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09-04-2015, 09:46 AM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by CATPart
a drain hole in the plate would be for fuel to escape
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That is correct. The drain hole is designed to get rid of fuel that may end up in the FAB where it doesn't belong. It should be placed at the low point.
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09-04-2015, 03:37 PM
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Join Date: Jan 2011
Location: Sydney, Australia
Posts: 370
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Quote:
Originally Posted by CATPart
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
__________________
[ Paid up on 3 Feb 2020 ]
RV-6 VH-SOL
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09-04-2015, 04:41 PM
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Join Date: Jan 2012
Location: up up and away
Posts: 312
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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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09-04-2015, 06:27 PM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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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
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09-05-2015, 06:21 AM
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Join Date: Sep 2009
Location: Port Orange, FL
Posts: 34
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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
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09-05-2015, 11:35 AM
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Join Date: Jan 2012
Location: up up and away
Posts: 312
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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.
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09-05-2015, 07:40 PM
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Join Date: Jan 2012
Location: up up and away
Posts: 312
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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.
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09-05-2015, 08:45 PM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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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
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09-05-2015, 09:17 PM
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Join Date: Jan 2012
Location: up up and away
Posts: 312
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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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09-05-2015, 11:19 PM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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Quote:
Originally Posted by CATPart
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.
__________________
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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