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prkaye

Well Known Member
OK very basic question about the carburetor. Why does it bolt onto the bottom of the sump? My understanding of the carburetor is that it meters fuel/air... there shouldn't be any oil in the carb should there? But the bottom of the sump should be full of oil (right?)... i'm picturing all this oil running out that big hole in the bottom of the sump into the carb which is bolted over this big hole.
Obviously I'm misunderstanding something very fundamental...
 
The inlet manifold tubes pass through the sump from the carburetor to the cylinder heads.

Options are limited for a boxer engine. VW bugs and Subarus have the inlet manifold and carb or FI on the top and the sump on the bottom. Lycomings and Continentals combine them both on the bottom so that the crank shaft and propeller thrust line are in the right place and the pilot doesn't have to crane to see over over the top of a great big carburetor and inlet manifold.

Jim Sharkey
 
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One MAJOR advantage to putting the carb on the bottom of the engine is to reduce the chance of an engine fire, if you had a leak.
 
And in addition to the explanation, the hot oil sump preheats the carb a bit, which helps prevent icing. This is why old Cessna's with Continentals always required carb heat on landing, and Pipers with Lycs didn't.

And since I'm a Piper guy, I don't use carb heat on landing either! :D

L.Adamson ---- RV6A
 
Thanks again guys! As always, so much to learn.
My engine came (from Aerosport) with a plate where the carb will go. If I remove this plate to install the carb, will I compromise any anti-corrosion "pickling" that has been done?
 
prkaye said:
Thanks again guys! As always, so much to learn.
My engine came (from Aerosport) with a plate where the carb will go. If I remove this plate to install the carb, will I compromise any anti-corrosion "pickling" that has been done?
Click to expand...

I had a plate on my pickeled Lyc, and installed the carb with no issues, such as gallons of oil running out...:) There shouldn't be much of anything in there. I did protect the carb opening though, just to keep critters and moisture out.

L.Adamson --- RV6A
 
It's been implied but not specifically stated that the fuel/air system passes through the oil sump in CLOSED tubes. Barring a crack or failure of some kind, there is no way for sump oil to enter the carb. In response to your second question, there is likely some preservative oil in the sump, but very little if any in the induction tubes. Removal of the plate should not be a problem, but as suggested you should probably keep it covered or plugged when you're not working on it.
 
With a preserved engine...

There will always be an insignificant amount of preservative oil in the intake. It got there through an open intake valve.
Because of this condition, the carb mount should be kept closed off as much as practical. If the engine won't be run for a while, it's a good idea to place a bag of desiccant in the intake. If you do this, be sure to hang a tag on the cover alerting you that it is in there and must be removed prior to installing the carb.
 
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