is the possibility of some parts being incompatible with mogas. The float needle valve being the primary one. There is a needle with a neoprene tip that swells when in contact with mogas causing it to not seal thus leaking. Switching back to 100LL eliminates the swelling so the valve seals again. Some mechanics like to use the neoprene tipped needles because it is easier to get them to seal. Judging by the response of the guy that worked on your carb I’d say he used a neoprene needle. I’d ask him, if he did and you want to keep running mogas you’ll have to get a metal tipped needle installed.
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Wed Oct 09, 2019 10:14 am

Re: Carburetor Question

whee wrote:Assuming you have a Stromberg carburetor there is the possibility of some parts being incompatible with mogas. The float needle valve being the primary one. There is a needle with a neoprene tip that swells when in contact with mogas causing it to not seal thus leaking. Switching back to 100LL eliminates the swelling so the valve seals again. Some mechanics like to use the neoprene tipped needles because it is easier to get them to seal. Judging by the response of the guy that worked on your carb I’d say he used a neoprene needle. I’d ask him, if he did and you want to keep running mogas you’ll have to get a metal tipped needle installed.


This guy Strombergs.
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Wed Oct 09, 2019 10:33 am

Re: Carburetor Question

pburns wrote: A couple minutes into the flight the tone of the engine changed and I immediately nosed over and headed back to the airport.


The most advanced maneuver in aviation - the 180 degree turn. Can't help with the carb issue, I just wanted to say "well done".
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Wed Oct 09, 2019 10:38 am

Re: Carburetor Question

aftCG wrote:
pburns wrote: A couple minutes into the flight the tone of the engine changed and I immediately nosed over and headed back to the airport.


The most advanced maneuver in aviation - the 180 degree turn. Can't help with the carb issue, I just wanted to say "well done".


Me too. Good job.
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Wed Oct 09, 2019 10:48 am

Re: Carburetor Question

aftCG wrote:The most advanced maneuver in aviation - the 180 degree turn. Can't help with the carb issue, I just wanted to say "well done".


Thanks aftCG, it definitely wasn't the "impossible" turn though. The plane was still running just not full power, and I had plenty of altitude. I did stay plenty high until I was sure I could glide in if needed. This all took about 2 minutes.

It is a Stromberg and Whee's explanation makes a lot of sense. The guy that re-built it did say they will leak when you use mo-gas and will clear up if you use 100LL, but didn't say why. Yes, the fire thing crossed my mind as well. Now I have to decide: do I fly low up the river or high over the runway...

Pete
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Wed Oct 09, 2019 10:55 am

Re: Carburetor Question

pburns wrote:It is a Stromberg and Whee's explanation makes a lot of sense. The guy that re-built it did say they will leak when you use mo-gas and will clear up if you use 100LL, but didn't say why. Yes, the fire thing crossed my mind as well. Now I have to decide: do I fly low up the river or high over the runway...

Pete


Instead of flying it and hoping for the best, why not remove the carb (have your mechanic do this for you since it's outside the scope of owner maintenance, as I have been advised via PM), have the needle and needle seat replaced with steel version? Reinstall carb and continue running either type of fuel.
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Wed Oct 09, 2019 11:19 am

Re: Carburetor Question

pburns wrote:
aftCG wrote:The most advanced maneuver in aviation - the 180 degree turn. Can't help with the carb issue, I just wanted to say "well done".


Thanks aftCG, it definitely wasn't the "impossible" turn though. The plane was still running just not full power, and I had plenty of altitude. I did stay plenty high until I was sure I could glide in if needed. This all took about 2 minutes.

It is a Stromberg and Whee's explanation makes a lot of sense. The guy that re-built it did say they will leak when you use mo-gas and will clear up if you use 100LL, but didn't say why. Yes, the fire thing crossed my mind as well. Now I have to decide: do I fly low up the river or high over the runway...

Pete


I understood your situation from your description. I commented because you made the timely decision to push the nose over and turn around when the temptation would be strong to continue and "see if it got better". Every year there are many people who didn't listen that little voice and we get to arm chair quarterback the wreckage.
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Wed Oct 09, 2019 11:25 am

Re: Carburetor Question

I agree with Zane. Put auto gasoline and ethanol resistant float, needle and seat in. I know of a Marvel that has had lots of E10 run through it for 13 yrs and hasn't had carb issues----just takes the right components inside.
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Wed Oct 09, 2019 3:09 pm

Re: Carburetor Question

Sounds like attention to the carb will cure the short term immediate issues...any concern about other material in the system braking down with this fuel mix...selector, lines or tank ..fuel floats...?
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Wed Oct 09, 2019 5:01 pm

Re: Carburetor Question

Zzz wrote:Instead of flying it and hoping for the best, why not remove the carb (have your mechanic do this for you since it's outside the scope of owner maintenance, as I have been advised via PM), have the needle and needle seat replaced with steel version? Reinstall carb and continue running either type of fuel.


aftCG wrote:I understood your situation from your description. I commented because you made the timely decision to push the nose over and turn around when the temptation would be strong to continue and "see if it got better". Every year there are many people who didn't listen that little voice and we get to arm chair quarterback the wreckage.


Zzz,

Thanks,
My comment was tongue in cheek. I do intend to have someone other than myself look at it. Sorry if that came across as anything different.

aftCG,

It's interesting you say that. My non-thinking response was to nose down, avoid a stall and turn around, but as soon as I did that my mind was saying hey it's your imagination, let's keep going and figure this out. The little voice then came in and said that's stupid, let;s get this thing on the ground and sort it out there.

Pete
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Wed Oct 09, 2019 6:40 pm

Re: Carburetor Question

Mogas will lead to carb ice a little earlier than it might show up with 100LL. It could be that you’re building ice, and the liquid you saw dripping from the carb was water when the ice melted out on the ground?

If you’re sure that it’s gas leaking, then I’m really off base, but I have to bring it up and have it considered. If it’s gas leaking, it won’t fix itself. Whatever the gas damaged in the carburetor won’t heal. I doubt you have a new carb issue.

Has something to do with latent heat of vaporization, the heat energy required to vaporize the fuel. When it vaporizes, it draws more heat from the surroundings than 100LL, and cools the carburetor more than 100LL might.
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Thu Oct 10, 2019 5:47 am

Re: Carburetor Question

Carb ice is an interesting possibility....I installed a carb air temp gauge in August and am amazed at how the temp varies based on flight condition...the 182 is a great Ice Machine....I guess you can sniff the liquid dripping out of the cowling ....
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Thu Oct 10, 2019 8:37 am

Re: Carburetor Question

-0-
Last edited by dogpilot on Thu Aug 06, 2020 12:20 am, edited 1 time in total.
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Thu Oct 10, 2019 9:06 am

Re: Carburetor Question

dogpilot wrote:Kind of like sex without protection, eventually you may get a surprise.


Image
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Thu Oct 10, 2019 9:10 am

Re: Carburetor Question

dogpilot wrote:... Kind of like sex without protection, eventually you may get a surprise.


The biggest surprise to me would be that I was having sex..
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Thu Oct 10, 2019 9:28 am

Re: Carburetor Question

Pinecone wrote:Mogas will lead to carb ice a little earlier than it might show up with 100LL. It could be that you’re building ice, and the liquid you saw dripping from the carb was water when the ice melted out on the ground?


This is why I recommend tasting all fluids that emanate from your airplane.
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