Alon exhaust and carb heat
Some important info for anyone with the Alon style muffler. If you have an Alon, and the carb heat shroud is not 16.5 inches long, then you probably do not have adequate carb heat to melt carb ice in all conditions.
"My carb heat made very little change in the rpm during run up, and I assumed it only barely met the 90° temperature rise requirement in the CAR's it was certified under. After installing a monitor with carb air temp I saw it was not the case at all. The temp only went up 20°, explaining the lack of rpm drop.
"On the test flight after the installation I noted the temp and planned to get it checked out after landing. It was an evening flight with little temp/dew point spread just barely above freezing. The short flight around the pattern was enough to get severe carb ice and it was my first time having an engine quit in flight. Pumping the throttle let to the accelerator pump blast enough ice off to get me to a safe landing. 0/10, do not recommend.
"YES, the flapper for the heat is in good condition, as are the hoses and other heater parts. The problem upon comparing exhaust pictures appeared to be the heat muff was too small, so I ordered a new one and as you can see there was a significant difference in size. 16.5" length on the new shroud, and I'll have to send the exhaust out to be changed back to the original dimensions for the new shroud to fit."
I discussed the modifications needed to get the original size heat muff to fit with Custom Aircraft Parts and possible reasons for the change. I was told it was likely a previous overhaul/repair person didn't have the correct size shroud ring to fit the 1.5" pipe, and instead decided to move the ring down to the 1.75" section and shortened the shroud to fit. The lower ring relocation was most likely for fitment issues from a warped exhaust in the past and a patched lower cowl backs that idea up.
Here is a picture of the new exhaust. I wrote the existing clearances on the old exhaust so when they built the new one they could change it to fit in the cowling better.
Come to think of it, I didn't have exhaust interference issues until after I replaced the engine mount rubbers, and after that the exhaust needed to be tweaked to fit correctly. If an airplane has exhaust interference issues new engine mounts should be tried before changing the exhaust dimensions. I'd bet that's what happened here, they changed the exhaust before refreshing the engine mounts.
I had them stud the exhaust for me while they were at it. My temp rise is improved, but it's still only between 90-100°F on the CAT display. It may be thrown off a bit by the probe location in the carb venturi, but the RPM drop is 100-125 now or 3-4 times greater.
Russell Booher