Leaning a Carbed Engine

PaulR · #1 02-25-2013, 03:14 PM

So I've read all these FI LOP threads and I can't go much LOP with my carbed O-320. I've tried carb heat and just about everything else that's been suggested. Probably need another PMAG.

Is there any definitive reading on leaning carbed engines?

I was always taught that at anything below 75% power you could pretty much lean to rough and enrichen to smooth, which is about what Lycoming said in the bulletin that was on this forum.

Are CHT's the determining factor?

When below 75% in cruise configuration, I can lean to peak and my CHT's only climb a few degrees, maybe 10 at most. Always now below 380.

Is 50 or 100 ROP better?

I just really want to economize the best I can, but also don't want to burn an engine up due to detonation.

I know that FI is more economical but that's not an option right now.

Any help will be greatly appreciated.

scsmith · #2 02-25-2013, 03:23 PM

A few things:

1) what fuel are you using? Although it is not necessarily "good" for it, I think there is enough detonation margin with 100LL that you can probably lean to peak without detonation.

2) CHT would then be deciding factor. Traditional Lycoming-recommended 'lean until rough, then richen until it just smooths out" probably puts you at peak or very slightly LOP. below 75% power (meaning WOT at about 8500 ft, or 2400 RPM at (I'm guessing a bit here) around 5000 ft) you can't hurt it running it at peak. Sure, LOP is better for it, and more economical, but there are a lot of Cessna's and Pipers with engines that make TBO when run according to the POH.

So, again, if you are running 100LL, just follow what lycoming says.

PaulR · #3 02-25-2013, 03:26 PM

Thanks Steve, running 100LL. 9:1 compression in an O-320.

uk_figs · #4 02-26-2013, 06:39 AM

For what its worth even with all the instrumentation in my carburated 0-360 -7 (D-100 and D-120) I use the same technique I used in my Pacer

. I lean until the engine gets rough and then richen until it smooths out, I then give half a twist on the mixture. The Dynon has a read out for ROP, Peak and LOP which I assume is computed from the various sensors and at the above settings it generally flip flops between PEAK and ROP. I occasionally have to tweak the settings to keep it in this spot.
At these settings at 55% power I burn about 6.9 gph at 165 mph and after 230 hours the plugs looked perfect during the annual this month.

RV10inOz · #5 02-26-2013, 02:37 PM

LOP ops really test out the ignition system, so plugs must be healthy and if you have a pmag keep it on the more conservative curve.

O-320 in an RV6 I fly at times will run smooth LOP not problem at all. But no two are the same.

Intake leaks are your biggest enemy, so this is a must for any engine, but it is surprising how many engines have them.

Try cocking the throttle plate a bit slightly pulling the throttle off a fraction, that and a small amount of heat.

PaulR · #6 02-26-2013, 03:13 PM

Dave,
Mine will run fine at 10 LOP but any deeper than that and it's rough. PMAG is on conservative curve now.

New engine at least less than 100 hours so I wouldn't think intake leaks would be the culprit? Have checked all the hose clamps on the rubber tubes and they are tight. Intake gaskets were torqued before the first engine start. How else do I check them?

I've tried small amounts of carb heat as others have suggested but honestly haven't tried WOT at altitude that's below 75% yet. I have tried it with the throttle partially closed though and 10 LOP is the best I have been able to do and was always afraid that wasn't lean enough. Though CHT's are always below 380 in cruise.

jrs14855 · #7 02-27-2013, 02:15 PM

The carbed 0 320's have a reputation of poor mixture distribution. A different carb may help. Another possibility is to determine the leanest cylinder and wrap the intake pipe with insulation. This supposedly provides a cooler/richer mixture for the lean cylinder. The ultimate fix if you wish to retain the carb, is the Superior sump which was designed to improve mixture distribution. Bart, the Canadian engine guy, claims he can do further rework to the Superior sump to further improve mixture distribution. The Superior sump was around $500 as of last summer.

Yen · #8 03-06-2013, 04:08 PM

I have been reading all the posts about leaning and one thing intrigues me. How does the mixture control work? Some people say it doesn't work while idling and others say that it does. I have looked at Marvel info and it appears to me that the mixture control is a sort of adjustable jet, upstream of the main jet. If that was the case I can't see how it would stop the engine as the idle jet would still be operational.
I can't find any details of fuel flow within the carby to shed light on the system. Does anyone know where that info would be available.

flyingriki · #9 03-06-2013, 04:53 PM

EAA has a webinar coming up on the subject of leaning.

Finley Atherton · #10 03-06-2013, 04:57 PM

The mixture valve is before the main jet/idle delivery vents. So close the mixture valve and no fuel flows to the jet/vents.
See the cut away diagram page 4 http://www.kellyaerospace.com/articl...essory_AMT.pdf. Read the whole article - lots of good info especially about setting the Idle mixture screw on P3.

Fin
9A
Australia