Hello,
New to the forum, asking for opinion on what we uncovered during my recent annual. I attached a photo so hopefully it is visible and it clear enough for everyone to see. The plane is a 1969 Cherokee 140 O-320 E2A. Basically, the section of exhaust under the shroud which sends warm air to the carb appears to be twisted and/or deformed. It has been several years since the shroud was removed but I never remember it looking like this. The slip joint was frozen so we dropped the two sections together and replaced. But the mechanic (who is new to the plane) has never seen anything like this. He said I appear to be leaning correctly based on how the plugs looked but I'm not sure if that would be conclusive on something like this. To me, it looks like it was hot, but why only under the shroud? The rest of the pipe appears fine, exhaust inspection was good as well. Any one out there have ideas?
The steel itself would have had to be bright red to deform like that (>1450°F). Although your mixture will have a large effect on EGT, your exhaust system should be able to deal with anything your engine is capable of generating. Even a ridiculously damaging mixture setting like peak EGT at full power should not be able to cause that high a temperature that far down the exhaust.
If it was in the area surrounded by the Carb Heat Shroud, it might have something to do with the shroud. Normally carb heat air either gets exhausted into the cowl or directed into the carburetor. Was your carb heat working properly? When carb heat is off, is anything preventing the air from flowing out of the shroud?
I would guess there is no air flow through that heat shroud over that section of pipe when the heat is shut off. If that's the case, the pipe will overheat and can deform. Ensure there is always good air flow through the heat shroud over a pipe even when the heat is off.
My mechanic did find something wrong with the carb heat mechanism but we didnt get into the details of what it was, only that he made repairs. I will reach out to him and probe that issue further. He did reinstall the shroud but I will investigate that further in case there is something else that could be blocking the air flow over it. That would make the most sense since this problem was found under the shroud only. Thanks for responding and the advice.
No indication of mouse nest, never saw any debris during preflight and the shroud looked clean on the inside, so I dont think that was the issue. But when I am back at the hanger, I plan to spend some more time looking this over more closely
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