Marvel Schebler MA4SPA Carburetor leaking fuel

[Ramiro Silveira]

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

[lr172]

Originally Posted by Ramiro Silveira

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.

[Ramiro Silveira]

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

[CAVU Mark]

Just wondering if you resolved the problem?

[Ramiro Silveira]

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.

[lr172]

Originally Posted by Ramiro Silveira

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.

[bhester]

Originally Posted by Ramiro Silveira

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/1089952...42358/CarbWork

Maybe this will help.

[Ramiro Silveira]

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

[bhester]

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.

[mahlon_r]

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