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My Carb has the Blues

C

CATPart

Well Known Member
I have observed something and seek an opinion. I have flown about 80 basically trouble free hours on this plane. About 10 hours ago I noticed some blue on the back bottom edge of my cowl between the exhaust pipes. There wasn't much, and it was wet with oil, as that is where any oil seepage ends up. I wiped it down, checked for leaks (ran the pump, ran the primer, ran the engine) and found nothing. Decided to monitor the situation. Today I saw the blue again, so I decided to pull the cowl. Found some sticky blue under my air box around the drain hole. The pictures tell the rest of the story. My gut tells me this is not a big concern, and probably just buildup of the leakage that happens every time I move the throttle 1/2" before starting the plane. But to be safe I figured I would get the collective opinion.

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not normal

I've only owned/operated one O-320, but I've seen a dozen; that is not common, and I don't think it's safe.
 
IMO, it takes a lot of fuel to evaporate and leave that much blue dye. There's something more going on there than startup issues.

My airplane is 14 years old and has never had a blue stain in the airbox.
 
That's not a standard filter arrangement. I also don't see a drain hole in the filter retainer plate, so fuel would have to creep over the filter rim to get out.
 
That is way too much dye to ignore.... It takes a lot of fuel to leave that much dye behind. Something is wrong. Do check it out.

BIG fire hazard......:eek:
 
I am guessing that you may have some issues with your carb's floats. If they are failing and sinking within the float bowl, the fuel will overflow the bowl and leak out as depicted. Your photos show a significant leak. IMHO, this needs attention ASAP.
 
aerhed said:
That's not a standard filter arrangement. I also don't see a drain hole in the filter retainer plate, so fuel would have to creep over the filter rim to get out.
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Actually that was the standard FAB design for O-320's up until some time around the year 2000.

Since there doesn't seem to be much evidence of leakage on the outside of the carb. (bowl gasket, accelerator pump seal, etc.), the first thing I would look at is to see if you have sinking floats. If they are ok, then look at the needle valve / seat condition.
 
There is zero evidence of leakage externally. It seems it is coming out of the throat. Would bad floats drip fuel through the throat?
 
CATPart said:
There is zero evidence of leakage externally. It seems it is coming out of the throat. Would bad floats drip fuel through the throat?
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Yes.

That is why I said since there is no evidence of leakage on the outside, I would look at the floats and needle valve / seat.
 
Carb

The float is a very sloppy fit on the pin that it pivots on. It is possible for the float to hang up occasionally and perhaps even slightly and ittermittently and cause this problem. The Marvel manual shows a guage to check side clearance on the float.
Could also be a needle/seat issue.
A separate recommendation is that if the throttle is being used to prime, engage the throttle first and then just one quick pump of the throttle and quickly back to near idle.
 
Bolt Safeties

Since you are likely going to have to remove the carburetor for inspection, make sure you do a better job of safety wiring the mounting plate bolts than the last guy!:eek: One completely negative, and the other 3 neutral at best.
Anyone attend Craig Catto's seminar at AirVenture, and hear his data on the effects of improperly safetied bolts?
 
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I'd put my money on a compromised float that is taking on fuel, have seen some blue plastic ones that had a flaw....
Keep us posted!!
 
Check your bowl to body bolts

I read 80 hours on the airframe, so how many hours are on the engine? Marvel carbs are nicknamed Marvel Dribblers with good reason. There is an SB /AD for inspection of loose float bowl body sections. Normal engine vibration WILL LOOSEN the fit of the 4 screws even if they are properly torqued and the saftey tabs properly locked. I have one on a 320 that showed no evidence of leakage other than the blue cowl stains and a strong fuel smell if I slipped the AC on final. I am not saying this is your problem here, but it could be. Just grab the bottom half of the carb and lightly try to wiggle it. I was not aware of the AD before I found mine loose:eek: Make that very loose. It is now on my check at every oil change check list.
 
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Scott, thanks for the clue on the older 320 FAB. Actually I kinda like not having the filter against the fiberglass. Shouldn't the plate still have a drain hole anyway? Gotta second the float/seat check and resafety though.
 
rvbuilder2002 said:
Actually that was the standard FAB design for O-320's up until some time around the year 2000.

Since there doesn't seem to be much evidence of leakage on the outside of the carb. (bowl gasket, accelerator pump seal, etc.), the first thing I would look at is to see if you have sinking floats. If they are ok, then look at the needle valve / seat condition.
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+1

I would also verify that your electric pump is not driving too much pressure. If it is running high and the float and/or needle/seat are not optimum, you will get this kind of leakage, as this pump is often run at idle or with engine off. I would expect most bowl overflow will get sucked into the engine at higher speeds.

Larry
 
You definitely have a carb float issue. In my opinion, this engine is not airworthy. plus it looks to be a major fire waiting to happen. Yes definitely bad floats.
I just had my o-320 carb rebuilt. Cost $650.00
PM me if you want the rebuilders contact info.
 
I do not believe that there is much fuel leaking since I have never smelled any, and I have a nose like a bloodhound. I run the pump, primer, and then inspect under the engine, stick my nose in the oil door, crawl around looking and sniffing. I have never seen or smelled a drop, other than discovering the slight blue at the base of the cowl, first time after sitting a few days, second time after a flight. There is no blue that I can see on the belly.

agreed on the safety wire.

The electric fuel pump makes 5-6 psi.

382 hours since new on the engine, o320-dia.

throttle is not used to prime, I have an electric primer. cold start procedure is" mixture rich, throttle fwd about 1/2", pump on, prime for 2 seconds, turn key, usually starts in in 1 revolution of prop.

I thought about the drain hole. aren't drain holes elsewhere, like the fab, usually for water to escape. a drain hole in the plate would be for fuel to escape
 
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CATPart said:
I do not believe that there is much fuel leaking ...
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The blue stains say otherwise. You do need to figure out where it is coming from. If you ever get a backfire you could end up very unhappy. I am with others, you should get this figured out.

Larry
 
CATPart said:
I have never seen or smelled a drop, other than discovering the slight blue at the base of the cowl,
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That isn't "slight" blue.

It doesn't matter what your nose is telling you or what you "believe" about it, that amount of dye means you have a major fuel leak. You shouldn't run that engine again until you've identified and repaired its source.

- mark
 
Obviously, I "believe" my nose enough to remove my cowl, ask every a&p I can find, and even start a vaf thread. The observations are given as data points to those interested in helping with a diagnosis. And I thank all who have thoughtfully contributed thus far.

Today I ran the fuel pump for 5 to 10 minutes. In that time I moved the mixture through its range, moved the throttle through its range, tapped the carb, raised the tail. I could never produce a single drop from the carb, except for the actuation of the throttle pump as expected. Then I ran the primer, and saw one brief drip to the floor. I discovered my copper primer line to #1 was broken at the manifold, at the base of the flare. Fuel was readily dribbling out, but never making it to the floor, just running down pipes and wires, leaving very little blue. I will replace that line in the morning.

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I would look while the engine is idling. It's possible your mech fuel pump is producing too much pressure. I am not sure that a strong overflow from the bowl would be overcome by the vacuum / flow at idle. Take off the FAB and do a bunch of ground tests while someone watches for drips from the carb. It's also possible that vibration is causing the poor seal at needle / seat.

Larry
 
You have a broken primer line and a bad carb. There is no way a broken primer line could put that much fuel in the carb throat. If you take the carb apart to check it be sure to check the hole the float hinge pin goes in, they wear. Replace it with a blue float they have a stainless hinge.

Chris
 
Correct Chris, the primer line has absolutely nothing to do with the carb. I do however suspect the blue at the back of the cowl is due to the primer line, as I saw no evidence of the blue from the fab traveling to the back of the cowl.
 
Fixed the primer line. Repeated my tests with the fuel pump on. This time I pumped the throttle multiple times to drain the bowl before turning on the pump. I figured this would let the float sink and then come back up as the bowl refilled. Again, no dripping from the carb. Then I tested the primer, and something new happened, this time the primer caused fuel to gush out of the carb throat. It didn't happen yesterday, but then again the primer line to #1 was not working then. I was surprised by how much fuel drained back from the top of the manifold as a result of priming. I filmed it actually, might post if I get around to it.

I think I have concluded that my carb is just fine, and the blue in the FAB is due to primer backwash (whatever it is called). I am going to adjust my start procedure to either priming while cranking the engine, or throttle pumping while cranking the engine.
 
Are you using the proper primer line terminations or just dumping the fuel into the intake chamber? They make special terminations that atomize the fuel and increase the pressure / velocity to avoid exactly what is happening to you.

Larry
 
Larry, I don't know. It looks like a typical blue AN fitting screwed into the head, and the flared pipe screws onto it. If you could point me to the correct type of fitting maybe that will help me identify it, thanks.
 
CATPart said:
Larry, I don't know. It looks like a typical blue AN fitting screwed into the head, and the flared pipe screws onto it. If you could point me to the correct type of fitting maybe that will help me identify it, thanks.
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If that is truly the case, then they likely are not primer nozzles and would feed way too much fuel like Larry said, and is likely the result of fuel staining in your airbox.

They should be THIS
Note: It does not use a flared end on the tube. It takes an 1/8" size ball and compression nut.
 
Thanks rvbuilder. I definitely do not have the fitting you posted. I will correct this situation at next conditional, where I planned to replace or anneal the current lines. Until then I will avoid using the primer. I will call the builder to see if he remembers what he used. He's a great guy, and it shocks me really what he remembers from 20 years ago.
 
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