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Old 01-26-2020, 07:55 AM
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DanH DanH is offline
Join Date: Oct 2005
Location: 08A
Posts: 10,695

Annual time, so as promised, I stuck a borescope down the fuel servo and intake throat to look in the sump plenum. Ran it for a hot leakdown, then took the photo 30~60 minutes after shutdown.

For those new to Lycomings, the horizontal intake sump has an upper chamber full of oil, and a lower chamber for intake air. Unlike the vertical draft sumps, the air passages do not pass through the hot oil. Instead, the intake tubes enter the air plenum from the sides.

Part #10 is the sniffle valve.

Intake tube #3 on the left, #4 on the right. Here you can see the threaded sniffle drain passage in the aft quadrant of a small dished depression centered in the plenum floor. That dish would be the lowest point with the aircraft level, like an A-model RV. In addition to the little bit of fuel in the dish, a tailwheel RV will collect fluid at the rear of the plenum floor. Here it's the fuzzy dark streak in the background, behind the tubes. I ran a stick with a swab back there, and got just enough to discolor the swab.

I remain comfortable running this particular setup without a sniffle. However, I shut down with a purge valve, and the airbox has a sump and a drain to divert rainwater when parked, which is not the case with a standard Vans snorkel. And, as Don points out above, not everyone is adept with engine controls.
Dan Horton
Barrett IO-390

Last edited by DanH : 01-26-2020 at 09:20 AM.
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