Carburetor Failure MA-4SPA

1001001 · #1 09-23-2017, 08:45 PM

Not in an RV but...

In the interest of safety I want to let you know I had a carburetor failure on my Warrior the other day. It is a Marvel Schebler MA-4SPA. Not sure, but this might be a standard model on RVs as well.

In this carb, the fuel bowl assembly is bolted to the venturi/throttle assembly via four body-bound bolts that mount through holes in the throttle assembly to sockets in the bowl assembly. The bolts are secured with locking tabs and not safety wire.

Due probably to vibration and in spite of the locking tabs, the bolts had loosened and then damaged the threads in the bowl assembly. The carb bowl was separated from the throttle assembly by about 1/32 to 1/16 inch and very wobbly. One bolt was almost completely loose.

I first noticed something was wrong when I shut down the engine on Sunday night at Sarasota. The engine continued Dieseling after I cut off the mixture. It finally coughed to a stop. It was late and dark, and so I decided to check it out in the morning. The mags did ground properly. In the morning we had to reposition from one FBO to another so I started up and taxied across the airport and had the same behavior when I shut down.

I did a close inspection of the engine compartment and found the carburetor wiggling when I pushed on it.

I got a mechanic at the FBO to look at it and he removed the carb from the engine and confirmed permanent damage to the bowl bolt sockets. He said it would not be safe to re-tighten the bolts and I agreed.

So, a new (overhauled) carburetor and a hefty maintenance bill later, I left Sarasota and got back home late Thursday night. The core is being sent back to the manufacturer for a failure analysis. When I receive that report, I will report it here.

Anyway, something to check on your airplane, and to pass around to other folks with carbureted engines. The carb is hard to see with the cowl on and this can only really be detected by grabbing it and shaking.

If this has happened in the past to a carb and the bolts have simply been retightened, it could be a time bomb. I was lucky I found it on the ground.

If you find this, make sure to check the integrity of the gasket and mating surfaces of the fuel bowl and throttle assembly, as pieces of the gasket or metal shavings could be ingested into the engine through the carburetor via this failure mode.

Also, note that if your engine gives you any unusual behavior, it is trying to tell you something. Check it out before further operations.

Ironflight · #2 09-23-2017, 09:14 PM

You're not the first - this has been happening to people for as long as these carbs have been around....many posts about it in the past. To my knowledge, no one knows how the bolts loosen up when they are secured by the locking tabs - but they do!

I had this happen on my O-320 powered Yankee twenty years ago.

1001001 · #3 09-23-2017, 09:26 PM

Quote:

Originally Posted by Ironflight

You're not the first - this has been happening to people for as long as these carbs have been around....many posts about it in the past. To my knowledge, no one knows how the bolts loosen up when they are secured by the locking tabs - but they do!

I had this happen on my O-320 powered Yankee twenty years ago.

Thanks. I hadn't done a search on this failure mode, because I sort of half figured that if it were common, it would have been recognized and fixed, maybe decades ago...

Too bad that the realization that the locking tabs aren't good enough hasn't, over many years, resulted in the application of a different locking scheme.

DNeufeld · #4 09-23-2017, 10:15 PM

Quote:

Originally Posted by Ironflight

You're not the first - this has been happening to people for as long as these carbs have been around....many posts about it in the past. To my knowledge, no one knows how the bolts loosen up when they are secured by the locking tabs - but they do!

I had this happen on my O-320 powered Yankee twenty years ago.

Just my opinion, and many years of wrenching, I would think the screws were not properly torqued to begin with. The gasket compressed in time and got the whole loosening thing going. The gasket frets away. During assembly the screws have to be re-tightened a few times to the proper torque until the gasket quits compressing. The gaskets are fairly thick.

snopercod · #5 09-24-2017, 04:35 AM

My carb is so old that it uses safety wire instead of those locking tabs.

bnt83 · #6 09-24-2017, 06:29 AM

I too have found those bolt alarmingly loose on an O320 powered Cherokee.

Caveman · #7 09-24-2017, 07:02 AM

We've come across this a couple of times on our field. Once on a Cherokee and once on a Skyhawk. We pulled the carbs and sent them to a certified repair station.

http://msacarbs.com/pdf/MSA-14.pdf

https://www.lycoming.com/sites/defau...w%20Insp_0.pdf

Of particular interest is the cautionary note:

IF ANY ATTACHING SCREW IS LOOSE, DO NOT ATTEMPT TO TURN THE SCREW WITH A SCREWDRIVER. IF YOU TURN THE SCREW WITH A SCREWDRIVER, IT COULD DAMAGE THE LOCK WASHERS AND INCREASE THE LIKELIHOOD THAT THE SCREWS WILL BECOME LOOSE.

I suggest adding this to your condition inspection checklist if you have a carb. As Paul said, it isn't that uncommon. The interesting thing to me is that the locking tabs were not bent out of shape. This is speculation on my part, but it appeared to be more of a gasket shrinkage problem followed by bowl throttle body interface fretting. This airplane had been a hangar queen and is only being flown rarely. I'm not sure if that is pertinent information so it's just a data point.

1001001 · #8 09-24-2017, 07:28 AM

Quote:

Originally Posted by Caveman

This is speculation on my part, but it appeared to be more of a gasket shrinkage problem followed by bowl throttle body interface fretting. This airplane had been a hangar queen and is only being flown rarely. I'm not sure if that is pertinent information so it's just a data point.

In my case, I have flown the Warrior frequently since 2013, but under previous owners, it had sat for long periods of time unused. The engine/carb had about 520 hrs since major overhaul at the time of failure.

birddog486 · #9 09-24-2017, 07:30 AM

I've had the same loose screws on my Cherokee. The local IA said he's seen this a few times a year for the past 40 years and a drop of Loctite has fixed them all.

1001001 · #10 09-24-2017, 08:20 AM

One could take away two different conclusions from this...probably among others.

1. The issue, having been known for decades, and resulting in few damaging or injurious incidents*, is mitigated properly by annual/condition inspections, and by operational checks of the engine.

2. The issue, having been known for decades, and resulting in few damaging or injurious incidents*, represents an example of normalization of deviation, and should be examined more deeply before additional close calls mount into more severe incidents involving loss of property, injury, or fatality.

* It would be interesting to see if there is any record (NTSB, FAA, or otherwise) that can tie this issue with aircraft incidents. Engine failure in flight, possibly catastrophic loss of power on takeoff, or other accidents caused by failure of the carburetor may or may not have been properly attributed.