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 Post subject: Wobble pump losing prime
PostPosted: Wed Jan 01, 2014 2:58 pm 
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Joined: Sun Oct 27, 2013 5:53 pm
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New to the forum and T6 community. I bought an SNJ5 in August and have been learning all I can on how it works. Really enjoy reading the posts here. When I bought the plane, the previous owner had just had the wobble pump rebuilt because they were having to pump for several minutes to get fuel pressure up. Their mechanic told them the problem was fixed but I have found that it was not. I have to pump for 2-3 minutes to get fuel pressure up to prime the engine. Once primed, the wobble pump keeps the pressure up with a single stroke. It will prime pretty quickly after an hour or two shutdown, but over night, I have to pump for minutes again. I am guessing I have an air leak somewhere in the system. No signs of fuel leak at the wobble pump. Any one have suggestions on where to start?

Steve Maehr
N3261G


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PostPosted: Wed Jan 01, 2014 11:31 pm 
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Joined: Sun May 02, 2004 1:16 am
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There are a lot of joints there! Is it the same regardless of fuel level or fuel selector position? HAve you checked all the joints to make sure they are tight?


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PostPosted: Thu Jan 02, 2014 12:38 am 
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Joined: Mon Jun 18, 2007 9:52 pm
Posts: 60
Location: Hong Kong
Hello Steve,
Congratulations on the purchase of your SNJ-5.
Once overhauled correctly the leather seals inside your type D-3 Fuel Unit should continue to work well for a very long time.
Air leaking in as you guessed is probably the reason it is taking you so long to wobble up pressure.
The large round plate at the bottom of the unit with the arm and butterfly nut holding it up allows access to the fuel strainer.
I have found in the past that the very thin and fragile gasket for this round cover plate can be cracked and or brittle. I have seen also that a thicker gasket might be required as some castings are not as smooth as they should be thus not allowing an air tight seal.
Please note that if you drain fuel from the drain cock on the bottom of the unit you will also be allowing air in and thus it will take a while to wobble up the pressure.

Strictly as an aside as long as we are talking about SNJ-5 fuel systems:
I reference the Navy AN 01-60F-2 Handbook, Erection And Maintenance Instructions for the SNJ-3,4,5,6
Fuel Diagram Figures 158 & 159

Notice the "T" fitting on the carburetor allowing fuel to flow to the carburetor from either the engine driven pump or the wobble pump. I have found at least 6 aircraft so far that did not have this "T" fitting and only an elbow coming from the engine driven pump. There is no guarantee if the engine driven pump fails that you will be able to get fuel to the carburetor from the type D-3 wobble pump without this "T" fitting installed. Make sure your system is plumbed properly!

Speaking of engine driven fuel pumps:
The only fuel pump allowed according to the Type Certificate Data Sheet is the type F-10
The type F-10 engine driven fuel pump does not have a fuel pressure adjustment screw on it. It doesn't need one as the fuel pressure is adjusted at the D-3 wobble pump to 4 P.S.I.

We all know that many 200 G.P.H. engine driven fuel pumps will work on the R-1340 engine, but try explaining to your insurance company why you have a different unit on your T-6/SNJ than what is called for on the Type Certificate Data Sheet when you have any kind of an emergency and they have to be called in.

This is all just my 2 cents worth coming from more than 36 years of T-6/SNJ maintenance and ownership.

I hope I have been of some help,

Vincent


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PostPosted: Thu Jan 02, 2014 9:33 am 
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Thanks Guys. :) I have an annual coming up next month and I will make sure all of the fittings are checked. I looked at what I could get at and saw no evidence of any leakage. I also checked the bottom of the fuel unit to see if the drain valve was the culprit and saw now leakage there either. My next candidate is the primer pump. I have gotten a little fuel coming out around the plunger and it is very hard to lock and unlock it. I will also verify the plumbing at the carburetor.


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PostPosted: Sun Jan 05, 2014 10:04 am 
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Joined: Sun May 30, 2004 9:56 am
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Location: Brush Prairie, WA, USA
There are two styles of fuel lines for the T-6/SNJ, one line and two line, the two line is the one with the T fitting. know which you have before making changes. There has been much discussion about them in our magazine.

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