Tuesday, August 11, 2009

Remember Carburetors?

Back in the day, there were carburetors everywhere! I remember owning at least six of them -- in two automobiles, one outboard motor, a snowblower, a lawn mower and a chain saw. Today I only have one, a Marvel-Schebler MA-4-5 bolted up to the underside of N631S' big Continental O-470 engine. Usually, it just goes quietly about its business of blending avgas with air and delivering the mixture to the engine's intake manifold but today it called in sick.

I went to the airport early this morning intending just to clean the windshield. I'd picked up more than enough bugs over Long Island yesterday morning. When I got to the airplane I saw an extensive blue stain on the top and sides of the nose gear fairing. That could only be the result of fuel dripping from the carburetor. This is clearly not a Good Thing. So I walked over to Three Wing's maintenance hangar and told Skip my tale. He said they'd have a look at it.

The carburetor in question looks just like this one:

Expecting the worst, I did some research on rebuilt carburetors. I learned that one can acquire a rebuilt for about $850 plus a $500 "core charge" that's refunded when you send in the old one.

Fortunately, it is looking like a complete overhaul will not be required. An e-mail from Skip states:

Frank, the metal float in your carburetor is worn and was chafing which we believe was the cause of the problem. The current float replacement kit [from the manufacturer] is $267.00 and takes about an hour to install. The labor to remove and install an overhauled carburetor would be the same to remove and install your original carburetor. We can have the float kit here tomorrow.

The float kit installation is straightforward in comparison with a full overhaul of the carb.It involves separating the bowl from the carburetor body and replacing the float, its shaft, and several minor parts and gaskets. The exploded view below (showing a similar carb) will give you an idea of which bits are involved.

While they have the cowling off they are going to change the oil (it's been about 40 hours) and they've already done the wear inspection of the pilot's seat rails required each 100 hours. Yep, N631S has flown 97 hours since the Annual Inspection back in March.

The good news is that it looks like I'll have an operational airplane to take me home on Friday.


5400AirportRdSouth said...

I was surprised when we pulled an old radial engine apart that the venturi openings on it were the same size as an old Holley or QuadraJet auto carb.

We wondered why this would be thecase when the engine eats a LOT more fuel... THe best theory we came up with is that maybe physics limits the size of the venturi being able to properly atomize the fuel.

Anyway, just found your blog, great stuff!

Frank Van Haste said...

I haven't got a theory on why the venturi bores on a big round engine's carb seem small. One would expect that the design's key dimensions would scale with fuel/air mass flow rate.

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