Overly Rich Mixture

[Richard Lanning]

My #1 cylinder temp has always been about 70F lower than #2, EGT about 100F lower in general. I always thought I had an issue with the #2 cylinder.

I fly a carbureted PA24-250. No engine add ons though I do have a fuel flow meter which has consistently oscillated about 2gph in cruise flight.

On a recent flight I went to a much higher altitude than usual. All of a sudden the engine was acting as if it was about to die or blow apart. I have a carb heat temp gauge which was in the yellow. Ahhh, carb ice even though I had not flown through any moisture. Adding carb heat calmed things down a lot immediately which is not what I would expect. I left the heat on for a few minutes then turned it off and things went south again and seemed to be getting worse.

At this point I noticed my #1 CHT and EGT temps were dropping quickly as if I turned the engine off. All other cylinders were normal. I began to further lean the mixture and voila the temps began to rise and things smoothed out considerably though now my other five cylinders were either at peak or lean of peak.

The airplane recently came out of annual. All compressions are good. Mags and plugs are all good.

On my next flight I watched the #1 cylinder closely. On climbout #1 CHT was 270F and #2 was 390F. Again, in cruise it was 70F/100F lower than #2 though #2 was pretty much in line with all the other cylinders. (I always assumed #1 and #2 would be slightly cooler due to their position thus my thinking I had a problem with #2 being too hot vice #1 being too cold.) Only #3 CHT seems to be a bit cooler by 30F than the other cylinders.

After flying for about 20 min and not touching anything #1 CHT began to drop down to 230F. Oh no, here we go again. However, what is puzzling to me is its EGT actually rose to 1300F from 1200F which is what the other EGTs basically read. It stayed this way for about 15 min then the temp climbed back to 240F and the EGT dropped to 1180F. I decided to apply carb heat to see the results and #1 climbed to 250F again and EGT to 1200F. Turning carb heat off it remained approx here for the remainder of the flight.

So does anyone have any ideas on this? It appears #1 is running overly rich. Possibly #3 as well. I have always had a slight vibration in this aircraft and I am beginning to think it is because one or two cylinders are too rich or the other three may be overly leaned. Not sure if my fuel flow oscillations have anything to do with this or not. Since it is carbureted I would not expect even fuel flow distribution but this seems excessive.

I always lean when I fly picking the hottest CHT/EGT. I usually fly at 20"/2300 rpm.

[William Hughes]

At high altitudes and lower power settings I am given to understand that the effect of adding carb heat can result in more even distribution of the fuel/air mixture (hot air evaporates fuel faster and distributes it more evenly). Which certainly seems to match what you are describing by adding carb heat at high altitude if #1 cylinder were far richer than the others.

It also matches what you describe on climb-out presuming the #1 cylinder is really really rich. As in fuel pooling in the intake tube rich. That almost sounds like fuel-air starvation or a restriction though. With the full power application there is far more air moving through there and far more fuel - a restriction in the intake would have a stronger effect at higher airflow.

A mechanic could pop that valve cover quick and have a look for obvious stuff, as well as check the plugs for obvious differences between #1's plugs and the others.

Might also be a leak in the other five intake tubes rather? Something involving gaskets where they are sucking cold air that hasn't been through the induction? So they are lean and #1 is rich.

I am having a hard time imagining what could squirt fuel up along one side of the carb so that it mostly went into #1, but perhaps there is something up with the accelerator jet?

My guesses:
- something up with a plug in #1 causing un-burnt fuel in rich mixtures that doesn't show up on a mag check - cheap and easy to check,
- something odd with the valves in #1 causing a restriction in flow - not cheap but easy to check,
- something weird and hard to imagine wrong with your carburetor (unlikely),
- five intake leaks on cylinders #2 through #5 that are all nearly the same (lottery grade unlikely),

You should be able to reproduce this for your mechanic on the ground in a low power leaned out run-up?

[Tim Weigle]

Check the steel tube swedged in the oil pan on #1 and make sure its not loose.

[Richard Lanning]

Thanks for the responses. Plugs were checked and they were good. Just came out of annual. Mags were checked good as well.

#1 being cooler than the rest has existed since I bought the airplane two years ago. Just recently did I really figure out why since I normally don't fly at higher altitudes. (Florida is pretty flat. )

I never really noticed #3 being low before so maybe something is developing. And #4 EGT is also concerning me. Not sure how hot I should allow it to get in a climb but it is my limiting cylinder on a climb right now.

Mechanic will do some more digging this weekend. The joys of owning an old airplane. Actually, it owns me.

Thanks again.

[Zach Grant L1011jock]

EGT is relative. There are no EGT limits, and all references to EGT in any technical manual are simply references to peak! So, that means the temps you are reading are not really important for anything. Depending on where someone drilled a hole in your exhaust, you will have a hotter or cooler reading. On a carbureted engine you will also se variations in distribution based on throttle position. The air/fuel mixture through the throttle valve is most even at wide open throttle. At less throttle settings, you will see different cyl become leaner and richer.

CHT is a limit. CHT is not soley dependent on mixture, as a matter of fact CHT is more dependent on airflow, and is minorly affected by mixture in the grand scheme of things. Think of it this way. Without the engine running the CHT will read ambient temperature. Put ice on one cyl and a heater on another and you haven't changed the "mixture, but you have decidedly different CHTs. The fact that your #1 cyl is cooler is normal. That is the first cyl that sees air coming in the nose, and sees the coolest air. I would hazard a guess that your right hand nose baffle probably is pulled under the cyl and is not adequately pushing cooling air up and over the front of the engine adding to your CHT disparity in cruise.

Now for the science behind these engines, or the lack thereof. In a carbureted engine, you cant have an overly rich cyl, only an overly lean mixture. If the intake has a leak it goes lean. If the plugs look good, then over lean isn't your problem. If the engine is running fine and you are burning at or near book FF, then you don't have a problem. This brings me to my complaint with new engine instrumentation being applied to these imperfect engine installs. It flew for 50 years before anyone realized there was a "problem", and most mechanics doesn't understand enough about what is really going on operationally to tell you you don't have a problem. They usually spend lots of time trying to find something that isn't wrong, and you pay the bill for it.