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Recent discussions an IA/A&P friend pointed out the repair on our carb air box was not a proper repair and that a rivet from that repair was missing. That rivet likely went into/thru the induction system and into the engine.
This mechanic mentioned how Air Box issues can cause partial or full loss of power by blocking the induction system.
We had not looked closely enough(most people don't) at the air box and no doubt ours in not usual. So look at yours closely checking for pivot bushing(fore/aft, up/down) wear, good sealing at the butterfly, proper hardware at the control arm/linkage connection.
After watching the induction video with Chase Duncan, I got motivated to have a look at mine this morning. The first thing I found, and should have seen long ago, was the missing seal on the back side of the NACA intake. My bearings are also shot on the carb air box butterfly shaft, but the butterfly looks ok from what I can see. My bellows between the carb air box and the carb adaptor looks original (1981).
BTW, I was the one who asked about my carb temp rising when I closed my cowl flaps at the live Ask the A&Ps. There are plenty of possible leak points, as it turns out.
Attached are photos of the NACA duct and the bellows.
Well the missing seal explains the CAT increase when closing cowl flaps and hopefully motivates others to look at the same. Somehow our airbox details are commonly overlooked at annual inspection so as owner/operators we must check this item.
Chase Duncan mentions the airbox seal material is available at your local auto parts store, but we used 1/2" thickness EPDM foam rubber from McMaster-Carr found at https://www.mcmaster.com/1566N248/ for $58.74
Not surprised your butterfly bushings are also shot, but is surprising your butterfly looks OK. Yes your bellows looks original so is worth closer look to make sure all's good.
My carb temp issue still persists, unfortunately I ordered a new heat box to carb adapter box duct from Airpower and I'm going to get my mechanic to re-route the fuel pump drain such that it isn't impinging on the duct.
I ordered the replacement seal from Cessna and glued it to the box - discovered the NACA duct and the air filter are not square to one another, so at the front side of the duct, the seal was being squished to a very short height, and by the back of the NACA duct, the seal was not tall enough to reach the air filter housing, so there was still an air gap that existed, although smaller than before. I glued on another seal on top of the first one in those areas to close the gap entirely. I have noticed no difference in perfomance (mp was unchanged at full power) and no difference in CHT temps, so at least in my case, it did not seem to do much . . . . But glad it is there!
