I spent a big chunk of my day reading John Deakin’s articles on Avweb about engine management. It is almost all geared toward fuel injected engines, but is still plenty interesting to those of us with carburetors. I’ve been noticing that in certain circumstances like high altitude, full throttle, the #2 cylinder is usually quite a bit leaner than the others. This results in a higher CHT on that cylinder and less leaning capability before the engine gets rough. One thing that sometimes helps is to bring the throttle off of the wide open stop. This sets up a bit of turbulence around the butterfly and mixes the fuel air mixture a little better. The next step is to experiment with the effect of carb heat on the consistency of the mixture to each cylinder.
So I went up for a little flight. The objective is to see if I can achieve a more balanced mixture just by tinkering with engine controls. The evening saw air that was smooth as glass so I could use lots of power and let the speed build. I pointed toward Llano and went up to 6500′ (DA just over 8500′) and left the throttle wide open. Watching the EGTs as I leaned, yep #2 is the lean cylinder by far and its CHT was about 25+ degrees hotter than all the rest. Flew like this for a few minutes to be sure I was stabilized and looking at real conditions. Then backed the throttle out about .2″ of MP and re-leaned. With the GRT engine monitor, I can watch a nice pretty graph of the EGT temps as well as their digital value. This is super useful for these explorations in engine analysis. Yep, #2 isn’t quite as lean as it was compared to the other cylinders, but is still not all that close either. After that stabilized I pulled the carb heat on while watching the EGT graph. Wow, the low EGT cylinders came up and the high EGT cylinders came down. All of the “absolute” EGT values lined up within a few degrees. About this time my heart started to beat a little faster as I’ve never been able to get my EGTs this close together in cruise. I let that stabilize. Yep, stable, so the original objective is about to be achieved, but of course we have to go for the gravy too (LOP), next step… (drum roll) I started a new mixture adjustment and all of the EGTs went up almost perfectly together, right to the top (peak), and before I knew it, they were coming back down the other side of the temp graph with no roughness but just a drop off of power as it came down the lean side of the curve. Holy cow! I’ve never seen anything like that before. Knowing I’m in potentially dangerous territory I chickened out and went back to the rich side. Once back on the sloppy rich side, cruising west at 150kts., I sat there considering what I just accomplished. Yep, I ran Lean Of Peak with a carburetor for just a minute. I didn’t really wait for it to stabilize because I was too chicken, so I don’t really know much yet, but it was an interesting experiment. For those that don’t know what I’m talking about or why it is so exciting, running a carburated engine Lean Of Peak is described as being almost impossible my many people. The advantage is cruising with minimum fuel burn. Like I said, my objective was just to get my fuel air mixture more even on the rich side. Obviously more trials will need to be done to be sure I have accomplished that. My personal conclusion about if I will be trying to run LOP is yet to be made.
