Pesky fuel leak.

Tom-D

Tom-D

Taxi to Parking
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Feb 23, 2005
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Tom-D
C-170-A with a 0-300-A rebuilt in 2005 to new standards, started to leave blue stains in the bottom cowl after about 100 TSOH, we rebuilt the carb, but really didn't see any reason for it to be leaking.
Blue stains continued.
Replaced the carb with a new yellow label Kelly rebuilt carb.
Blue stains continued
Capped off the primer system at the gascolator.
blue stains continued.
Engine does not run rich or fowl plugs.
Engine preforms well, makes static RPM, no hesitation during throttle advancement.
Gives a very slight rise during idle mixture check.

I'm out of ideas.
 
Your owner is a throttle pumper and fuel is dropping out the bottom before he starts it up?
 
Stewartb said:
Your owner is a throttle pumper and fuel is dropping out the bottom before he starts it up?
Click to expand...
At first that is what I thought, but Nope, I've watched him, 1 stroke of the primer while cranking, he's been trained. :)
 
There is always fuel stains around the carb body, and inside the airbox.
Accelerator pump is at its lowest setting.
 
Stains on the outside of the carb body?

Not coming from the fuel line connection to the carb or a crack in the fuel line itself?

Drip or damp spot if it sits overnight with the fuel selector left on?

Pictures?
 
stand and watch while someone primes.....I'll bet there is a crack or a leak at a fitting. Seen that.
 
Checkout_my_Six said:
where the blue part is....and the drippy stuff comes out.o_O
Click to expand...
I wish I could see where the blue stuff is coming from.
It only shows up in the lower cowl after a flight.
Plus it is in the airbox every time also.
 
Capt. Geoffrey Thorpe said:
Stains on the outside of the carb body?
Yes

Not coming from the fuel line connection to the carb or a crack in the fuel line itself?
Always dry

Drip or damp spot if it sits overnight with the fuel selector left on?
Aircraft does set a lot, 10 hours last year,

Pictures?
Click to expand...
The owner cleaned it up prior to me getting there this morning.
 
I had a leak on my cummins that I could not locate ... wrapped the damn thing up in diapers and did a ground run. Then un-diapered to look for the stain. Found a power steering leak well away from where the drip was showing up ...

Might his leak show on a short ground run?
 
Based on the information given.
Well, updraft carbs are always fighting gravity. Back pulses on the intake can cause spitting out the carb - saw a 327 that was famous for that. Or the carb pedestal on the intake is warped and it is wicking out past the gasket. Or there is a pin hole on the intake where fuel puddles and then drips out. Or the throttle shaft on both carbs have wallowed out bushings and the fuel is wicking out there.
 
Is he shutting down with mixture or mags? I've seen Continental shut down, throttle closed with mags end up with gas running out the airbox because it is drawing gas as it winds down. Maybe a bad carb to intake sump gasket leaking?
 
N3368K said:
Is he shutting down with mixture or mags? I've seen Continental shut down, throttle closed with mags end up with gas running out the airbox because it is drawing gas as it winds down. Maybe a bad carb to intake sump gasket leaking?
Click to expand...
always with the mixture, and if we had a intake leak at the carb to sump, we would have other symptoms, like several cylinders running lean.
 
Tom-D said:
C-170-A with a 0-300-A rebuilt in 2005 to new standards, started to leave blue stains in the bottom cowl after about 100 TSOH, we rebuilt the carb, but really didn't see any reason for it to be leaking.
Blue stains continued.
Replaced the carb with a new yellow label Kelly rebuilt carb.
Blue stains continued
Capped off the primer system at the gascolator.
blue stains continued.
Engine does not run rich or fowl plugs.
Engine preforms well, makes static RPM, no hesitation during throttle advancement.
Gives a very slight rise during idle mixture check.

I'm out of ideas.
Click to expand...

Is there some stuff, some goop or something that you can put on every fitting and joint under the cowl that reacts somehow with gas. Like changing color. There will still be the blue scattered around, but it could identify where it is originating from.
 
Dr. O said:
Based on the information given.
Well, updraft carbs are always fighting gravity. Back pulses on the intake can cause spitting out the carb - saw a 327 that was famous for that. Or the carb pedestal on the intake is warped and it is wicking out past the gasket. Or there is a pin hole on the intake where fuel puddles and then drips out. Or the throttle shaft on both carbs have wallowed out bushings and the fuel is wicking out there.
Click to expand...
This is low time over haul with 6 new ECI cylinders that runs smoother than spandex.
 
just thinking out loud, could you have a hose wicking or a fitting leaking somewhere between the firewall and carb and it wicking down a hose to a low point and dripping on cowl during operation? check all the hoses for any blue stains.
 
I have a theory, so I'm having the owner shut down by shutting the fuel off at the fuel selector valve.
See what changes over time.
we will see if in fact we do have a static fuel leak forward of the firewall.

This AM I disconnected the primer line at the manifold, then plugged it, then primed the engine the primer pump was rock solid it would not creep down like it would if there was a leak.
luvflyin said:
Is there some stuff, some goop or something that you can put on every fitting and joint under the cowl that reacts somehow with gas. Like changing color. There will still be the blue scattered around, but it could identify where it is originating from.
Click to expand...
It's called a strip of white rag. tie it around a fitting see if it turns blue.
 
unsafervguy said:
just thinking out loud, could you have a hose wicking or a fitting leaking somewhere between the firewall and carb and it wicking down a hose to a low point and dripping on cowl during operation? check all the hoses for any blue stains.
Click to expand...
I've washed the 1 fuel feed hose with electronic cleaner catching the run off in a white rag,, nothing.
 
Well the owner got it started by the throttle, and flew it back to it's hangar we will go see the results in the morning.

I am going to place a white rag around the primer line that I have disconnected, lock the primer pump. and turn on the fuel, just to see if the primer pump may be leaking.

theory is, the primer pump has an internal leak, which after setting long periods will fill the airbox with 100LL that has been evaporated to a blue gooo. so then the engine is started the engine shakes the goo out into the cowl.
We will see what happens and report back.
 
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