The last time I flew an airplane with a carburetor was the mid 1970's. How easy we forget.
Anyway I have continued to investigate all the potential areas to lower cruise and climb CHT's. Since getting my Carbon Cub last April I have noticed how the CHT's move around as the mixture setting was changed but never really took the time to understand what was going on.
So today I decided to run a lean test. The last time I flew a carbureted airplane there was no CHT or EGT, much less a multichannel version. Pretty much just looked at the oil temperature and leaned for peak RPM. How things have changed. Anyway after flying for years behind fuel injected engines with tuned injectors where every cylinder was running within .1 or .2 gallons of the same mixture I am enjoying a walk down memory lane with Carbon Cub's old carburetor. Yes, it is light and simple but very crude.
So here is the lean test today.
First thing that jumps out on this test is that the front half of the motor is in a different mixture world than the back half. There is a .9 gph difference between the leanest cylinder #2 and the richest cylinder #4. So if you were to lean this motor to best power (50 dF rich of peak EGT on the leanest cylinder) you would be running around 6.1 gph no where near the "best power" available to the engine at this throttle setting. The good news is you would see lower CHT's.
Since there is such a gap between the leanest and richest cylinders "best power" (based on the RPM readings, I have a cruise prop) is somewhere around 5.1 gph. At this fuel flow setting #1 and #2 are running near peak EGT while #3 and #4 have not yet reach best power EGT levels.
Of interest in the airplane I was able to run without missing as #1 and #2 went 20 to 30 dF lean of peak, just as #3 and #4 were hitting peak. So it looks like at this power setting I don't need all the fancy equipment, just lean until #1 and #2 are so lean LOP they miss, then richen until smooth and you are probably at the best point.
It would certainly be good news if we could figure out a way to get the same mixture to each cylinder since it would appear you could then run 20 or 30 dF LOP and get very efficient power and much lower CHT's.