Carburetor Failure MA-4SPA

[YellowJacket RV9]

When I had my carb rebuilt at Performance Carb, Ron let me watch the process and warned me about those tabs. He said that they will always be able able to wiggle a bit, and that you had to make sure they were held as far CCW as possible when the screw was tightened, otherwise they are able to turn a tiny bit and let the screw loosen. Not saying this was the cause, but seeing it with my own eyes, it made sense.

Chris

[jpowell13]

Same thing happened with the O320 in my 6A. I had dieseling too. Also, had momentary engine hesitation on acceleration. With RVs you can reach into the carb air inlet during preflight. If the airbox wiggles when you press against it, that carburetor bowl has probably loosened since the airbox is bolted directly to the carb bowl.

John

[snopercod]

Is there some reason that one shouldn't Loctite those screws?

[smokyray]

Originally Posted by 1001001

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.
Check it out before further operations.

Dave,
As Paul stated, this is an old occurrence and really should be an AD.
It's on my RV pre-buy look see list as well as I encountered it numerous times on inspections and on my own RV4.

My fix was to use a very thin film of Permatex black form-a-gasket (non silicone) on the gasket and loc-tite on the screws. I also drilled the screws and safety wired them.
No worries afterward...
V/R
Smokey

[fixnflyguy]

It happened on my RV-4, and I directly associated it to the engine torque/ snorkle configuration typical of R/V's. I "loosened" the joint for the snorkle/cowl interface by increasing the clearance and slotting the rubber seal a little more. I have advised several other R/V owners, who also found the loose bowl issue and corrected it before it was a problem. I often do a periodic "wiggle" check of the snorkle before flight and during un-cowled maintenance.

[sblack]

An excellent addition to the list of things to look at with the cowling off. Thank you very much for posting. Many of us would never have known otherwise. I will also be double checking the clearance between the airbox and cowling to ensure I am not putting a lot of load on the airbox.

[Low Pass]

Originally Posted by smokyray

Dave,
As Paul stated, this is an old occurrence and really should be an AD.
It's on my RV pre-buy look see list as well as I encountered it numerous times on inspections and on my own RV4.

My fix was to use a very thin film of Permatex black form-a-gasket (non silicone) on the gasket and loc-tite on the screws. I also drilled the screws and safety wired them.
No worries afterward...
V/R
Smokey

Same story, +1. Loose screws. Locktite on screws, drilled heads, safety wire. No more problem.

Not a surprise in hindsight. The carb sits quite a way from the centerline of the engine, rigidly mounted, meaning it moves/vibrates a fair amount. No pins for lateral location of the cast parts as I recall. No shocker that a dry thread in a casting would work loose in those conditions.

Not sure if previously mentioned, but it will result in a rough idle, hard to start condition well before the bowl falls off.

Ultimately removed the problem switching to fuel injection.

[rvbuilder2002]

One factor not always considered is the influence that the seal between the FAB and the induction inlet of the cowl can have.

If you have a rather tight seal between the two, engine start-up / shut down movement and operational vibration induces loads across the joint in the two carb. halves.

This is also a factor that influences the lifespan of the adapter plate that connects the FAB to the carb. (or servo).

(post, post I noticed that Bill already mentioned this.....)