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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
 
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.
 
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.
 
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.
 
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
 
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
 
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
 
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?
 
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.
 
Bob,

As far as I understand the carb operation you are correct for the second part of the test that I described earlier. I agree with you that with engine off, if I operate the throttle many times, the accelerator pump will throw a jet of fuel up through the venturi, but this fuel will fall down by gravity back through the venturi and will accumulate and leak from the filter air box.
But, for what reason fuel would leak with engine off, elec pump on, mixture open?
It never happened before.
I have never seem this as a normal condition on other airplanes.
Could not it be because of float or needle?
Regards,

Ramiro
 
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?

I agree with Bob, when you pull the throttle, the accelerator pump will squirt fuel in the throat and wind up in the filter box. However, if you leave the throttle closed, turn on the pump and pull the mixture, you should not get fuel coming out of the carb. The first item to check is the float mechanism. Most likely is debris blocking the needle from fully closing. Second likely would be a saturated float (pin hole leak allows fuel in float, reducing buoyancy). Third likely would be a mis-adjusted float level.

Given that this problem disappears whenever the carb is rebuilt, I would guess a pin hole in the float or debris. do you get noticeable sediment in your gascolator?

I would pull the FAB, run the pump and watch. If the fuel leaks until you turn the pump off that is different than only some initial leaking that stops while the pump is still on.

Larry.
 
Larry,

I check gascolator every annual and have never found an abnormal condition.
Every time I drain tanks before flight never find dirt.
Today, with engine cold, I turned elec fuel pump on, there was lots of leak with mixture closed and open, throttle at full power and idle.
Then I run the engine for 15 min, very normal , except by a reduction in 50 rpm at full throttle, but the same I used to get before carb overhaul. Shut engine down and waited for 20 min, no leak. Turned elec pump on again, no leak, mixture closed and open, throttle at full power and idle.
It seems that if the cause were debris they have gone elsewhere.
Every part in the carb, except the core, is brand new, less than 10 h, the float is the last model.
I still suspect of mistake during assembly.
The shop will give me the money back and I will probably send it to another shop.
Regards,

Ramiro
 
No solution for the problem by now.
I suspected the primer system because the only way I could reproduce the failure was turning the electrical fuel pump on while commanding the primer valve open at the same time, then I replaced parker solenoid valve, switch and circuit breaker.
Everything looked very good for the last flight hours, I was convinced that I had got the problem solved and with confidence I planned my first cross country.
Last sunday (april 24 2016) the problem appeared again in 2 consecutive flights.
When I set throttle to idle position the engine did not quit completely but remained operating rough close to 500 rpm when the normal would be 800 rpm.
 
No solution for the problem by now.
I suspected the primer system because the only way I could reproduce the failure was turning the electrical fuel pump on while commanding the primer valve open at the same time, then I replaced parker solenoid valve, switch and circuit breaker.
Everything looked very good for the last flight hours, I was convinced that I had got the problem solved and with confidence I planned my first cross country.
Last sunday (april 24 2016) the problem appeared again in 2 consecutive flights.
When I set throttle to idle position the engine did not quit completely but remained operating rough close to 500 rpm when the normal would be 800 rpm.

This would support Bob's suspicions of an internal leak. Fuel is finding a way to the carb throat other than via the idle circuit.
 
When I set throttle to idle position the engine did not quit completely but remained operating rough close to 500 rpm when the normal would be 800 rpm.

If the mixture valve does not get seated properly it can cause this. I recently had to replace my mixture valve because it had a bend in it and was worn in an area on it. If you look at the last 6 pictures in this album you will see the bad mixture valve and the new one. In a couple of the older pics you can see the area that the valve has to slide into in the bowl. My engine got to the point where it would not shut down when I pulled the mixture all the way back, it would do like you described. After I replaced the mixture valve and insured that it seated properly during assembly, all is well.
https://picasaweb.google.com/108995216147102542358/CarbWork

Maybe this will help.
 
Bobby,

I can?t believe that it is related to mixture control because the issue happens with mixture set full rich.
It is an intermittent failure in which, with mixture rich and throttle in idle, the rpm dropped from normal (close to 800 rpm) to abnormal (500 rpm or lower).
It happened inflight.
In the worst case I had a complete engine stoppage inflight and had to perform a deadstick landing.
And why a few amount of fuel would leak through the airbox few minutes after flight during engine cooldown period with mixture in cut off and throttle closed?
Regards,

Ramiro
 
Agree

I agree the other problems you list sound like something else. The only problem I was referring to was the engine not shutting down with mixture pulled off. It does sound like an internal problem.
Hope your able to find the problem.
 
Where both accelerator pump check valves replaced? I think one of them is intermittently defective. Obviously, the problem is in the carb if you have disconnected the primer system. If you can't figure it out, replace the carb, regardless of the cost before it damages you or your aircraft. Not a situation to be penny wise on, in my opinion.
Good Luck,
Mahlon
 
MA4SPA overhauled in 1994, with almost 930 h since then.

Marvels with a few (900 hours) hours of service often vibrate the float bowl section of the carb loose. There is an AD on this. With the cowl off ( at every oil change) just lightly try to wiggle the bottom body of the carb sideways. You will know instantly if it is loose. Any time a Marvel has a fuel leak of any kind show up, it is the first thing I look at.

Mahlon 's opinion / advice is 100% good. ( post #20)
 
Mahlon,

I have always checked and it is perfectly attached to the engine.
I agree with you that I should not run the risk to damage my plane and my self saving money on a decent carb.
I am looking for a new one.
Thanks,

Ramiro
 
And why a few amount of fuel would leak through the airbox few minutes after flight during engine cooldown period with mixture in cut off and throttle closed?
Regards,

Ramiro

I can only think of three reasons a carb would leak into the airbox after shut down, assuming you're not moving the throttle. First is a crack. Second is continued fuel flow into the carb under high pressure or low pressure with a bad needle/seat. Third would be a poorly adjusted float level (level too high causing overflow out of vent into throat), but you would be running overly rich without agressive leaning. Someone needs to check this carb for a crack. Given your previous problems, I would be looking in the area of the accel pump chamber.

Larry
 
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My guess would be a sinking float.

Causes engine to run overly rich and will dump fuel into the FAB.
It can get bad enough that the engine will quit on roll-out after landing because it has been idling on final with a very rich mixture.
 
To make a long story short: I sent the carb to Marvel Schebler and no issues were found.
I am not surprised with this result because the technical problem was intermittent, it happened to me few times, but it did not showed up at manufacturer shop flow test bench.
I preferred to not run more risks test flying this supposed fault prone carb then I ordered a rebuilt one and took US$600.00 as core change refund.
By now it is installed but tested only on the ground.
It is running fine until now.
Flight test to happen soon...
 
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