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  #1  
Old 06-16-2014, 09:35 AM
Ramiro Silveira Ramiro Silveira is offline
 
Join Date: Apr 2011
Location: Sao Jose dos Campos
Posts: 24
Default Marvel Schebler MA4SPA Carburetor leaking fuel

Hi All,

I have a Van?s Aircraft RV-6, powered by a Lycoming O-320-H2AD, equipped with a Marvel Schebler Carburetor, model MA4SPA, P/N 10-5135, S/N CK 1 1828, overhauled in 1994, with almost 930 h since then.
The other day I flew for about 40 min. About 20 minutes after the flight, with the aircraft already stored in the hangar, I detected fuel dripping by the underside of the cowl. The leak ended soon. Later I found fuel spraying through accelerator pump stem when reducing throttle to idle.
I purchased the standard repair kit plus the float kit and sent the carb to the shop for a complete overhaul.
The carb returned as new.
Installation was straight forward and operation was smooth as before removal.
2 months later, after 10 flights, only 5.5 flight hours, the carb is leaking again on the same point.
I suspected of a mistake durind assembly that could have affected the accelerator pump plunger leather boot.
Removed carb again under shop warranty.
The carb came back from the shop and unfortunately it is leaking again.
I flew for 40 min, 15 minutes after the flight I detected fuel dripping on the floor, the leak ended soon, as before.
How can fuel get to the upper side of the accelerator pump chamber?
Why this would happen while the engine is cooling?
Looks like that fuel gets hot then expands.
Marvel Schebler online support told me they believe that the pump stem packing may not have been staked tight enough into the casting at overhaul.
Someone can help?
Thanks,

Ramiro
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  #2  
Old 06-16-2014, 10:03 AM
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scard scard is offline
 
Join Date: Jun 2005
Location: Cedar Park, TX
Posts: 3,229
Default

It sounds to me like your overhaul shop never replaced the accelerator pump as it is leaking. It is a simple operation once you have it apart. The packing on the top is just there to keep the dirt out and isn't designed to keep fuel in.
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  #3  
Old 06-16-2014, 10:33 AM
fixnflyguy fixnflyguy is online now
 
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Location: Winston-Salem, N.C.
Posts: 1,376
Default Fuel pressure? I have same engine

I am also running an H2AD in my -4, with no mechanical pump. I have 2 electric low pressure pumps in series, but only run one at a time except take off and landing... both on at same time is slightly high on fuel pressure and I have seen seeps when I accidently ran both for long periods. I dont know what your fuel pump scenareo is, but it me be causing your situation. the accelerator pump (as mentioned) may also be the culprit. I have same carb as you, and its about as simple as they get to rebuild.
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  #4  
Old 06-16-2014, 12:17 PM
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rocketbob rocketbob is offline
 
Join Date: Jun 2006
Location: 8I3
Posts: 3,683
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I had a MA3 carb on a C150 that absolutely stymied me for weeks because I couldn't figure out why the engine was running rich. Turned out a very small crack was in the bowl casting that could barely be seen was causing the problem.
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N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
N8155F C150 flying
N7925P PA-24-250 Comanche, restoring
Not a thing I own is stock.
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  #5  
Old 06-17-2014, 02:46 PM
Ramiro Silveira Ramiro Silveira is offline
 
Join Date: Apr 2011
Location: Sao Jose dos Campos
Posts: 24
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Scott,

The pump plunger was replaced, they sent me the old one. Maybe some mistake during assembly.

Bob,

If the cause were a crack it would leak all the time, it would not develop leakage after some flight hours.

Shop technician may show up any time in the near future to take look. When we discover the problem cause I will post here.

Living in Brazil makes things more difficult and expensive, if I were in the USA I could buy a rebuilt carb for US$800.00, but here it would cost me US3,000.00.

I need to find a way to save mine.

Regards,

Ramiro
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  #6  
Old 06-17-2014, 03:21 PM
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rocketbob rocketbob is offline
 
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Quote:
Originally Posted by Ramiro Silveira View Post
Scott,
If the cause were a crack it would leak all the time, it would not develop leakage after some flight hours.
In my case it did not leak externally.
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Bob Japundza CFI A&PIA
N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
N8155F C150 flying
N7925P PA-24-250 Comanche, restoring
Not a thing I own is stock.
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  #7  
Old 06-18-2014, 01:33 PM
lr172 lr172 is online now
 
Join Date: Oct 2013
Location: Schaumburg, IL
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I am rebuilding my MA 4 for my O-320. There are two check valves in the accelerator pump circuit. If your problem only occurs with heat and expanding gas, it is possible that one of these is adjusted improperly. One has an adjustment, but I forgot which one. It seems somewhat unlikely that they got the packing wrong twice. You could also look for scoring or gouges in the wall of the pump chamber that would allow fuel to leak past the accelerator pump seal and put more pressure on the shaft packing.

I disagree with the previous poster. That shaft packing should hold the gas in unless it is coming under a decent pressure, as would be caused by a problematic check valve. If the gas in the chamber can't get through it's designed orifice, it will leak past the pump seal and then must go through the shaft packing. Do you notice any stumbles during moderate to aggressive throttle application? That would indicate that the gas is not getting from the pump chamber to the throat. The pump arm is going down either way, so if it can't follow the designed path, it has to go past the seal and out the shaft packing.

Good luck,

Larry
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  #8  
Old 06-18-2014, 06:02 PM
Ramiro Silveira Ramiro Silveira is offline
 
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Location: Sao Jose dos Campos
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Larry,

Thanks!
The fuel sprays through the packing when the throttle is retarding to idle, I mean, when the pump plunger is going up, not down.
For some reason the other side of the chamber is becoming full of fuel.
The shop technician commented with my A&P mechanic about adjusting a valve to reduce internal pressure, but I don't know which one.
They also changed the pump stem adjustment from hole no. 1 to no. 2, reducing its stroke.
Anyway, engine operation was not affected, it is as good as before, start is easy, idle is stable, max RPM the same, rapid accelerations are smooth, no problems with extreme attitudes or low g (above zero obviously).
There was no report of cracks of excessive wear on the core.
Regards,

Ramiro
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  #9  
Old 06-19-2014, 02:11 PM
Ramiro Silveira Ramiro Silveira is offline
 
Join Date: Apr 2011
Location: Sao Jose dos Campos
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Today,when I arrived at the hangar in the morning, I went to check the carb and I could see some blue stains like fuel residue from evaporation.
Then I performed more tests, with the airplane sitting still, engine cold, I turned the electric fuel boost pump on.
Results:
- mixture closed: no leak
- mixture open: lots of leak from the filter air box
Then, with engine off, I operated the throttle rapidly from idle to full power then back to idle many times, lots of leak as I should expect from the filter air box, but could not see any fuel spraying from the pump plunger packing.
It seems to me that float adjustment is incorrect or the float needle is not working properly, right?
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  #10  
Old 06-19-2014, 02:27 PM
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rocketbob rocketbob is offline
 
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Quote:
Originally Posted by Ramiro Silveira View Post
Today,when I arrived at the hangar in the morning, I went to check the carb and I could see some blue stains like fuel residue from evaporation.
Then I performed more tests, with the airplane sitting still, engine cold, I turned the electric fuel boost pump on.
Results:
- mixture closed: no leak
- mixture open: lots of leak from the filter air box
Then, with engine off, I operated the throttle rapidly from idle to full power then back to idle many times, lots of leak as I should expect from the filter air box, but could not see any fuel spraying from the pump plunger packing.
It seems to me that float adjustment is incorrect or the float needle is not working properly, right?
No, if you pump the throttle the accelerator pump will shoot fuel into the venturi, and if the engine isn't running, fuel will dribble out.
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Bob Japundza CFI A&PIA
N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
N8155F C150 flying
N7925P PA-24-250 Comanche, restoring
Not a thing I own is stock.
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