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A note on intake connections for ULS models


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We've got two CTLS's in our fleet, one of which is mine. Recently I had a report by someone who usually flies my CT about the other one, that it didn't seem to be developing engine power. It was usually written off because they have slightly different prop pitches, and my engine is brand new. However, I make it a habit to check out squawks anyways. I jumped in and noticed the same problem, and played with the controls to start eliminating possibilities. Upon pulling the carb heat while at full power, I noticed an instant rise of over 200 RPM. Engine was still running smooth, so I let it run a short time and was watching the RPM climb slowly.


For a short moment I thought it was carb ice. Pull carb heat, gain RPM over time right? Well, that didn't explain the 200 RPM rise the moment I pulled carb heat. Next I thought it could mean there is an induction problem and the carbs were running rich, but then again that's the purpose of the venturi, to act as a measuring device to meter fuel. Lower intake, lower fuel. Hmmm... Maybe the carb heat control isn't functioning properly. So I brought it back to the hangar, popped the cowl, slipped a borescope behind the plenum, and watched them actuate. The levers worked fine. I was at this point, going to pop the scat tubes off at the shroud and intake box to see if the butterfly valves were working properly, but I decided to read the section on the manual a bit more closely to see if there's something I missed before I go taking things apart.


I was reading the section on the intake, the carb heat, etc. Nothing that I haven't read before. Wait a sec... that picture of the plenum, how did they take that photo? (it was a photo of the pilot side of the plenum, that's buried against the firewall) Must have had the engine off or something. Wait, carb heat is on the right side of that photo, fresh air on the left...  Why would they... but that means... HUH? I better go check this! Went back out to look at the plenum again. Followed the scat tubes to their respective intakes. Left side went to fresh air, right side into carb...


But do you see the error?



I'm looking at the plane from the prop side. The photo is from the pilot side. That means the tubes are reversed! So I ran over to my CTLS, popped the cowl, had a quick look... yup those are in the correct order! The other one has the tubes reversed! Someone forgot to account for the fact that the photo shows the plenum from the opposite side that a mechanic stands while looking into the engine. The fresh air intake side actually crosses over/under the carb heat tube on the way to the intake on LS models. We had not had any major work done that required an engine pull since we got the plane, so it was probably like that for a long time!


After correcting the problem, the regulars who fly that CT noticed that performance was quite a bit better!


Why didn't I notice this before? I don't fly that CT. I've only flown it one time, and that's when I noticed the carb heat control wasn't working properly and it was because of a shroud alignment problem. I didn't have my A&P at the time, so I couldn't correct it myself. Why didn't anyone else notice this? Good question, but to be fair, carb heats on either of them never really did make any noticeable effect on the ground run, only once you got things warmed up and at a high power setting did it really make a noticeable change. Carb heat intake is unfiltered, so there isn't much of a pressure drop in the carb heat intake, and there isn't a lot of heat while on the ground.

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This sounds very familiar only my problem (2008 LS) was that suddenly I had lost 150-200 RPM in the early months of this year so the hoses being in the wrong location could not be the issue.  Lots of theories, explanations, and suggestions were thrown around from fuel flow, carb balance, intake, etc...  Problem was, the engine  still as ran as smooth as silk through all RPMs but, was just 200 RPM less than what it should have been which does not add up with what the typical application of carb heat would change it.  After checking and fiddling with many of these checks over the last several months, I gave up and re-pitched the prop because the loss made climb out uncomfortable and my 5 year rubber change is due at the end of the month.  I figured since the rubber change would include the carbs and fuel lines etc..., it might be solved then or???  One of the checks I had done was the operation of the carb heat cable.  All seemed OK but, it is very difficult to see the linkages that move the butterfly valves to verify their full travel...  After re-pitching (and some fiddling with these linkages even though the cable "appeared" to be actuating these OK) I gained 400 RPM.  What the ???  I should have only gained 200 RPM so why 400?  I pulled the carb heat and some of this loss came back but not all of it.  Still stumped, I decided that somehow this linkage was definitely the cause but not entirely sure yet since the numbers were just not adding up.  Not liking this new RPM (I was quite happy with where the prop was before this mysterious RPM loss and was reluctant to change it), I again re-pitched back to very close to where it was before the RPM loss and made sure the carb heat was completely off (since cycling the carb heat was no longer moving anything).  Then I opened the panel to find that the cable had kinked (between the knob/rod end and the cable housing) and finally broke (which explains why the previous flight it all of a sudden was easy to cycle in/out. Now, on the ground, it was not moving anything).  Long story short, the new pitch puts me back where I want to be with respect to RPM with the cable broke (and needing fixed or replaced) we made sure the valves were full open for fresh air and closed for heat.  The only thing I can figure is that the cable (which is very flimsy) got kinked over time during my pre-flight run-up checks and finally was not allowing the fresh air to be fully open (mostly blocked and not getting enough air flow).  Post pitch and carb heat/fresh air valve tweak, all is back to normal which is a huge relief!

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