unfiltered airbox flapper valve operation

[Joel Severinghaus]

Can one of you other purists out there who still has an unfiltered airbox help with this?

The mechanic who's working on my prop strike engine repairs phoned today to say that the flapper valve lever on my replacement airbox appears to be brazed on to the hinge at the wrong angle. We both got more confused as he tried to describe the problem over the phone, and he didn't have a digital camera handy to email me a photo. And he and my ship are 65 miles away, so I can't just run out there to have a look.

Photos below show my old airbox before and after my nose-over incident. My recollection is that, if you were looking at the left side of the airbox (from the airplane's nine o'clock):
lever at four o'clock - flapper valve closed - carb heat cold
carb heat cable pulls the lever up and back counterclockwise, and
lever at two o'clock - flapper valve open - carb heat hot

Is that how your unfiltered airbox works? I think my mechanic was saying this replacement airbox has the lever pointing at ten o'clock rather than four o'clock when the flapper valve is closed - carb heat cold. That don't sound right...

[Robert Lees]

Re: unfiltered airbox flapper valve operation

If it works it woeks, eh?

Mine is different to yours, in that t he operating cable approaches the lever from the front, so the cable loops past the carb and almost loops inside the nosebowl. No photos at the moment.

I don't know which is right, but mine works great, so I'm leaving it alone.

[mulwyk]

Re: unfiltered airbox flapper valve operation

Joel:
picture of my airbox, not hooked up yet. Shown in the open position with the lever at 2 o'clock. Closing the valve (hot air to carb) rotates the lever almost 90 degrees CCW to the 12 o'clock position. As Rob says, the cable approaches the airbox from the front.

[Joel Severinghaus]

Re: unfiltered airbox flapper valve operation

Well, now I'm scratching my head, wondering why different airboxes would have the lever arm set at different angles. I guess I prefer my old set-up, where the shorter cable approached the airbox from the rear to pull the arm up, rather than a big bend in a longer cable to approach the airbox from the front and pull the arm forward. That's also what an engine installation drawing shows. So now my mechanic and I are talking about the feasibility of re-welding the flapper valve to the shaft at a different angle (like my old one) without destroying the airbox in the process.

Anyone else have an unfiltered airbox? How does yours work?

[Robert Lees]

Re: unfiltered airbox flapper valve operation

Bob, that's exactly what mine looks like.

Joel, I think the arm is brazed.

Rob

[Joel Severinghaus]

Re: unfiltered airbox flapper valve operation

Rob, yes, you're right -- brazed, not welded. Slip of the typing fingers.

My mechanic, whose favorite phrases this week seem to be "let sleeping dogs lie" and "it is what it is" (he's anxious to finally get my plane out of his shop) is now looking for a longer carb heat cable to route like yours and Bob's, having ended any talk of modifying the airbox.

[Robert Lees]

Re: unfiltered airbox flapper valve operation

I bought a carb heat cable from Spruce. Plus the label (not original, I know). Trim the outer & inner separately to fit.

[Joel Severinghaus]

Re: unfiltered airbox flapper valve operation

My mechanic had a spare new cable on hand, but it would have required replacing the original carb heat knob in the cockpit, so that was out of the question. We looked at removing the wire from the original knob, but it's swedged in the knob shaft really well. And we both still preferred the shorter cable routing and mechanical advantage of having the arm rotate from four o'clock to two o'clock.

So he melted the brass brazing with a torch, twisted the arm ninety degrees, and TIG-welded it back on. Now it works like a charm.

Photo below is after sand-blasting and welding, before repainting.