IowaRV9Dreamer

IowaRV9Dreamer

Well Known Member
I need some help from a Lycoming guru. I have an O-235-L2C engine with a stock carb, stock mags. Just installed multichannel EGT/CHT and I see something interesting:

At cruse (75% power) I can lean to peak EGT (#2) per Lycoming and I get very even EGTs & CHTs. All 4 EGTs are within 30-40 degrees. Engine runs smoothly. Amazingly good, I thought!

But at full power, full rich I see a 150-200 degree difference in EGTs, with cylinders 3 & 4 (the back 2) both lower than the front two. The CHTs are also both lower for the back 2 cylinders. The engine has always run noticably rough at full power / full rich.

There are no problems with probes, etc. Compressions are all OK, timing is on spec, and I just set the valve lash.

I know that a 150 degree EGT difference is not unusual for a carb'ed engine, but what I think is weird is that this engine can operate in a very balanced manner at lower power / leaned, but is more out of kilter at full power.

Does anyone have an idea what might cause this? I know it might not be fixable, but it seems like at full power / full rich (like in a climb) the back 2 cylinders aren't making as much power as the front 2. This would explain the roughness. I could sure use a bit more power, especially when climing at 300 fpm on a hot day :)

Thanks for any insight. My next test is to climb up high (8500, that will take a while) where I can try full power (full throttle) and lean to peak EGT and see if the EGTs level out. In other words, is the carb throttle plate causing the problem.
 
I see similar results with my carb'd RV-8 although the differences aren't quite as dramatic. If you have one on the original carbs, as I do, they use more fuel at full throttle than the newer ones. Jon Thocker's engine is the same as mine, built at the same time by the same engine builder and he has one of the newer carbs. His engine runs hotter than mine at about 1.5 gal/hr less fuel flow at full throttle. This is good for engine cooling, but when I get above about 4,000' on a hot day it starts to 'pig out' - too much fuel for it to swallow and maybe the intakes tubes on the back two cylinders are slightly shorter allowing an easier path - and more fuel - to those two cylinders. Just a guess, but when I feel this roughness, I just lean a little, even in a climb at full throttle and things balance out. If you try this, watch your EGT's and don't lean too much in the climb. When I'm cruising at 60-65% power and leaned to lean of peak on all four, everything cools off a little (EGT's & CHT's) and all temps are within a couple degrees of each other.

Scott Hersha
RV-8 FB
 
I saw the same thing in my previous O-235 powered plane, although I don't remember which cylinders were hotter/colder.

Everyone told me that carburetors do not evenly distribute fuel to the individual cylinders.
 
One possible explanation...

When at less than full throttle, the throttle plate on the carbs that we use, rotates to act as a deflector slightly biasing the mixture flow towards the front cyls.

You probably really do have the missmatch that you see when at full throttle, and partial throttle compensates for it.
 
My O-320 has higher EGT's at WOT on the front two, although not as dramatic as yours. I believe it is due to uneven fuel/air mixing just above the carb. Backing off the throttle just a little helps and brings things reasonably even. I am within a few degrees at cruise.

You might try pulling carb heat. While it does cut the power a little, it also evens things out. I often cruise with carb heat on to be able to lean better. I would guess that the O-235 would be similar, but you probably want a little higher percentage power than the bigger engined RVs. I often use 55-65% unless I am in a hurry. One of the trade-offs with different engines. . .

Bob Kelly
 
Same for me

I see the same thing consistently with my O-360 RV-8. The EGTs are spread out while running rich, and come closer together when leaning toward peak. My guess is that with the richer mixture the fuel doesn't get vaporized as completely in the intake header, resulting in more tendency for droplets to be deflected unevenly.
 
Thanks for all the helpful info guys. I don't think there is anything "wrong" - I'm just trying to understand this weird effect. I want to know if the uneven fuel distribution is due to full rich (mixture control) or full power (throttle control)......
 
Your carb. may have an econmizer valve. Check if there is a -32 suffex on data plate. This carb is design for cooling a full throttle.
 
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